Copyright © 1997, by Bert Pool

This article describes a bare minimum of Tesla coil theory and complete descriptions of the electronic components used.  Plans for building a Tesla coil capable of sparks over six feet in length are given.

Warning and disclaimer:  

Tesla coils are high voltage devices that incorporate voltages and currents that can be lethal.  This file is intended for persons over the age of 21, who are familiar with the construction and safe operation of high voltage apparatus and who know how to safely build and operate Tesla coils.  Extreme caution is advised, especially when building and operating coils which utilize potential or distribution transformers as a power source. Every attempt has been made to insure that the information contained in this file is both accurate and safe.  The author assumes no liability whatsoever in the event of injury or death or damage to property resulting from the construction or operation of apparatus built using any of the information contained in this file.  The builder should be aware of the many risks involved and should take reasonable personal precautions to insure his or her own safety, as well as the safety of bystanders and personal property.  If you are careless, or if you are stupid, or if you have no experience working with high voltage apparatus, then you should not build Tesla coils or attempt to work with high voltage.

Tesla coil theory in a nutshell

The Tesla coil is an air core resonant transformer.  The power source is a step-up transformer, which drives a capacitor and primary coil, which together form a resonant tank circuit.  A spark gap is used to commutate the high voltage in the primary circuit.  As the tank circuit oscillates, voltages and currents in it reach phenomenal values.  The Tesla coil’s secondary winding is in close proximity to the primary, where it is inductively driven.  The secondary has been carefully built so that its inductance, self-capacitance, and top load capacitance form a resonant circuit whose frequency exactly matches that of the primary tank circuit.  The voltages at the top of the secondary coil can easily reach hundreds of thousands of volts, and in some large coils can reach a potential, measured in millions of volts.  The electrical currents produced are high frequency, high voltage, and are relatively low current.  When the spark length exceeds four feet, or so, the display becomes quite spectacular.

The components in a Conventional Tesla coil

Tesla coils, in their simplest terms, are made from only five components: a power transformer, a capacitor, a spark gap, a primary coil, and a secondary coil.  Many successful coils have been made using only this handful of parts.  However, there are several additional items required if the circuitry is to protect the transformer from destructive over-voltages.  Some of these same transient spikes and the radio frequency noise produced by the Tesla coil can get back into the house wiring and cause problems, so line filters should be used to stop this unwanted electrical noise. 

Figure 1. 
A schematic diagram with all the required parts for a good Tesla coil – “good” meaning a coil which will produce impressive sparks, but which stands a reasonable chance of surviving the incredible electrical stresses which are generated by its operation.

First, we will discuss each of the components used in a Tesla coil, and then we will build and test the coil.

Power Transformers

The first item we will examine is the power transformer.  In small systems, this can be one or more neon transformers connected in parallel.  In larger systems, it can be a potential transformer or a power distribution transformer, often referred to as a “pole pig”.  The purpose of the transformer is to step the line voltage at the wall plug upward to several thousand volts.  Tesla coils typically operate at transformer secondary voltages ranging from 5,000 volts to over 20,000 volts.   Power ratings on transformers used in Tesla coils can range from 360 watts to over 120 kilowatts.

Neon transformers, luminous tube transformers, and furnace ignition transformers

Neon or luminous tube transformers, and furnace ignition transformers are similarly built.  All are self-limiting in current.  In other words, the transformer has been designed to supply only a maximum amount of current under a fully loaded condition.  If you short out the secondary of a 60-milliamp neon transformer, only 60-ma of current will flow through the secondary windings, and only a few amps of current will flow through the 120-volt wall outlet into the primary of the transformer.  I generally figure on about 6 amps per transformer, so a standard 30-amp wall fixture will safely operate up to about five neon transformers.

Figure 2.  
Several neon or luminous tube transformers, and a furnace ignition transformer.  All use a similar method of construction, and are filled with tar or similar potting insulation.

Beginners often start with one neon transformer in the 9 kV to 15 kV range.  The secondary or high voltage winding on neon transformers supplies high voltage at relatively small currents.  Most neon transformer secondary windings can supply 30, 60 or in a few rare transformers, 120 milliamps of current.  A current of 30 milliamps will shock you, but is not very likely to kill you, unless you manage to pass the current directly through the chest, where it could stop the heart.  Any current over 50 milliamperes is very dangerous.  Neon transformers usually have a VA rating on the nameplate.  VA stands for Volt Amps, kVA stands for Kilovolt-amps (thousands of volt-amps).  The terms VA and kVA indicate how much power the transformer can deliver under full load.  For example, a 15,000-volt transformer, which is rated at 120 ma, can deliver 1.8 kVA, or 1,800-Volt amps.  KVA ratings, for our purposes, are the same as watts.  You will often hear a 1,800 VA transformer referred to as a 1.8-KVA or 1,800 watt transformer.  Neon transformers are almost always less than 1,800 VA in rating.  These transformers are often filled with a tar compound that acts as high voltage insulation.   The tar is easily carbonized, especially when exposed to high frequency currents like those found in a Tesla coil.  Unless properly prepared and protected, neon type transformers will not last long in Tesla coil operation (but even when protected, it is just a matter of time before they expire.)  You may find used neon transformers at local neon sign shops.  Often, just for the asking, owners of sign companies will give away their bad transformers.  Many of these “bad” transformers can be opened up and the tar removed, instantly fixing a short or ground condition caused by the carbonized tar.  Never buy a new neon transformer, as new units are very expensive, and you can expect every neon transformer to eventually fail in Tesla coil service.  Because neon transformers have such a high morality rate when used for coiling, it is a good idea to use transformers that you have gotten for free or for which you have paid very little.  Hint: coil builders have found that 60 milliamp transformers hold up under Tesla coil stresses better than 30 milliamp transformers.

Potential and distribution transformers

Potential transformers are used by your local the Power Utility Company to precisely monitor power line voltages.  They come in a variety of sizes, usually ranging from 1 to 5 kVA.  Output voltages can range from about 4,500 volts to over 34,000 volts.  Potential transformers are NOT self-limiting for current.  For example, if you take a potential transformer that has no load connected to the secondary, then connect its primary directly to a wall outlet, only a couple of amps will flow through the primary.  This small current is the amount of power that is required to magnetize the iron core.  But, if you connect a load to the secondary, then very large currents will flow in the primary of the transformer, often hundreds of amperes!  All potential transformers require some method of limiting or controlling the current, which will try to pass through the transformer once it has a load connected to it. A large variable transformer, also known as a variac, can be used to vary input power to the transformer.  Along with the variac, a variety of methods of controlling or ballasting current to the transformer are used.  We will cover these methods of current control in more detail later.  Potential transformers are very robust.  They are designed to handle the abuse that comes with power line service, including the voltage transients induced by lightning strikes.  The high voltage winding is usually designed to withstand 110,000 volts!.  Potential transformers are an excellent choice for operating medium to medium-large Tesla coils.  Internally, they are often insulated with a high quality silicone rubber potting insulation, or a very good epoxy.  You can find potential transformers at salvage yards, and at electric utility auctions.  Bring a buddy along if you think you might buy one, as they are very heavy for their size, usually weighing anywhere from 150 to 250 pounds.

Figure 3.  
Shown are a 2 kVA and a 3 KVA potential transformer.  They are compared in size to a typical 360 VA neon transformer.

Again, be aware that potential transformers require the use of large variacs and massive resistive and inductive ballasts to properly operate them.  That $35 bargain potential transformer you find at the salvage yard will probably cost you upwards of $300 for inductive ballasts and big variacs before you can use it.

It is very important that the Tesla coil builder understand a key safety difference between neon transformers and potential or distribution transformers.  A neon transformer will shock you, and you’re likely to live to coil another day.  A potential or distribution transformer will kill or seriously maim you – there are no second chances.  Great respect and extreme caution are required when working with these large transformers!

Distribution transformers

Distribution transformers, also known as “pole pigs”, are the ultimate Tesla coil power source.  Like potential transformers, they have been designed to operate outdoors in very harsh conditions, and can take severe electrical abuse without failing.  Pole pigs are filled with transformer oil.  Be careful when purchasing old pole pigs, as the oil may contain PCB’s.  PCB (poly chlorinated biphenyl) is a nasty additive, which once was put into transformer oil to improve its dielectric properties, but PCB’s have been proven to be both carcinogenic and mutagenic.  Federal and state environmental agencies have banned PCB’s totally since 1977.  If one of these agencies finds out that you have a transformer or capacitor filled with PCB’s, it can cost you several hundred dollars to have it disposed of at a certified toxic waste incineration site.  Make sure your capacitors and transformers are PCB free before you buy them, else you might end up with hundreds of pounds of highly toxic (and costly) material.

Figure 4.  
A variety of distribution transformers, or “pole pigs” as they are known.

Like potential transformers, pole pigs require external current limiting.  This is accomplished with a combination of resistive and inductive devices placed in series with the primary of the transformer.  A large variable transformer, also known as a variac, is used to control power to the transformer and the ballasts.  Pole pigs are rated in both voltage and kVA.  Typical pole pigs are 5 and 10 kVA, with big ones going to 25 kVA and up.  The input voltage is most often 220 volts, controlled by variacs and ballasts.  You can expect your input current on a pole pig setup to be at least 50 amps, and you might as well plan on putting in one hundred-amp breaker service to your controller.  Output voltage on pole pigs can be anything from 4,500 volts to over 35,000 volts.  Most Tesla coilers use 12,000 to 14,400 volt pole pigs.  If you use a transformer with a voltage less than 7,200 volts, you will have difficulty making a spark gap work.  Voltages higher than 14,400 will require higher voltage rated capacitors, which are much more expensive.

Controlling and Ballasting Potential and Distribution Transformers

As mentioned earlier, potential and distribution transformers require external current limiting.  This is provided by a combination of resistive and inductive elements placed in series with the transformer.  Usually, part of the inductive ballast consists of a large variable transformer, better known as a variac.  The resistive ballasts range in value from a few ohms at most, down to a tiny fraction of an ohm.  Power ratings for the resistance will typically be several thousand watts.  Most often, nichrome-heating elements from electric heating furnaces or ovens are used.  Heating coils out of clothes dryers work very well.  

Figure 5.  
Some heater resistive elements used as current limiting ballasts for large Tesla coils.  Note the adjustable slide tap on the nichrome wire element.

The inductive ballast is most often an electric welder.  The output cables of the welder are shorted together.  The primary of the welder is connected in series with the pole pig.  Because the current going to the pole pig has to go through the welder first, the maximum current that can go through the transformer is the same maximum current that the shorted welder would normally pull.  A shorted welder will typically pull a maximum of 60 amperes.  By changing the current taps on the secondary of the welder, the amount of primary current through the welder can be controlled.  Thus, by changing the current settings on the welder, you can vary the current in the pole pig from a few amps all the way up to about 60 amps.   Multiple welders may be paralleled for more current, or larger welders used.  Some coil builders have made their own inductive ballasts.  Large laminated iron cores are wound with 8 gauge or 6-gauge wire, creating an inductance of about 8 millihenries.  Taps can be placed on the turns of the coil so that different inductances can be chosen from about 8 millihenries down to about 2 millihenries. Typically, for a 10-KVA transformer, an inductance of about 3 millihenries works well when used in series with a resistance of about 0.3 ohms.  Some coil builders prefer connecting the resistive ballast in parallel with the inductive ballast instead of in series.  In this case, the resistance will usually be higher in value.  Using inductive ballast improves performance over using pure resistive ballast because of a “kick” effect on the controlled voltage provided by the inductor.  

Figure 6.   
Typical methods of connecting resistive and inductive ballasts to limit current to potential and distribution transformers.

If you do not have a welder handy, you can take microwave transformers and substitute them as inductive ballasts for small potential transformers.  Depending upon the construction of the microwave transformer, you may or may not need to short the secondary of the microwave transformer(s).  Depending upon current demands, you might need to dunk the microwave transformers in a bucket of oil for cooling.

