The images referenced in this article have not been scanned and placed into the file yet, although a couple of schematics have been placed into the file.

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.

Above, schematic for the Power Controller

   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 moralit y 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 transf ormers 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 larg e 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 ver 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 sev ere 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 h ad 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 fre quency 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 transf ormer 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 th e 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 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 Archimed ian or flat pancake coil. The pan cake 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 b e 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 initia lly 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 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 detai ls, 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 o nly 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 an d 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 incre ase 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 s ress 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 tim e 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 r equired 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