Coilers have found that using pure inductance, with no resistive ballast, their transformer “bucks” or suffers from large current surges.  When this happens, adding some resistive ballast smoothes out operation.  Most large coils use both resistive and inductive ballasts, and the ability to finely adjust the best combination of inductance and resistance in the ballasts is one of the things that can separate a good coiler from a mediocre coiler!

Variable transformers

Figure 7.  
Shown are several styles of variacs.  Note the dual and triple transformers on one common shaft.  Dual 120-volt transformers can be wired for 220 volts.  The triple transformers are originally designed to be used with 3 phase, 220-volt power, and can still be salvaged for single-phase use.  

Variacs are rated in both voltage, and current, usually 120 or 240 volts, with current ratings ranging from 15 amps to 50 amps or more.  You can generally run a variac beyond its nameplate rating by a factor of up to 100% for short periods of time.  Look for large variacs at salvage yards, theatrical equipment auctions, and inside heavy electrical power equipment, especially radio frequency induction furnaces used to heat treat steel.  Expect to pay $50 to $200 for 50 to 100 amp variacs.  You will be better off buying 220-volt variacs instead of 120 volt if you are planning on operating potential or distribution transformers.  Keep a lookout for 120-volt variacs where you find two variacs mounted on one common shaft.  These double 120-volt variacs can be wired to work on 240 volts!  Avoid 600-Hertz surplus military variacs like the plague.

Figure 8.  
Methods of wiring variacs and ballasts to control current to transformers.  Note the method of connecting two 120-volt variacs to control 240 volts.

Line filters

Tesla coils can produce a lot of RFI, or Radio Frequency Interference.  A coil that has been tuned to produce maximum spark will radiate little RFI through space.  However, you can still get a lot of RFI and transient voltage back through the transformer into the house wiring where it can play havoc with sensitive electronic equipment.  The long “Tesla trail” is littered with dozens of dead radios, computers, VCRs, garage door openers, and other expensive gadgets ruined by severe RFI and voltage transients produced by a Tesla coil, which were not kept out of the house wiring.  It is highly recommended that you purchase and install “line filters” between your wall outlet and Tesla coil power supply.  A line filter can be made from capacitors and inductors, but there are many suitable commercial line filters available on the salvage and surplus market at dirt-cheap prices.  I usually pay $5 for 50 amp rated filters at ham fests.  Even with a line filter installed, I still always unplug sensitive electronic equipment throughout the house before I run any Tesla coil.

Figure 9.  
Several line filters, ranging is size from 5 amps to 50 amps.

Protecting the transformer

There are three devices used to protect the transformer from an early death.  It is recommended that all three be used together.  The first protection is a safety spark gap placed across the transformer.  The gap is set so that normal operating voltage will not jump across the gap, but whenever a high voltage transient appears, the gap will conduct, shorting out the voltage and preventing it from damaging the windings inside the transformer.  On neon transformers, which always have a grounded center tap on the secondary, it is recommended that a safety spark gap be placed on the output of each of the two high voltage bushings.  

Figure 10.  
A safety spark gap constructed of 1-inch ball bearings mounted on ceramic insulators and connected to the transformer.  Note that the center ball is connected to the grounded case of the transformer.

The second protective device is the bypass capacitor.  This is a small high voltage capacitor that goes from the high voltage bushing to ground.  Typical values are 150 to 300 pf.  The 60-Hertz line frequency current will ignore a small capacitance like this, but the cap will appear as a low resistance path to ground for RF noise that might damage the transformer.

Figure 11.  
Shown are typical bypass capacitors.  These are small ceramic and mica “door knob” capacitors.  

The third transformer protection is a choke. Chokes will pass the 60-Hertz low frequency power but will block any high-frequency components.  Chokes are coils wound either on a plastic pipe, or on ferrite toroidal cores.  The chokes are placed in series with the wires leaving the high voltage bushings of the transformer. The ferrite core chokes have the advantage of high inductance in a small space, but a disadvantage is that they must be carefully constructed and insulated to prevent electrical shorts between the layers of wire. Air core chokes must be physically large in order to have enough inductance to be effective.  Because the chokes are located in the low current circuit, prior to the tank circuit, they may be wound with small gauge wire.  A few millihenries are a minimum value for an effective choke.  Many electronic supply houses sell ferrite toroidal cores 2 to 4 inches in diameter, usually for $3 to $6 each.
 
Figure 12.  
4 inch diameter, 6 inch long air core choke wound with 28 gauge enamel wire, and a 3 inch ferrite core choke, also wound with 28 gauge wire.

The safety spark gap, bypass capacitors, and choke coils are mandatory for protecting neon transformers against an early death.  Many coilers do not use such protection with potential and distribution transformers, because the transformers are rugged enough to survive without such protection.  Indeed, the author has never heard of a single distribution transformer failure when operating a Tesla coil.  On the other hand, the author has personally ruined 30 neon transformers; most before he had learned about safety gaps, bypass capacitors, and chokes!

Figure 13a, b, c.  
Typical transformer protection circuits.  The third circuit is a “pi” circuit and is very effective.

One of the things that the coiler has to be aware of when using bypass caps and chokes is that it is possible to accidentally wind a choke that resonates at the same frequency as the tank circuit of the Tesla coil.  This is bad news.  Should this happen, the choke no longer protects the transformer; indeed, a resonating choke can actually create high voltage transients that can damage a transformer!  One sure sign that a choke is resonating at the coil’s primary frequency is that it will get very hot, so hot, in fact, that it can smoke or burn the insulation off the wire!  It is always a good idea to run a Tesla coil that has new chokes for several seconds, then power down the coil and feel the chokes and see if they are getting hot.  If you have a choke that is getting hot, try changing the inductance by using fewer or more turns, or rewinding it on a longer or shorter form, or try a form with a different diameter.

Spark gaps

Tesla coils use a main spark gap to commutate the high voltage from the transformer to the tank circuit.  It works something like this: during the first ½ cycle of the AC current, the transformer moves charge from one capacitor plate to the other.   The voltage appearing across the spark gap at this time is only the peak voltage of the transformer, which is not quite enough to jump the gap.  On the next ½ cycle of the AC current, when the current flows in the other direction, the transformer tries to move the charge from the second plate back to the first plate of the capacitor.  But this time, we have two voltages involved; the voltage of the transformer PLUS the voltage of the charged capacitor.  This summed voltage is enough to jump the spark gap.  When the voltage does jump across the gap, the capacitor discharges its entire charge through the gap into the Tesla coil primary.  This primary current can reach hundreds of amperes in small coils and thousands of amperes in large coils.  Once ionized, the air across the spark gap becomes very conductive, and forms a "closed" switch.  The current stored in the capacitor continues to bounce back and forth between the plates, passing through the primary coil, again and again, each time giving up energy.  This passing back-and-forth occurs at a frequency determined by the value of capacitance in the capacitor and by the inductance of the primary coil.  These two electrical components form the resonant tank circuit of the Tesla coil.  The alternating current passing through the primary, when observed on an oscilloscope, starts out very large and drops every cycle until it reaches zero, forming a damped sine wave on the oscilloscope’s display.  

Figure 14.
The damped wave observed in the primary circuit.

Once the voltage stored in the primary circuit has dropped nearly to zero, the spark gap extinguishes, and the transformer again begins charging the capacitor and the cycle begins anew.  

Cylindrical gaps

Beginning coilers always make the mistake of using a spark gap made out of two bolt heads separated by a short air space, or, even worse, two nails pointed at one another.  Such a spark gap gets hot very fast and quenches the spark very poorly.  The ions from the hot metal will prevent the gap from quenching which will prevent the capacitor from recharging completely.  Several ways of making spark gaps quench faster have been developed.  One of the best methods is to use several copper cylinders in a row, each cylinder separated from the next with a small gap.  This “series static gap” works very well if you use copper cylinders 1 to 3 inches in diameter and 3 to 4 inches in length, mounted side-by-side.  The large mass of copper, open on the inside, allows for good air circulation and excellent heat dissipation.  A large copper cylinder gap by itself is good for power levels up to 4 kilowatts.  For better quenching and higher power, the cylinders can be mounted in one end of a box, called a plenum chamber, and a vacuum cleaner blower motor mounted at the other end of the box.  The motor is positioned so that air is sucked into the box through the gaps between the cylinders.  This “vacuum quenched” gap performs exceptionally well even at power levels of several kilowatts.  

Rotary gaps

A rotary gap is made of a fiberglass or plastic disk with evenly spaced screws or tungsten studs placed around its periphery.  A variable speed motor spins the disk at speeds up to a maximum of 10,000 RPM.  The electrodes on the disk spin past two stationary electrodes.  Every time a moving electrode comes between the stationary electrodes, the gap can fire.  The high-speed movement of the flying electrodes allows for excellent spark quenching.  The flying electrodes stay cool, and the air they stir up helps cool the stationary electrodes a bit.  The disks are usually made out of G-10 epoxy-glass composite material, polycarbonates such as Lexan, or if precautions are taken, they can even be made of acrylic.  The epoxy-glass material has the best thermal and tensile strength properties.  Acrylic is not recommended, as it can shatter.  The polycarbonates are very tough, but could possibly melt; though I personally know of no disk failure due to the flying electrodes melting a plastic disk.

Figure 15.
A Lexan disk is shown with electrodes.  The hole in the center of the disk mounts onto the end of the drive motor shaft.

Many novices make the mistake of building their first coil using one neon transformer and a rotary spark gap. A good cylindrical gap is the only gap that you will need to build, unless you are using a potential transformer, a pole pig, or more than 2,000 watts of paralleled neon transformers.  However, if you do use a big transformer, a rotary gap will be necessary to insure adequate quenching of the high-power spark.  In some cases, especially very high power coils or magnifiers, both a rotary gap and a series static gap with vacuum quenching are used together.   

There is seldom a problem cooling the flying electrodes.  Usually, eight or more electrodes are used, and each one is in the arc only a fraction of the total time.  The electrodes are also moving very fast through the air, which provides very good cooling.  For example, a 9-inch disk spinning at 5,000 rpm will move the electrodes through the air at a speed of over 196 feet per second, or about 133 miles per hour!    The two stationary electrodes, however, have an arc on them almost all of the time, and airflow past them is meager.  The accumulation of heat by the stationary electrodes can be partially alleviated by using large electrodes, though this in itself can cause problems because a large diameter electrode increases “dwell” time.  One of the best methods to deal with heat is to mount the electrodes in massive blocks of brass, which act as heat sinks.  Some rotary gaps have their stationary electrodes cooled with a continuous blast of compressed air.

Some advanced coil builders use salient pole synchronous motors with their rotary gaps.  A synchronous motor runs at a precise speed that is determined by the 60-Hertz power main frequency and number of poles in the motor.  A salient pole synchronous motor will lock onto the 60-Hertz frequency so that the electrodes are always at the exact same position on the sine wave every time the motor is run! This precision allows the user to position the stationary electrodes such that the spark gap fires exactly at the optimum point (phase) on the 60-Hertz sine wave on each cycle. An 1800-RPM synchronous motor will require 4 flying electrodes and two stationary electrodes.  A 3600-RPM synchronous motor will use 2 flying electrodes and two stationary electrodes to obtain one-gap conduction at the peak of each half cycle.  It is generally accepted that a synchronous spark gap is easier on neon transformers than an asynchronous rotary gap, but salient pole synchronous motors can be hard to find.  It is possible; however, to modify a standard synchronous motor into a salient pole synchronous motor, if you have access to a machine shop and milling machine.  Converting a synchronous motor to a salient pole synchronous motor is beyond the scope of this article. 

Figure 16.
Rotary spark gap, showing the Lexan disk, electrodes, and motor.

As a rule, you should use the smallest diameter electrode material that can withstand the power level at which you run your coil. The smaller electrodes will insure shorter dwell time and thus better spark quenching and better coil operation.

Primary coils

The primary coil in a Tesla system should meet several important criteria.  It should be of low resistance, and it should be “tappable” for tuning purposes.  The particular type of primary, i.e., helical, pancake, or inclined pancake, should match the type of secondary coil.  There are three main categories of primary.  The first is the helical or cylindrical design.  This design allows very high coupling between the primary and secondary coils.  This type of primary is most often used on vacuum tube Tesla coils and on the drivers of modern magnifiers.  Because the typical disruptive (spark gap) Tesla coil only needs a K (coupling factor) of about 0.1 to 0.2, the helical primary will often over-couple the primary and secondary, causing damaging sparks to race up and down the windings of the secondary.  Helical primaries also are seldom used because arcing easily occurs between the primary and secondary.  The most frequently used primary design is the Archimedian or flat pancake coil.  The pancake coil offers reasonable coupling, and is normally immune to primary/secondary breakdown.  The third type of coil is the inclined pancake, where the outer edges of the coil are about 5 to 15 degrees higher then the inside turns.  This increases coupling, but still keeps the turns away from the higher voltage windings of the secondary.

Figure 17.
A Helical primary, a flat pancake primary coil, and an inclined pancake primary.

Because of its advantages, I only recommend that the pancake type primary be used on most conventional Tesla coils.  On coils driven with a single-bushing potential transformer, it is possible to connect the inner turn of the primary to ground.  Since the bottom of the secondary is also connected to this same ground, there will be zero volt difference between them, virtually guaranteeing that no arcing between the two coils will occur.  Neon transformer driven coils will have high voltage on the inner turn of the primary, and sufficient space must be allowed between this turn and the grounded bottom of the secondary coil.

Because of the “skin effect” which is present in currents at RF frequencies, copper tubing makes a superb primary coil conductor.  Coil builders use soft refrigeration copper tubing, available in 50-foot coils at “Home Depot” and similar hardware stores.  Do not attempt to use rigid copper pipe!  Soft aluminum tubing also works well, though it is seldom available except as a surplus item.  Bare tubing must be space wound to prevent shorts between adjacent turns.

The primary coil support frame can be made out of Plexiglas (acrylic), phenolic, or even good dry wood that has been varnished.  The purpose of the frame is to hold the primary turns equally spaced.  Some primaries are made out of large insulated coaxial cable, close wound, with no space between turns.  If your coil uses coax for the primary, and the coil is the flat pancake design, then no special support frame is necessary, and the turns do not need to be spaced.

Taps on turns are usually best accomplished by using a copper strap that wraps completely around the tubing or stripped coaxial cable and which has a screw and nut to allow tightening.  It is important that tap connections have large surface area and low resistance, as very large currents will flow through the connections.  Poor connections will get very hot, due to large resistive losses.  A large alligator clamp can be used to initially tune the coil, but should be replaced with a clamp, once good tuning has been achieved.

Secondary coils

The secondary coil is usually wound on PVC or acrylic pipe, although many excellent coils have been wound on cardboard pipes, also known as sonotubes.  Cardboard is not the material of choice, as it is hydroscopic, and the more moisture the tube has in it, the poorer its performance will be.  If you do use sonotube, bake it dry, then coat it thoroughly inside and out with polyurethane and let it dry completely before winding your wire.  Perhaps the ultimate coil form material is polystyrene plastic, as it has a very low dissipation factor, and coils wound on this material will have a very high Q, or quality factor. Polystyrene pipe is difficult to find, however.  Acrylic is very good, but is expensive.  PVC pipe is somewhat lossy, but is readily available and it is very cheap.  More coils have been wound on PVC pipe than any other material.

Figure 18.
Typical secondary coil.

There are some general rules to follow when building secondary coils.  

•	Use the lowest loss coil form material available to you, the thinner the wall thickness, the better.  
•	Coils should be at least 400 turns, but no more than 1,000 turns. 
•	Except for some giant coils, there is no reason whatsoever to space wind a secondary.
•	Inductance translates into voltage, i.e., the higher the inductance, the longer the spark.
•	Coils should not be more than 3 to 4 times longer than they are wide.
•	Enamel magnet wire will allow more turns per inch, meaning more inductance.
•	Do not use Teflon coated wire – it is hard to work with and windings will loosen with time.
•	Do not drill holes in the ends of the tube and stick the ends of the wire through!  To do so 
will invite internal arcing disaster!
•	Do not wrap aluminum tape all the way around the ends of your tubes to “make 
connections easy!"  The conductive tape will form a giant single shorted turn and will steal 
huge amounts of power.
•	You will get much better performance out of larger diameter coil forms.  I never 
recommend winding a secondary less than eight inches in diameter.  Wimpy three and 
four inch diameter coils just will not perform as well as an eight or ten inch diameter coil 
using the same form length and wound with the same number of turns of wire.

Capacitors

Capacitors are one of the most important components of a Tesla coil.  There are several different kinds of capacitors which can be used in the resonant tank circuit, and some will perform much better than others.  One of the oldest capacitor designs is the salt-water and glass capacitor.  All it takes is a galvanized tank filled with salt water, several beer or champagne bottles, some wire, and a bit of oil.  I’m not going to go into construction details, because, simply put, this capacitor sucks.  It is messy, it is not easily moved, it suffers from serious dissipation losses, and you have to build several and connect them in a series/parallel arrangement to achieve a capacitor at a reliable voltage rating.

Figure 19.
Several capacitors suitable for Tesla coil use.  They are shown in the preferred order, best to worst.  Polypropylene commercial cap, homemade polyethylene capacitor, mica transmitting cap, ceramic doorknob, glass plate capacitor, beer bottle capacitor. 

Glass plate capacitors are only marginally better than salt-water caps.  They have very large dissipation losses that will cause them to get hot.  They are very heavy.  The glass is fragile.  I once built a 600-pound glass plate cap with a rating of 0.1 ufd, at 40,000 volts.  Never again!

The surplus market has a lot of ceramic doorknob capacitors available.  Some people disparage them, but I have parallel/series connected 20 of them and put them in transformer oil, and they drove a coil which produced 7 foot sparks.  High voltage Mylar capacitors (these are usually red plastic inside a glass oil-filled cylinder with metal ends) are worthless for Tesla coil use.  Mica transmitting capacitors work well on small coils, but mica has only a moderate dissipation factor.  Mica capacitors will get quite warm in a Tesla coil, due to dissipation losses, but a mica transmitting cap is designed to handle a lot of heat, so they seldom fail. Paper capacitors, such as the ones used by power utility companies are totally unsuitable for coil use.  The last type of capacitor, the pulse discharge capacitor, which is the best cap for coil work, uses modern, low loss plastics as the dielectric.

A general rule of thumb, often used with 15 kV neon transformers, is that you can plan on using 0.01ufd for each 60 milliamps of current.  Thus, if you have four 60 ma transformers, or 240 milliamps total, then you will require about 0.04 ufd of capacitance. 

Let me tell you right up front that you need to purchase a rugged commercial polypropylene impulse capacitor suitable for Tesla coil use, made by Maxwell, NLW, Plastic Capacitors, or any one of the other major Tesla coil capacitor manufacturers.  Get a cap that is rated for at least 25 kilovolts AC, preferably 40 kV or 45 kV.  If you can find a working used capacitor, then good for you!  They are very, very difficult to find.  Search surplus yards and junkyards, and electronic surplus stores long enough and you might find one, but it’s not likely.  So, you’ll most likely end up buying one.  A new 0.025 ufd, 25 kVAC capacitor will cost you somewhere around $200.  The larger the capacitance, the more it will cost you.  The higher the voltage rating, the more it will cost you.  But the commercial caps are durable, they are lightweight, they have extremely low losses, and they will make a Tesla coil kick butt!  The number one rule when ordering such a capacitor is to make sure the voltage rating is sufficient.  You should get a voltage rating that is a minimum of two to three times higher than the RMS voltage rating of your transformer output.  If you can afford it, get FOUR times the transformer voltage.  Remember that resonant circuits produce incredible peak-to-peak voltages, and you don’t want your expensive capacitor to fail because you skimped on the voltage rating!

Now, having told you that you need to purchase a good commercial cap, I have to tell you that you can build your own oil filled polyethylene capacitors.  A 0.01 ufd capacitor that can withstand about 7,500 volts AC will cost you about $85 or so for each unit you build.  

If you decide to build your own capacitors, be aware that it is futile to try and increase the voltage rating to more than 7,500 volts by using thicker insulation.  At voltages above 7.5 kV, corona will form on the edges of the metal plates.  This corona will exist even in oil, and it will slowly break down the polyethylene dielectric, no matter how thick you make it.  Rather than trying to use thicker dielectric, you will have to build several capacitors and connect them in series to increase the total voltage rating.  Because connecting capacitors in series effectively divides the capacitance, you will have to parallel several sets of caps to bring the capacitance back up to the value needed for your coil.  For example, assume you are using a 15 kV neon transformer, and the required capacitance has calculated to be 0.01 ufd.  Two rolled 60-mil poly caps in series will operate safely (barely) on 15 kV, though I recommend placing three in series to reduce the stress on the caps.  Placing three of these caps in series will reduce the capacitance to 1/3 of 0.012 ufd, or 0.004 ufd.  You will need to build three sets of three caps to bring the capacitance back up to 0.012 ufd.  Since these caps will cost you anywhere from $60 to $85 each to build, the required nine caps will cost you between $540 to $765, not including labor, to build!  A commercial 0.012 ufd, 45 kV capacitor will cost you much less than $540.  Even if you decide to series connect only two caps, and risk blowing them, you'll still need to build four capacitors at a total cost of somewhere between $240 to $340.  You can buy a commercial cap for this same amount of money, and it will be more rugged than the ones you build.  Now you can see why I highly recommend buying a commercial capacitor!  If you use several paralleled neon transformers, or if you use a potential or distribution transformer, even more capacitance is required, driving the cost and construction time of home made caps even higher.

Building your own capacitors is an interesting, though costly, exercise, and the capacitors you build will be comparable in performance to commercial capacitors, except your capacitors will probably leak oil, will weigh many times more and be physically much larger than similarly rated commercial caps.  I have included plans for building a polyethylene capacitor at the end of this paper, for those of you who really like to punish yourselves.  Personally, I’ve found using my time to make big sparks to be much more fun and rewarding than making capacitors.

Figure 20.
A homemade polyethylene capacitor, after rolling but prior to installation in the oil filled case.

I always place a safety gap across each of my capacitors to protect them against over-voltages produced by kickbacks.  You can make a very simple gap from two pieces of 12-gauge solid copper wire or small copper tubing.  Connect one wire to each capacitor terminal, and placed the other end of the wires a couple inches above the cap, with about a one-half inch gap between the ends.  Run the coil, and see whether the gap fires constantly.  If so, turn off the coil, and increase the gap slightly and re-test.  Keep increasing the gap until it only fires only once every few seconds.  This setting will allow the coil to run normally, but any excess voltage will jump the safety gap and bypass the capacitor, protecting it.  Safety gaps are very loud when they fire, because the capacitor will discharge into the gap once it conducts.  Your beautiful capacitors are worth the expense and bother of installing a fifty-cent safety gap!

Figure 21.
Safety gap installed on a commercial capacitor.

All connections from the capacitor to the main spark gap and to the primary coil should be made with very heavy gauge cable.  How big a cable should you use?  Well, whatever the largest gauge cable you can successfully connect will be just about right!  Copper tubing works very well as a primary tank connecting cable.  Large coax works well, but is a little harder to connect.  Buy some “split bolt” copper connectors from Home Depot or an electrical supply center – they are wonderful for connecting large gauge cables together.  You can even use small hose clamps, in a pinch.

IMPORTANT:  Keep all cable connections from the capacitor to the primary coil and spark gap as short and direct as possible.  These components form part of the resonant tank circuit, and long cables will form “off axis” inductance, which is pure loss, plain and simple.  Careful thought on the short, neat layout of the wiring on these components will result in better performance and longer sparks.

Figure 22.
Safety gap across transformers

Top toroid capacitance

The final stop for the high voltage, before it leaps into space, is the top load capacitance, situated at the top of the secondary coil.  Old Tesla coil designs did not use large capacitive top loads and their performance generally suffered from this lack.  Research over the last few years has shown that Tesla coils love large top load capacitances.  The top capacitor appears to allow much higher voltages to form before the spark channel is initiated, and it helps to sustain the spark channel once it is formed.  The most perfect physical shape for a capacitor is a sphere, but unfortunately, this is NOT the ideal shape for a Tesla coil top load!  Trial and error has shown that a large toroid shape provides an excellent capacitor, but more importantly, it also provides critical electrostatic shielding to the top windings of the coil.  If you run a Tesla coil with no top load, or with a small sphere, you will find that the top few turns of the coil will be covered with a massive blue hairy corona, robbing your system of long sparks and damaging the insulation on the wire.  Simply placing a suitably sized toroid above the top of the secondary, at the proper height, will completely eliminate all such unwanted corona!  And your output spark will be much longer and brighter.

Figure 23.
Some toroids made out different materials.

How big should your toroid be?  Well, the size of a toroid is dependent upon two key parameters: the physical size of the secondary coil and the amount of power you are putting into the system.  Toroids have two important dimensions.  The first is the outside diameter of the toroid.  The second dimension is the cross-section or chord.  Cross-section is how “fat” the tire part of the toroid is.  The larger the cross section, the higher the voltage required to “break out”.  It is possible to put such a large toroid on a given coil that the available voltage is insufficient to exit the toroid; so no spark will issue at all.  When this happens, you can try to improve the situation by placing a small bump of aluminum tape on the side of the toroid to allow the spark to exit, or you can increase the total input power to the system.  A three-inch diameter coil, twelve inches long, will readily use a toroid sized 12 inches by 3 inches.  A six-inch diameter coil could handle a toroid 24 to 36 inches by 4 to 6 inches chord in size.  I have used a 40-inch by 5-inch toroid on an eight-inch diameter by 24-inch long coil with excellent results.  You will generally get the biggest sparks out of the largest toroid that your coil will successfully operate. 

Figure 24.
Figure showing toroid mounted above top of coil, and spiraling wire attachment.

The height of the toroid above the top winding on the secondary can be critical to optimum performance.  I have seen height changes of ½ inch make huge differences in coil performance.  You need to make allowance for some method of raising and lowering the toroid in small increments so that the optimum height can easily be determined.  Your toroid mounting method should also provide a secure mount for the toroid – if you just balance the toroid on top of the coil it will fall off, sooner or later, and when it falls, it will be damaged.

Tesla coil toroids almost always have a center disk.  This center disk allows you to support the large diameter donut and the disk also forms part of the electrostatic shielding that the top load provides to the coil.  The disk can be made of aluminum, or an insulating material covered with aluminum tape or aluminum flashing.  There is usually a small hole in the center of the disk, which is used for a rod or bolt mounting on the toroid so that it will not fall off the coil.

What should your toroid be made of?  Just about anything that is suitably shaped, conductive, and smooth.  I have seen the following used as good toroid stock material:

•	Large inner tube from inside a truck tire inflated and covered with aluminum tape.  Advantages: cheap, light.  Disadvantages: can be punctured, changes size with temperature. Cost: < $20

•	Styrofoam Christmas wreath covered with aluminum tape.  Cheap, durable, very light, 
readily available.  Unfortunately, really large ones are not available.  Cost: < $10

•	Styrofoam life preserver for swimming pool covered with aluminum tape.  Good 
medium toroid.  Cost: < $30

•	Aluminum flexi-duct used for heating and air-conditioning.  Available in sizes up to 20-
inches in diameter!  Reasonably priced, but ribbed aluminum is very soft.  Best used if 
you do not stretch it out, but leave it compressed when you make the toroid.  Beautiful 
toroids can be made from this material, but they have to be handled and stored with care.  
Cost: < $60 for 36 inch by 8-inch toroid.

•	Flexible corrugated black sewer drainpipe covered with aluminum tape.  Available in sizes up to about 8 inches in diameter.  Very, very rugged; makes an indestructible toroid.  Unfortunately, not available in large diameter pipe.  Very cheap, and is the medium size toroid material of choice.  Cost: < $10

•	Aluminum heating vent covers.  These are the flying saucer-looking things you see on the roofs of commercial buildings.  They make excellent top loads for Tesla coils.  Can be found in scrap and salvage yards.   Cost:  negotiable, but usually for the cost of scrap aluminum.

•	Aluminum tubing formed into an open framework the shape of a toroid.  Lightweight, and material is relatively cheap.  Welding can be an expense.  Smoothness of shape is important.  Open frame toroids are found to be most useful on very large coils.

•	Ring of aluminum spheres formed into a toroid.  If you can get a bunch of small conductive spheres, they can be arranged around a disk into a toroid shape.  Works well, but looks weird.

•	Commercial spun aluminum toroid.  This is the most beautiful toroid available.  Very expensive; a 30-inch by 7-inch toroid will cost you $350.  A spun aluminum toroid 8 feet in diameter with a 36-inch cross section will set you back about three grand.

Tesla coil grounding system

Most amateur coilers overlook one of the most important parts of a Tesla coil system, the grounding system, and in so doing, they limit coil performance, and they even risk burning the building down because of an electrical fire!

Figure 25.
Do not connect the secondary to the wall outlet ground!

DO NOT USE THE ROUND CENTER PIN ON YOUR WALL ELECTRICAL OUTLET TO GROUND THE SECONDARY OF YOUR TESLA COIL!  

The ground connection on your electrical outlet is connected to a bare copper wire that is bundled together with the hot wire and the neutral wire, which all go back to the electrical panel or breaker box.  Usually, this wire is several yards in length.  The impedance of the wire can be very high at Tesla coil frequencies, and any high voltage currents impressed upon the wire can arc over to the other wires in the “Romex” or conduit and cause a short, resulting in an electrical fire.  I have heard of small Tesla coils causing purple corona to shoot out of nearby electrical outlets because of this poor practice!  The use of a good line filter, and always connecting the secondary to a good dedicated ground eliminates the problem.

Well, what constitutes a “good dedicated ground?”  A copper clad ground rod driven six feet into the soil, or a metallic cold water pipe are good grounds.  The conductor from the bottom of the coil to this ground connection should be a heavy copper conductor, or wide aluminum strip; the wider the better.  Wide metal strips form very low impedance conductors for Tesla coil currents.  Standard aluminum flashing works very well for this.

Serious coil builders should install their own low impedance ground system, designed for Tesla coil use.  Choose a spot in the yard or garden as close as possible to where the coil will regularly be operated.  

Constructing the Tesla coil

NOTE: These Tesla coil plans will produce a working Tesla coil, providing you build the exact sizes of coils, toroids, and capacitors with the values shown.  If you vary any of the parameters to any significant degree, you may not be able to tune the coil to resonance.  Build the coil as shown and it will work.  If you change the size of the primary, or the secondary, or the capacitor, or the toroid, then the coil will not tune to the expected design frequency, and in fact, you may not be able to tune the Tesla coil at all without making other compensating changes.  Suggestion: build it as described, tune it for best performance, then you can start changing it to suit yourself!

Parts list:

The toroid will require either black plastic sewer pipe or aluminum flexi-duct.  The material, size and length will be determined by how many transformers you will be using with the coil.  Below is a chart that you can use to determine the toroid size needed, and what materials to purchase:

One or two 60-ma transformers:	Toroid will be 24 inches in diameter with a 5-inch cross 
section.
You will cut a 14-inch disk and wrap around it a length of five-inch diameter, flexible black sewer pipe covered in aluminum tape.  You will need about four feet of flexible sewer pipe.

Three or four 60-ma transformers: Toroid will be 24 inches in diameter with a 7-inch cross section.
You will cut a 14-inch disk and wrap around it a length of seven-inch diameter aluminum flexi-duct.  One piece of duct should be sufficient.

Five or six 60-ma transformers:	Toroid will be 36 inches in diameter with an eight-inch cross-section.  You will cut a 20-inch disk, and wrap around it a length of eight-inch diameter aluminum flexi-duct.  Three lengths of duct will be required.
 
Two kVA potential transformer: 	Toroid will be 36 inches in diameter with an eight-inch cross-section.  Cut a 20-inch disk, and wrap around it a length of eight-inch diameter aluminum flexi-duct.  Three lengths of duct, carefully taped together with aluminum tape, will be required.

One to six neon transformers, all must have the same voltage rating, preferably 9 kV or 
12 kV, at 60 milliamps.  If you can locate 120 milliamp transformers, fewer will be 
required.

120 volt, 30 ampere line filter

120 volt, 30 ampere variac

120 volt, 30 ampere DPDT on-off switch

Four 250 pf, 30 kV ceramic doorknob bypass capacitors

Four 2 inch diameter ferrite choke coil forms

200 hundred feet, 28 gauge enamel wire for chokes

Electrical tape

20 feet of high voltage wire, 18 or 14 gauge

20 feet insulated, 8 gauge stranded wire

Capacitor:  0.01 ufd per 60 milliamps of current available, 40 KVAC voltage rating
For example, four 60-ma transformers will require a 0.04 ufd, 40 KVAC capacitor
A 2 kVA potential transformer will work with a 0.04 ufd capacitor
A 5-kVA-pole pig will need at least 0.06 ufd capacitance
A 10-kVA-pole pig will need about 0.1-ufd capacitance

100 foot roll of .25 to .5 inch diameter coaxial cable, computer printer cable, etc., which 
has an aluminum or braided wire ground jacket inside.  Outside of cable must be 
insulated.

9 pounds of 18 gauge enamel wire, approx. 1800 feet

One sheet of 1-inch thick foil covered Styrofoam core insulating sheathing.

10 copper splice sleeves for 2 inch diameter copper pipe.  These copper cylinders will be about three inches long 

Base for the cylindrical spark gap, can be wood, phenolic, or acrylic piece, 5 inches wide, 24 inches long, ¼ to ½ inch thick	

Cylinder support pieces, fiberglass rod or bar stock, ½ inch diameter, 120 inches in length 

Fast setting epoxy for gluing fiberglass bar stock

Rosin core solder, 200-watt soldering iron

1 foot length of 1 inch diameter PVC pipe

Three foot length of 10 inch diameter, thin wall PVC drain pipe

1 inch length of 10 inch diameter, thin wall PVC drain pipe (cut one inch off long pipe)

4 foot by 2 foot piece if ½ inch thick plywood

Two by fours, total of 50 feet length, enough to build box frame 4 feet long by 2 feet tall

Sono-tube, acrylic, or PVC pipe, 10 inches in diameter, 32 inches on length.

Decking screws, 1 and ¾ inches long, one box

Hot glue

Piece of ½ inch plywood at least 30 inches by 30 inches square, or ¼ inch thick phenolic or acrylic sheet, 30 inches by 30 inches, to be used for primary support

Wood strips, 1-inch wide by ½ inch thick, 15 inches long, 6 required

2-inch long nylon screws with nylon nuts, at least 18 required

One quart of polyurethane, or three cans of spray polyurethane.  
You can substitute Behr 50 decoupage two-part epoxy coating, but
it is quite expensive, though it does make a beautiful coating for a coil!
Do not use a water-based product!  
Do not use spray acrylic, as it will crack and craze over time.

J.B. Weld epoxy (requires 24-hour setup)

Large alligator clip for making tap connections when tuning primary coil.

16 standoff insulators for bypass caps and choke coils (optional).
	
5 binding posts for transformer protection circuit.

Assortment of copper “split-bolt” connectors for splicing large gauge cable together

120-volt AC meter

20-amp AC current meter.

20-amp AC power switch

120-volt AC power indicator panel lamp

120-volt, 20-amp variac

If you use a brush-type AC rotary gap motor:
200-watt light dimmer control, or a 120-volt, 5-amp variac

If you use a DC rotary gap motor:
120-volt, 5-amp variac to control power to the DC power supply

DC power supply: suitable low voltage transformer for motor, full wave bridge rectifier, 2 MOV’s for DC power supply, 

If you are using a potential transformer, you will also need:

Lincoln “cracker box” welder, 150-amp output rating, or similar adjustable inductor

Assortment of 1,000-watt or larger heating elements

Fan or blower for cooling heater elements

240-volt, 50-amp “range” outlets and pigtails for ballasts

	Three 8-foot long copper-clad ground rods 

	28 feet of 2-inch wide copper strap

Now that we’ve become familiar with all the parts of a Tesla coil, let’s build one.  The plans presented here will allow you to build a coil powered by up to six neon transformers connected in parallel.  The six-transformer configuration can produce sparks at least four to five feet in length.  Also, we will go over how you can change out the neon transformers with a 2 kVA potential transformer, complete with variac and ballasts, to produce sparks up to six or seven feet in length, using basically the same core system.

Figure 26.
The Tesla coil in operation, producing sparks in excess of four feet in length

Making the box frame

The Tesla coil is best built on an open box framework, with a ½ inch thick piece of plywood on top.  The primary and secondary coils mount on top of the plywood.  The capacitor(s) and spark gap mount on a shelf underneath the plywood.  The plywood top is 24 inches wide and 48 inches long.  Plywood sheets, already precut to this size are available at most lumber supply companies.  The box frame supporting the plywood is made from two by fours, connected together with decking screws, which are also used to attach the plywood on top (there are areas of the plywood which must NOT have metal screws, see Figure 27).  The box frame should support the plywood at least 24 inches from the floor, to prevent the magnetic field produced by the primary from interacting with the ground or with steel rebar in a concrete floor.  After building the box frame and attaching the plywood, use a good polyurethane varnish to coat the entire assembly.  The coating will help keep moisture out of the wood.  If you have esthetic sensibilities, you can stain the wood first.

Figure 27.
The box frame and location of components

See Figure 27 for the location of the primary, secondary, gaps and capacitor on the plywood and on the shelf.  The shelf is made from three 2 by 4 boards which are mounted across the bottom of the box frame and covered with a piece of 1/2 inch plywood.  You can stack the neon transformers on top of each other on the shelf, or mount them side-by-side, whichever allows for easiest paralleling of the primary and secondary connections.  If you stack the transformers vertically, you will want to add a couple of vertical braces to prevent the transformers from tipping over during transport.  You may also mount an axle and wheels to the box frame so that it may be rolled about.  If you will be using a homemade capacitor, you will want to add a capacitor shelf and strap to the side of the box frame for the capacitor – see Figure 27.  

Once the transformers have been mounted, you will want to drill two holes in the plywood above the transformer stack, through which you will run the two high voltage leads.  Place rubber grommets inside the holes in the plywood to insulate the wires away from the wood.

Making the primary support disk and primary coil

See Figure 28.  Take a piece of ½ inch plywood (¼ inch thick acrylic, if you want to really show off) and cut out a disk 36 inches in diameter.  Using a hole saw, drill a 1-inch hole in the exact center for the secondary coil support pin.  If you make your disk out of plywood, stain and varnish it with polyurethane.
Let the polyurethane thoroughly dry.  The primary is held to the disk with four radially placed sticks.  Cut the ½ inch by one-inch sticks into 9.5-inch lengths.  Drill three 1/8-inch holes in each stick, two near the ends and one in the center.  Place the sticks on the disk as shown in Figure 29, and mark the holes on the disk.  Drill the 12 holes through the disk, using a 1/8-inch drill bit.  Next, countersink the holes using a ¼ inch drill bit to a depth of about ¼ inch.  Take a very small amount of epoxy, and carefully glue the 12 nylon nuts into the bottom of the countersunk holes.  Do not get epoxy in the threads of the nuts.  Let the epoxy dry.  Temporarily screw a two-inch nylon screw into each of the middle holes.  Place 4 strips of double-sided carpet tape on the disk as shown in Figure 29.  This tape will help hold the flat primary in place as it is wound and it will help whenever you place taps on the cable.

Figure 28.
The layout of the disk.

Cut a 1-inch long ring off the end of your 10-inch diameter PVC pipe.  Place the ring in the center of the disk. Use a few drops of hot glue to fasten it in place, so that it may later be removed.  This ring will serve as a temporary guide for the winding of the primary, and it will later be removed.  Lightly tack one end of the coaxial cable to the disk, right up against the plastic ring.  Carefully wind the cable into a flat disk, forming 17 turns. When you reach the four nylon screws, work around them as neatly as you can.  When you’ve wound 17 turns, cut off any excess cable.  Finish off by tacking the remaining end of the primary cable to the disk with a couple drops of hot glue.  Now carefully pry the ring off of the disk.  We will need some space between the secondary coil and the inner most turn of the primary, so you need to carefully remove the inner two turns of coax, then tack it back down to the disk with hot glue.  Remove the four nylon screws from between the primary windings, taking care not to cover up the center holes.  Take the four sticks that you previously drilled the three holes in, and align them with the matching holes in the disk.  Fasten the sticks to the nuts using the 2-inch long nylon screws.  Use wire cutters or a Dremel and cut-off wheel and cut the nylon screws off flush with the backside of the disk.  Strip one inch of insulation off each end of the coaxial cable.  Except for taps, the primary is done.

Figure 29.
Double-sticky tape layout of primary

See Figure 29.  On the coil end of the top of the box frame, mark a spot in the center of the plywood, 24 inches from the end of the plywood.  Use a 1-inch diameter hole saw and drill a 1-inch hole at this spot.  This hole is for the secondary coil support pin.  Set the primary disk on top of the box, and align it so that the secondary pin holes in the disk and in the top of the box exactly line up.  Make sure that the outside end of the primary cable ends up near the center of the box, as shown in Figure 30.  Secure the primary disk to the box using four decking screws.  Place the screws as far away from the primary cable as is practicable.  If you have made your disk out of acrylic, be very careful not to crack the acrylic by over-tightening these screws!

Figure 30.
Hole positions in plywood.

A final 1-inch hole now needs to be drilled through the disk and plywood top.  Use Figure 30 as a guide for placing this hole.  It is near the end of the box, centered exactly 10.5 inches from the middle of the disk.  This hole will be used for the ground connection for the secondary coil.

To protect the primary from high voltage strikes, a “strike rail” needs to be placed on insulators above the primary.  The strike rail can be fabricated out of ¼ inch copper tubing which is formed into a “broken” circle which has the same diameter as the outside turn of the primary.  DO NOT CONNECT THE TWO ENDS OF THE TUBING.  The tubing MUST NOT form a complete circle.  There must be a gap of one inch between the ends of the tubing.  Mount the copper tubing on plastic standoff insulators about two inches tall above the outside turn of the primary.  Run a ground wire from the strike rail to the main dedicated ground.

Building the secondary

Cut three 24-inch lengths of 1-inch diameter PVC pipe.  Drill two holes at one end of each pipe, the first is one inch from the end of the pipe, and the other hole is three inches from the end.  Place three marks on your 10-inch PVC pipe 120 degrees apart.  Using these three reference marks, drill six holes, one inch and three inches from the edge of the pipe as shown in Figure 34.  Counter-sink these holes. Do not place any hardware in these holes yet.

Coat the 10-inch pipe it inside and outside with a good polyurethane coating.  Do not use water-based polyurethane.   After the tube has dried thoroughly, we can begin to wind the 18-gauge enamel wire on the tube.

It is recommended that you use a coil-winding jig something like the one illustrated in Figure 31, or you can wind the tube by hand.  Keep the windings tight, do not kink the wire, and do not have any “crossovers” in the wire.  It is highly recommended that you stop every six inches of winding length and stick a piece of box tape on the windings to hold them in place, just in case the wire breaks or if you should accidentally drop the wire.  

Wind the 10-inch diameter secondary coil from the bottom of the tube toward the top of the coil.  Start one-inch from the bottom of the pipe and stop when you reach the holes which are three inches from the top of the pipe.

When your winding reaches the holes that are three inches from the top of the pipe, stop winding.  Using the NYLON screws and nuts, attach the three 1-inch diameter, 24-inch long PVC pipes to the INSIDE TOP of the 10-inch pipe.  The screws must be flush-mount NYLON SCREWS AND NYLON NUTS.  Refer to Figure 34.  Under no circumstances use metal screws or nuts!

Hand wind the last two inches of wire over the flush nylon screws, and finish off with a bead of hot glue around the end of the winding.

Figure 31.
Hand-crank operated coil winding jig.

When you have finished winding the coil, tape the ends of the wire to the tube.  Coat the windings with several coats of polyurethane, allowing proper drying time between coats.  If you are using a winding jig, it helps to turn the coil while the polyurethane is drying to prevent runs and sags in the polyurethane coating.  Let the coil dry thoroughly before using!  The polyurethane will help to physically protect the windings and keeps them from loosening, and finally, it keeps moisture away from the wire.

After the coating on the secondary has dried, you can finish off the connections on the ends of the wire.  First, cut and shape a ½ inch wide, 2-inch long piece of copper strap to fit the curvature of the coil form.  Next, drill a hole in the copper, and fasten a flathead, ½ inch long screw through it and secure with a nut.  Epoxy this strap and screw to the side of the tube, below the bottom of the winding, with a generous dollop of J.B. Weld epoxy (J.B. is used because of the soldering heat, which will be applied later).  After the J.B. Weld has cured (approx. 24 hours), solder the bottom end of the secondary coil wire to this copper ground plate.  

Hint: enamel wire is easily stripped if you will take a Bic lighter and first burn the insulation off the end of the wire.  Then take a pocket knife and scrape the carbon off, and you’ll have beautiful clean wire.

The screw is used to connect the large copper or aluminum ground strap, which runs to the dedicated ground rod.  See Figure 33 for details on this bottom coil ground connection.  You will align the screw over the one-inch hole, which was drilled through the disk and top of the box.  Do not attach the top end of the secondary wire to anything at this point of construction.

Figure 32.
Disks with centering pins

The final step of construction on the secondary will be to build and attach the top and bottom disks with their centering pins.  Refer to Figure 32 for details.  The disks should be cut to fit snugly within the ends of the secondary tube.  Use a hole-saw and drill a 1-inch hole in the exact center of each disk.  The center pins are six-inch long pieces of 1-inch diameter PVC pipe.  Use epoxy to glue these pieces of PVC pipe into the holes in the center of the disks.  These centering pins will be used to align the secondary and prevent it from falling over, and also to hold the toroid assembly above the top of the coil.  After the epoxy on the pins has set, we can mount the disks in the ends of the secondary tube. Do not attach the disks into the ends of the tube with metal screws!  Use nylon screws, or drill holes and peg with plastic pins secured with epoxy. If you want, you can coat the edges of the disks with epoxy before inserting them into the ends of the tube, to further insure a solid, secure bond.

Building the toroid

Refer to the beginning of the parts list at the start of this construction article.  A chart has been provided to help you choose the toroid that is likely to work best with your transformers.  

Black sewer pipe

Figure 33.
Sewer pipe toroid details

If you build the smaller toroid, the flexible sewer pipe is best.  Cut the 14-inch disk out of the Styrofoam core panel.  Wrap the sewer pipe around the disk and cut it to fit.  The two ends of the pipe can either be hot glued together, or they can be laced together with strong nylon line.  Wrap the pipe in its entirety with aluminum tape.  Take a screwdriver handle and rub the tape down smooth, leaving no protruding wrinkles.  Place the disk in the middle of the toroid and hot glue into place.  Run some lengths of aluminum tape all the way across the toroid and disk to insure everything is electrically tied together.  See Figure 33 for details.

Flexi-duct:

On the larger toroids, use the aluminum flexi-duct.  This duct comes in lengths about 32 inches long. Leave the duct in this compressed state; it is much stronger than if it is stretched out.  When the duct is stretched out, the duct becomes very soft and is very easily damaged, even by regular handling.   

Figure 34.
Flexi-duct toroid details

On the three toroids, we will use aluminum flexi-duct.  This duct comes in lengths about 24 inches long. Leave the duct in this compressed state; it is much stronger than if it is stretched out.  When the duct is stretched out, the duct becomes very soft and is very easily damaged, even by regular handling.   

Making foam disks and covering them with aluminum

Note: it is very important to exactly align parts that have been sprayed with contact adhesive.  Once the parts touch, they cannot be adjusted.

Cut six 19-inch diameter disks out of the Styrofoam core panel.  Spray one side of two disks with spray contact adhesive and let them dry for ten minutes.  Carefully align the two foam disks and stick them together, forming one thick disk.  Do this to the other two pairs of disks, forming a total of three thick disks. 

We will now cut aluminum disks to cover both sides of the foam disks.

Cut six 19-inch diameter aluminum disks out of the aluminum flashing.  Spray one side of an aluminum disk and one side of a foam disk with contact adhesive, and let them sit ten minutes, then stick them together.  

Cover the other side of the foam disk with another aluminum disk using the same technique.  Do this to all three foam disks.  When you finish, you will have three aluminum-covered disks 19-inches in diameter, one for each toroid.

Connect three lengths of 8-inch diameter aluminum duct together with aluminum tape, forming a long straight tube.  Do not rub the tape down yet! The duct will require some bending when it is formed around the disk.  Wrap the duct around the disk and cut it to fit exactly.  The two ends of the duct can be taped together with aluminum tape. Test the toroid for a good fit around the disk.  Once the duct has been cut and fitted properly, take a screwdriver handle and rub the tape down smooth, leaving no protruding wrinkles.  Finish up by aligning the disk in the middle of the toroid and hot gluing it into place.  Run some lengths of aluminum tape all the way across the toroid and disk to insure everything is electrically tied together.  See figure 34 for details.

After you have made the toroid, you will need to cut three 1-inch diameter holes in the center of the disk.  These holes should be 120 degrees apart, and must exactly match the positions of your three support pins at the top of the secondary coil.  The toroid disk will slide down over the PVC support pins, which stick out of the top of the secondary coil.  The pins through the disk will prevent the toroid from ever falling off the top of the coil.   A row of small holes can be drilled through the support pins, and a plastic cotter key put in place to lower or raise the toroid.  You may also cut and use short cylinders cut from 10 inch PVC pipe to set the height and spacing of the two toroids above the secondary coil. 

The capacitor

Place your capacitor on the shelf, near the spark gap and below and to the side of the primary coil.  Refer to Figure 27 for parts placement.  The goal is to place the capacitor and spark gap close to the primary, where connecting wires are as short as possible.  Note:  To prevent accidental arcs, do not place either the capacitor or spark gap assembly closer than about four inches to the primary.

Figure 35.
Capacitor shelf and strap.

If you decide to build a capacitor, refer to the separate set of capacitor construction plans attached to this file.  The homemade cap will stand about 18 inches tall, and should be mounted on a separate box beside the box frame, or on a shelf attached to the side of the box frame.  See Figure 34 for an example of how this mounting can be done.  Do not let the top of the capacitor sit higher than about six inches above the edge of the plywood top, else it will become an attractive lightning target.  Be sure to place a safety gap across the spark gap, as shown in Figure 21.

Building the static gap

These gaps are easy to build, and are not expensive. Start by cutting a base for the cylindrical spark gap out of wood, phenolic, or acrylic.  It should be eight inches wide, and thirty inches long. 

Figure 36.
Initial placement of fiberglass rods for static gap.

Be sure to wear a respirator when cutting and grinding the fiberglass rods!  The dust is extremely damaging to your lungs, and is only slightly less dangerous than asbestos.  

Use an abrasive cut-off wheel in a Dremel moto-tool, and cut 22 pieces of ¼ inch diameter fiberglass rod about 1/8 inch longer than your copper cylinders.  The exact length of the rods will depend on the exact length of your cylinders, which should be about four inches in length.  Cut four more fiberglass rods to a length of 24 inches.  Using fast-set epoxy, glue two of the 24-inch long rods and two of the short rods together on the stand as shown in Figure 36.  Let the epoxy cure.

Place two strips of double-sided sticky foam tape on the top of the gap stand, down the length of the two 24-inch rods, about ½ inch inside the rods.  See Figure 36 for clarification.  The purpose of the sticky tape is to temporarily hold the remaining short rods in place so that the gaps spacing can be set.

Figure 37.
Placement of double-sided sticky foam tape on static gap.

The remaining rods are arranged in pairs.  Place them on the sticky tape as shown in Figure 37, using the approximate spacing shown.  Once all the rods are in place, set all ten copper cylinders on top of the pairs of the rods.  Adjust the spacing between the rods so that the cylinders are perfectly parallel, and each pair of cylinders is separated with a 1/32-inch gap.  You can use 3 or 4 stacked business cards to make a handy feeler gauge that is just about the right thickness.  The sticky tape will hold the rods in place, but will still allow you to change the spacing between them until all your cylinders have perfect gap settings between them.  Once you are satisfied that the gaps are all even, then carefully lift the cylinders off of the rods.  Mix up some J.B. Weld epoxy, and glue the rods to the stand so they will no longer move.  After the epoxy has set up for about half an hour, set your cylinders back on top of the rods and verify that the rods are exactly where they need to be.  The epoxy will still be pliable enough at this time that you can make slight adjustments, if necessary.  Be sure to take the cylinders off the rods again, so that the rods will not be slowly moved by the weight of the copper cylinders!  It will take 24 hours for the epoxy to fully cure.  Once the epoxy has hardened, your gap will be perfectly aligned every time you set your cylinders on the rods.  As you use the gap, the copper cylinders will discolor because of the heat.  Discoloration is not a problem, but over a period of time, a black oxide scale will build up on the outside of the copper cylinders at the area of the narrow gaps.  If this should happen, simply rotate the cylinders until fresh copper is in the gap area.  This neat trick will allow you to run your gaps many times before they need to be re-polished!

You need to solder a length of the 8 gauge stranded wire to the inside of the cylinders at each end of the row so that you can connect this static gap to the rotary gap, and to the rest of the Tesla coil circuit. You will need a heavy soldering iron; I have found that a 200-watt unit works well for this.  You can use an acetylene torch, but a touch of skill and a good rosin flux will be necessary.  

Building the rotary gap

A rotary gap incorporates a plastic disk spinning at very high speeds.  The disk can shatter and throw the rotating screws and shards of sharp plastic at high velocity in all directions.  It is recommended that you either build the rotary gap into a box or place a Lexan “scatter shield” around the gap to protect yourself and bystanders from shrapnel, should anything ever go wrong.

Because the disk will be spinning very fast, as much as 10,000 RPM, the disk needs to be cut and machined to a fair degree of precision.  This is best done in a machine shop on a lathe.  The disk can be cut and “faced” on both sides.  Once the screws have all been mounted and ground to length, it is possible to have the disk dynamically balanced; though this is an expensive proposition.  I do not personally know anyone who has spent the money to balance a rotary gap disk.  If the disk is cut and trued-up on a lathe, and if identical hardware is used and accurately placed around the disk, then balance is almost always very good.

I have build excellent rotary disks using no tool fancier than a drill press.  If you have a good drill press available, go ahead and make your own disk.  If you do not have a drill press available, then spend  $25 or $30 to have your plastic disk trued up on a lathe.  Buy the machinist a few beers after work, and he’ll likely do it for free.

Figure 38.
Layout of rotary disk

Use Figure 37 as a guide for the layout and cutting your disk.  I recommend eight rotary electrodes made out of common 10-24 screws and nuts.  The disk should be made out of G-10 epoxy-glass composite, perhaps the strongest material available for this application.  If you cannot locate G-10, or cannot afford it, then use ¼ inch thick Lexan sheet.  Lexan is virtually indestructible, but has a relatively low melting point.  As a last resort, you can use acrylic, but it cracks easily, and I’m always nervous when I’m near a spinning disk make of acrylic.

G-10 is a glass composite, and is very hard.  I have worn all the teeth off a metal-cutting saber saw blade almost instantly trying to cut this material.  Use carbide or diamond coated saw blades.

Gathering the materials for the rotary

First, you need to find a suitable AC/DC motor.  Let me warn you that most motors will be unsuitable for rotary gap use, because they do not have carbon brushes in them.  No brushes, no speed control.  For example, the motor in a cheap box fan will not work, because it only runs at a fixed speed.  The best motors are 115-volt brush-type motors, like the ones that once were used in sewing machines.  They will operate on either AC or DC, and because they are brush type motors, their speed is adjustable.  Best of all, you can control their speed using nothing but a cheap $5 light dimmer control, eliminating the need for power supplies or variacs!  As an example of what you can do, I once pulled the brush-type motor out of a rotary skill saw and made a rotary gap with it.  The shaft turned at 5,000 RPM, and the end of the shaft was already drilled and tapped with a bolt.  To use it, all I had to do was take the motor from the saw, unbolt the saw blade, and bolt in my plastic disk.  It was very powerful; the motor speed was controlled with a light dimmer control, but gosh was it loud!  

Figure 39.
Simple controllers for AC/DC motors and a DC power supply for a DC motor.

After AC/DC brush-type motors, the next best rotary gap motors are low voltage (6 to 50 volt) DC motors.  These can be found at surplus electronic catalog suppliers, etc.  These type motors have been extensively used in old IBM mainframe reel-to-reel tape decks.  You will need to build a DC power supply to convert the 120 volts AC to the appropriate DC voltage for your motor.  A simple step-down transformer and bridge rectifier is all that is needed here.  The easiest and most robust way to control the DC motor speed is by controlling the 120 volts from the wall outlet to the power supply through a small variac.  Do not attempt to control a power supply transformer with a light dimmer control!  Always use a variac to control power to a transformer.  See Figure 38 for a diagram of a suitable power supply and controller for a DC motor.

When you are looking for a suitable rotary motor, it is important to try and find a motor that already has a mounting screw/bolt/flange/hub-thingy on the end of the shaft that allows you to easily bolt your disk onto the motor.  If the motor does not already have this feature, then you’re going to have to make or pay someone to make a suitable shaft-adapter that will allow you to bolt your disk onto the shaft.  Once you have a motor with some kind of disk mounting arrangement, then you can make the disk.

Start the disk construction by marking off a 9-inch circle on your disk material with a compass.  Use a jigsaw or saber saw to cut as accurate a circle as is humanly possible.  If you are using the G-10 fiberglass board, you’ll have to use special blades because of the hard glass fibers.  The plastics are much easier on saw blades, but they can be hard to cut without melting the plastic.  The key to cutting plastic is to use the a course blade, adjust the blade speed and cut at just the right forward speed to prevent the plastic from melting back together behind the blade.  Sometimes I’ve been lucky in that department, sometimes not.  I have one friend who cuts plastic outdoors while cooling the blade of the saw with a trickle of running water from his garden hose, but I consider doing that tantamount to attempting electrical suicide!  Maybe a friend could instead direct a cooling blast of air from an air-compressor tank?

Figure 40.
Method of truing up edge of rotary disk.

After the disk has been cut out, drill a hole exactly in the center of the disk.  This hole should match the mounting hardware on the end of your drive motor.  If you have access to a lathe, chuck the disk up and true off the outer edge as well as the front and back face of the disk.  If you don’t have a lathe handy, but do have a drill press available or a router and router table, then you can true up the outside edge in a different way.  Refer to Figure 39.  Mount a bolt in the center hole of the disk. using a couple of flat washers and a nut.  Make sure the bolt you choose is a very snug fit in the hole – you don’t want any slop here!  Put the bolt through a board, so that you can slowly spin the disk. Clamp the board down so it does not move.  You are going to bring a milling bit or a router bit up against the edge of the disk, just where the bit is cutting away material.  Lock the router or drill down so it cannot move anymore. Slowly turn the disk by hand, until you’ve turned the disk 360 degrees, slowly milling the edge as you go.  After the first go-round, the high spots on the edge of the disk should now be milled off.  If the disk is not yet perfectly round, move the router bit closer to the disk until it starts cutting the plastic, and rotate the disk slowly around again.  After two or three passes, your disk should be almost as perfectly round as it would be if it had been turned in a lathe.  The key here is to make sure that your mountings are tight, and that the disk turns freely with no wobble or slop at the center pivot.  I suppose one could bolt the disk to a steel bearing assembly and clamp that down and rotate the disk, but I’ve never had a bearing assembly handy.

After truing up the disk, you want to carefully rule off the circle into eighths.  On each of your eight lines, exactly ½ inch from the edge of the disk, make a mark.  After all eight marks are made, drill holes at these marks, using a drill bit suitable for #10 hardware.

Figure 41.
Detail of flying electrode

Take eight #10 screws, 1 and ½ inches long, and cut the heads off with your Dremel moto-tool and cut-off wheel.  Mount the screws on the disk, using lock washers and nuts on both sides of the disk.  See Figure 40.  Try to perfectly center the screw, so that an equal amount of screw sticks out of both sides of the disk.  On acrylic, do not over-tighten the nuts, or the acrylic will crack.  If the acrylic cracks, throw the disk away and start over, because the disk will be unsafe to use!

Figure 42.
Detail of stationary electrodes.

The stationary electrodes should be tungsten tipped steel electrodes, ½ inch diameter, about 2-inches long.  They are available at any large welding supply center.  They are the “high freq.” electrodes used inside TIG welders.  Mount them in blocks of phenolic or G-10 fiberglass at least ½ inch thick.  The mountings need to be very rigid so that the clearances between these stationary electrodes and the flying electrodes remain precisely as set.  See Figure 41 for clarification.

Figure 43.
Complete rotary spark gap.

You will need to build a rigid motor mount for the rotary out of wood, phenolic, or acrylic.  Because no one else in the whole world will be using a motor exactly like the one you have, you’ll have to make up the design of the mount for your motor yourself.  Refer to Figure 42 for a complete rotary design example.

Here are some key things to keep in mind when assembling the motor mount and stationary electrodes:

•	The disk will probably be a bit out of balance, so there will be vibration.  The platform for the rotary should sit on a thick foam rubber gardener’s pad (available at K-marts everywhere) or install rubber feet to prevent the unit from “walking”.

•	The motor should be mounted so there is NO movement or play in the motor with respect to the mounting platform.  There will only be an air gap of a few thousandths of an inch between the stationary electrodes and the flying electrodes, so there can be NO play between the stationary electrodes and the flying electrodes mounted on the disk!

•	The mounts for the stationary electrodes have to be made out of high temperature material, such as G-10 or phenolic.  Do NOT make the stationary electrode holders out of acrylic or Lexan, because they will instantly melt.  

•	The stationary electrode mountings have to be very, very rigid, because if they move, and the flying electrodes hit them, all hell will break loose!  Lock the electrodes into the holders with setscrews, and put locking nuts on the setscrews.

•	Keep the motor and the motor wiring away from the high voltage electrodes, or better yet, build acrylic high-voltage shields to prevent the high-voltage from arcing from the electrodes to the motor.

•	Ground the motor!  There will be thousands of volts on the disk and on the stationary electrodes just inches away from the motor and its wiring.  Protect yourself by grounding the motor housing.

•	Place the entire rotary gap in a protective box, or install a Lexan plastic scatter shield around the disk to protect yourself from flying shrapnel should the rotary disk ever fail.

My rotary spark gap of this design, using a 9-inch disk, with acrylic motor stand and high-temp phenolic electrode mounts has switched 20,000 volts at 10+ kilowatts for over a year with no problems.

Building the transformer protection circuit

First, you will have to wind your choke coils.  If you decide to use an air-core choke, wind it on a piece of four or six-inch diameter PVC pipe, at least 8 inches long.  Close-wind the form with 28-gauge or similar enamel wire.  Make sure you have no crossovers or kinks in your wire.  Coat the final coil with a couple coats of polyurethane to hold the windings in place.  

If you decide to wind a choke on a ferrite core, it is wise to wrap the core with a layer of electrical tape first to prevent arcing to the core.  End the first layer of wire at least ½ inch away from the starting point of the layer, also to prevent arcs.  If you wind more than one layer of wire, be sure to insulate with electrical tape between each layer of wire.  Winding ferrite donut cores is simplified if you will fill a small bobbin with wire, which you can shuttle through the hole in the core as you wind it.  I usually use a Popsicle stick wrapped with a quantity of wire as a bobbin when I make my chokes.

After you have built your chokes, cut a piece of Lexan or acrylic sheet about 12 inches-square.  Mount the four bypass capacitors and four choke coils as shown in the circuit diagram in Figure 13c.  To prevent arcing between parts, do not mount the parts too close together.  You can mount the parts on the plastic sheet with standoff insulators, or you can glue them in place with hot glue.  All high voltage solder connections should be smooth and round, to reduce corona losses.  You may also use heat-shrink tubing over your connections.

I recommend mounting a set of two input and two output binding posts to facilitate connecting the transformer leads as well as connecting the tank circuit leads.  You will also need a fifth binding post for your ground connection. 

Build a transformer safety gap as shown in Figure 21. 

Figure 44.
Neon transformer controller schematic.
Note that the ground on the line filter connects to the ground wire on the three-wire electrical outlet ground.  The AC voltmeter and amp meters allow you to measure input power.  The light dimmer control varies the speed of the rotary spark gap motor, while the variac will control power into the transformers.  If your rotary gap motor uses a DC motor and power supply, you will have to replace the light dimmer control with a small variac. 

Figure 45.
Controller parts layout

Figure 46.
Potential transformer controller schematic

Building the remote power controller box

Because this coil can produce long sparks, the power controller must be connected to the transformers and rotary gap by means of long extension cords so that the operator will not be struck by the sparks.  A remote power controller box will need to be built.  In this box, you will mount your line filter and variacs to control the power going into your Tesla coil.  The power controller box is built as shown Figures 43, 44, and 45.  The power controller is built into a box, 12 inches tall, 18 inches wide, and 12 inches deep.  It has a metal front panel (which is grounded, by the way,) upon which are mounted a power switch and indicator lamp, a rotary gap light dimmer control or variac, and the main power control variac.  An AC voltmeter and AC amp meter may also be installed on the panel.  Two AC receptacles are mounted on the back of the controller. One receptacle is for an extension cord for the rotary gap motor (or power supply) and the other is for an extension cord for the main power transformers.

You should mount your power switch, variac, dimmer control, etc., on a metal panel, which is grounded.  Use the green wire in the main power cord, which runs to the wall outlet as the main panel ground.  If you fail to ground your power panel, it will very likely pick up radiated energy from the Tesla coil and give you a nasty shock!

Mount your variacs, dimmer control, switch, lamp, and meters on the metal panel.  Wire them together as shown in Figures 43 - 45.  You absolutely must use a grounded, three-wire power cable to connect this panel to a 115-volt receptacle!

If you are using neon transformers, then your controller panel will not require provision for ballasts.  However, if you are building a power controller that will operate a potential transformer, you will have to incorporate some 240 volt, 50 amp “range” power receptacles into which you can plug the welder and heater element ballasts.  These plugs are paralleled with each other, but they are connected in series with the main power variac.  See Figure 45 for specifics on the wiring.  

Figure 47.
Adjustable ballast which uses nichrome wire.

Building an adjustable resistive ballast: Appliance repair shops sell replacement nichrome heater element wire kits.  The nichrome wire is coiled up like a very long spring.  Take some three-inch tall ceramic standoff insulators and mount two 36-inch lengths of the wire side-by-side.  In the middle, strap them together with a slideable copper strap.  Your electrical connections will be on one end of the wires and on the strap.  You can change the resistance of the load by changing the location of the slideable strap.  This ballast will allow you set the resistance of the ballast very precisely.  You will want to mount a fan or blower to cool the nichrome wire.  If you mount the insulators and wire on plywood, you will want to place a sheet of aluminum flashing between the nichrome and wood to act as a heat shield to prevent a fire.  See Figure 46 for details. 

For the tests described below, it is assumed that you already have the ballasts plugged into the control panel.  Since the ballasts will be in series with the load, failure to plug them in will prevent the transformer from receiving power.

Once you have wired the control panel, plug a 100-watt incandescent lamp into the main power outlet on the back of the controller and test the on/off switch, power lamp, and main power variac.  Monitor the output voltage and current on your meters.  You also need to plug an incandescent lamp into the AC rotary gap outlet and test the rotary motor controller as well.  If you are using a DC motor and associated power supply, use a voltmeter to measure and verify that the DC output voltage of the DC power supply is within reason and that it varies from zero to full rated voltage when controlled by the rotary gap variac.  This power controller should be operated on a fused or circuit breaker protected wall outlet with a current rating of no higher than 30 amps.

Figure 48.
Stack of neon transformers

Connecting the neon transformers together

Once the power controller is operational, you can start connecting your neon transformers together in parallel. You can safely parallel connect several neon transformers of the same voltage rating together for more current, but you have to connect them together one at a time and verify the phasing on the windings.  A transformer connected out of phase with the other transformers will buck the other transformers and will produce little or no power.

Always make sure power is off before touching the high voltage connections on the transformers!

Take a 12-inch plastic rod and attach an 18-inch long clip lead to one end.  Connect one end of the clip lead to one of the neon transformer’s high voltage bushings. Connect the first transformer’s 115-volt primary terminals to the main power extension cable. Using the power controller variac, power up the transformer.  Use the insulated rod to bring the other end of the clip lead near the other high voltage bushing of the neon transformer.  You should be able to pull a two or three inch arc off the bushing.  Turn off power.  Parallel the second transformer’s primary connection to the first transformer’s primary connection.  Next, parallel connect the two neon transformer secondary connections together.  Connect the clip lead, and perform the arc test again.  You should get an arc at least as long and hot as the previous test, and in fact, the arc should be hotter and longer because the second transformer’s power aids the first.  If you get little or no spark, then the second transformer’s primary is wired wrong.  In this case, turn off power, and swap only the two wires going to the primary of the second transformer. Re-perform the arc test, and you should now have your nice hot flaming arc on the secondary bushings.  Keep parallel-connecting transformers, one at a time, and testing them one at a time, until all transformers have been connected.  

•	I do not recommend parallel connecting more than six identical transformers together.  Do not attempt to parallel connect transformers with different voltage ratings!  

•	Avoid using 30-milliamp transformers because of their small-gauge secondary wire

•	Do not attempt to “series connect” neon transformers for higher voltage.  The secondary windings are center-tap grounded to the case, and attempting to series connect the transformers can put several thousand volts on the case of the transformer!  

Six 15,000 volt, 60 milliamp neon transformers can supply 5,400 watts, which is a very substantial source of power for a Tesla coil.

Building a dedicated Tesla coil ground system

Start of lecture:
Before you dig, or drive ground rods, contact your local water, gas, electric, telephone, and cable TV companies, and tell them you are going to be doing some serious digging in your yard, and request that they come out and flag all their buried utility plant on your property.  Get the person’s name that you talk to and write it down.  If you fail to have the utilities marked, and then you drive a ground rod through their buried line, you can (1) get killed by electrocution, gas explosion, etc., and/or (2) you will be responsible for the full repair costs, and I can assure you that you don’t want to pay for a cut utility line!  For example, and I’m speaking from personal experience here, a cut phone cable can cost you a grievous bundle of money!  The utility company can charge you for the trucks (at about $65 per hour, each), the barricades (maybe $50), the backhoe ($150 per hour), the crew (maybe $50 per hour per employee, and there will be several), their overtime pay (time and a half, minimum, maybe double time if it gets to be night or a weekend), the replacement cable (depends on the size, but a large one can cost $100 per foot), the buried splice enclosures (as much as $100 each, and two will be required), and on, and on, and you can even be assessed charges for the estimated lost revenues in long distance calls should you cut a telephone toll cable!  So please, get your utilities marked, and stay away from them.  
End of lecture.

Measure off a triangle eight feet to a side, and drive a six-foot long copper clad ground rod at each corner of the triangle.  It is recommended that you rent a jack hammer and use it to drive the rods into the ground. If you want a lot of exercise, and don’t want to rent a jackhammer, you can instead rent or purchase a special length of steel pipe with handles attached which is used to drive steel fence posts into the ground. Drive the rods into the ground until the tops of the rods are a couple of inches below ground level.  After your ground rods have been driven, dig a trench from ground rod to ground rod to ground rod.  Lay a heavy copper strap in the trench, connecting all three rods, and solder the strap to each of the three ground rods.  Bring a length of the copper strap up above the surface of the ground, so that it sticks up and provides you with an electrical connection to the buried strap and ground rods. Now, cover over your trenches, burying the copper strap and the tops of the ground rods, leaving the one strap-end sticking out of the earth.  You may substitute wide aluminum flashing for the copper, but you are more likely to have a corrosion problem because of the dissimilar metals.  This grounding construct provides 18 feet of deeply buried copper clad rod and 24 feet of buried copper strap, and it is a very good ground connection for coil use.

Figure 49.
Good triangular grounding system

Whenever I operate my coil, I run a length of aluminum or copper strap from the bottom of the secondary coil, and connect it to the ground strap sticking out of the ground by clamping the two straps together with a pair of Vise-Grip pliers.

Bringing it all together

Let’s review just where we are, at this point.  You built a box frame upon which to mount the Tesla coil components.  You have wound a beautiful secondary coil, and built a nice toroid to place on top.  You have wound a flat pancake primary coil, which is mounted on a flat wood or plastic support disk.  You wound either four air core chokes, or four ferrite core chokes and mounted them along with some bypass capacitors on a plastic plate and connected them to five binding posts.  You built a cylindrical static gap.  Finally, if you are using over 2-KVA of neon transformers or if you are using a 2-KVA potential transformer, you have also built a rotary gap.

The time has finally arrived to connect all these components together and to test them in the Tesla coil!

Step 1.
Connect one end of the static gap to one of the stationary electrodes of the rotary spark gap.  The remaining stationary electrode and the other end of the static gap are now the two connections, which will be referred to as “the spark gap” below.

Step 2.
The primary and secondary should have already been mounted on the box frame.  Connect the bottom of the secondary to the dedicated ground, using a 2-inch wide aluminum or copper strap.  Keep this ground run as short as possible.  

Step 3.
If you are using a potential transformer, connect the inside end of the primary to the same ground connection as the bottom of the secondary.  If you are using neon transformers, connect the inside turn of the primary to the bottom side of the spark gap, using a piece of the heavy 8-gauge wire.  Keep this wire as short as practicable.

Step 4.
Connect a 15-inch length of 8-gauge wire to the end of the capacitor nearest the primary.  Connect the alligator clip to the other end of the wire, then clamp the alligator clamp onto the eighth turn of the primary, counting from the inside turn outward.  

Step 5.
Connect the capacitor safety gap to the top of the capacitor.  See Figure 21.   Connect the transformer safety gap to the transformer, as in Figure 21.

Step 6.
The remaining end of the capacitor connects to the top end of the “spark gap” with a piece of 8-gauge wire.  Because this is part of the tank circuit, it is important to keep this wire as short as possible.

Step 7.
Connect the grounding binding post of the “transformer protector” to the case of the neon transformers.  Make absolutely sure that the cases of the neon transformers are all connected to each other and to the green grounding wire, which goes to the main power cable.

On the “transformer protector”, connect the two binding posts marked “OUT” to the two spark gap connections.

On the “transformer protector”, connect two lengths of high voltage wire from the two binding posts marked “IN” to the output of the neon transformers.  Run these wires through the rubber grommets in the top of the plywood.  If you are connecting a potential transformer, make sure that the “IN” ground connection on the protection circuit goes to the ground connection of the transformer, and that the “OUT” ground binding post of the protection circuit connects to the grounded side of the spark gap!  See the schematic in Figure 1 for details.

Step 8.
Connect the transformer primary extension cable to the main power receptacle on the back of the power controller box.  If you are using a potential transformer, make sure that ½ of your total ballasts are plugged into the controller to supply a moderate amount of power to the transformer (once the coil is tuned, you can add the remaining ballasts to increase power.)

Figure 50.
Controller, Tesla coil, and the cable locations

Step 9. 
Connect the rotary gap extension cord to the rotary gap receptacle on the back of the controller box.

Step 10.
Make sure the power switch on the controller is “off”, and the variacs and controls are turned to minimum.

Plug the main power cable from the power controller box into the 115-volt wall outlet.

Step 11.
On the controller box, turn on the power switch.  Verify the panel light is “on.”

Step 12.
Test the rotary gap control by slowly turning up the control.  The rotary gap motor should slowly come up to speed, approximately 80% of the maximum voltage on the control.  Listen for unusual vibration or signs of other mechanical problems with the rotary gap. Turn the rotary gap off, after performing this vibration test. 

Step 13.  
Make sure power is turned off.  We want to set the safety gap across the transformer and the safety gap across the capacitor at this time.  With power turned off, adjust the gaps fairly close (1/2 inch).  Run up the spark gap to about 50% speed.  Bring up the main power variac until the rotary gap begins to fire. The safety gap on the transformer should fire very frequently.  Turn off power and open the gaps slightly, and re-test.  You want the gap setting to be such that the safety gaps on the transformer and capacitor fire once every several seconds.  This setting will allow normal voltages to pass unimpeded, but will bypass excessively high voltages to ground, protecting the transformer and capacitor.

Step 14.
We will now test the main power circuits.   Turn on the rotary spark gap and bring it up to about 70% speed. 

Step 15.  
Slowly bring up power on the main power variac.  At some point, usually at about 40% power, the rotary spark gap should begin to fire.  The sparks should be bright white, with a very sharp report.  If they appear as a yellow flame, then your capacitor may not be connected properly.  At this relatively low power setting, and if in tune, sparks should just begin to break out of the toroid on the top of the secondary coil.  

Note: always make sure power is turned off before making adjustments to spark gaps or the primary!  The voltages and currents in the primary circuit can be lethal.

Step 16.
If you see no sparks, increase the variac setting about 1/8 turn.  If you still see no sparks from the secondary, note the power setting, then turn the variac down to zero, and turn the rotary control down to zero, then turn off the power switch.  Move the primary tap either in or out one turn, and retest, using Steps 11 through 13 as a guide.  If you still get no sparks, create a “breakout-point” on the toroid, by adding a small (1/4 inch) bump of aluminum tape to the side of the toroid.

If the sparks get shorter, when adjusting the primary tap for best tune, try choosing a tap in the opposite direction.  Tune the primary number of turns for maximum spark length at this reduced power setting.  

NOTE: These Tesla coil plans will produce a working Tesla coil, providing you build the exact sizes of coils, toroids, and capacitor with the values shown.  If you vary any of the parameters to any significant degree, you may not be able to tune the coil to resonance.  Build the coil as shown and it will work.  If you change the size of the primary, or the secondary, or the capacitor, or the toroid, then the coil will not tune to the expected design frequency, and in fact, you may not be able to tune the Tesla coil at all without making other compensating changes.  Suggestion: build it as described, tune it for best performance, then you can start changing it to suit yourself!

Troubleshooting - If you have NO sparks

Note:  If, after trying all primary turns, you still get no sparks, carefully do the following:

•	Try a smaller toroid, especially if you are only using one or two neon transformers.  You will not be able to make a spark break out of large toroid with a small neon transformer.

•	Verify that you have wired the circuit, exactly as shown in Figure 1.  There are several opportunities where you could have miswired the circuit.

•	Discharge the main capacitor with a clip-lead, and test the capacitor with an ohmmeter, then a capacitance meter.  You may also be able to substitute the capacitor with another capacitor that is known to be good for Tesla coil service.

•	Test the output of the transformer(s) with an insulated rod and test-lead.  You should be able to draw an arc at least three or four inches long from between the transformer high voltage terminals.

•	If the transformer(s) test ok, try the arc test between the output terminals of the transformer protection circuit.  An open choke coil can prevent operation, as can a shorted bypass capacitor.  You can test each of the chokes for continuity with an ohmmeter; you should measure a very low resistance, perhaps a few ohms at most.  

•	The bypass capacitors should measure an infinitely high resistance as measured with an ohmmeter.  If you measure any DC resistance across a bypass capacitor, then it is shorted internally and should be replaced.  Their actual capacitance value can be verified with a capacitance meter. 

If you have substituted a known good tank capacitor, and have verified the circuit is wired correctly, and the spark gap is producing bright, loud sparks, and you still have no toroid sparks, then the primary is still not tuned to the secondary.  It could be that the toroid is too small or too large, and has made the secondary resonant frequency above or below what can be tuned to using the existing primary coil and tank capacitor.  Many times, you can try a toroid of a different size and can re-tune the primary to resonance.  You can also try a larger or smaller tank capacitor value, or even add more turns to the primary.

Tuning tips

•	Once you have tuned the coil by setting the number of primary turns, you need to experiment with the number of static gaps.  Several things determine the quenching of the spark gap, one of the most important being the number of gaps in series.  Try reducing the number of gaps, and see whether performance improves.  The sound that the spark gap makes is a critical tuning clue.  A spark sound that is ragged, and which spits and sputters is a clue that performance is suffering.  An experienced coiler can instantly recognize a coil which is in tune, and whose spark gap is optimized, simply by the clear, pure, singing note of the spark gap when it is firing.  The speed of the rotary gap will affect the output of the spark as well.  If you run a rotary gap too fast, the tank capacitor will not have a change to recharge between the 60 hertz peaks of the line frequency.  Also, the faster you run a rotary gap, the harder you are pounding the tank capacitor.  If you run a rotary gap way too slow, you risk blowing your tank capacitor, as low rotary speeds cause kickback voltages that will fire your safety gaps.

•	If you are using a potential transformer and its ballasts, you will need to optimize the amount of inductance (current settings on the welder) and resistance (number of paralleled heater elements) for best operation.  As a general rule, the more ballasts you have paralleled, the more power that will flow into the transformer.  A balance of inductance is required in order for the transformer to operate best, with no bucking.  A rough guideline is that 3 millihenries and 0.5 ohms resistance is a good starting place.

•	Raising and lowering the secondary a fraction of an inch will change the coupling between the primary and secondary, greatly affecting operation.  A Tesla coil whose coupling is too low will not produce maximum spark.  If the coupling is too high (i.e., primary is too high up), you will see sparks racing up and down the secondary.  If you see this happen, immediately kill power and raise the secondary ¼ inch and try again.  You want the maximum coupling that you can get without sparks racing up and down the sides of the secondary.

•	The height of the toroid above the secondary will also affect operation.  Get it too high, and sparks will shoot out of the top windings of the secondary.  Get it too low, and spark length will drop.

•	If your coil is running well, you will probably get occasional strikes down towards the primary.  That is why we placed a grounded copper strike rail above the primary.  If you get frequent strikes to the capacitor, or to the rotary spark gap, then you may have to mount the capacitor and gap lower down or move them further out away from the secondary.  Sometimes, it becomes necessary to place grounded strike wires above the rotary and capacitors to protect them from long sparks!

•	If you get sparks from the primary to the secondary, you can try removing one inside turn on the primary.  Sometimes, you can put a sleeve of polyethylene around the bottom of the secondary to insulate it.

Good luck with the construction of your Tesla coil, and remember: safe coiling!

End of Conventional Tesla Coil Construction File