The images referenced in this article were never scanned and placed into the file.   Refer to pictures of magnifier system at Bert Pool’s web site -



Copyright © 1997, by Bert Pool


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 build and operate Tesla coils. Extreme caution is advised, especially when building and operating those coils using potential or distribution transformers as a power source. Every attempt has been made to insure that the information contained in this file is 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 a re 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.



Theory in a nutshell

The Tesla Magnifier is an air core transformer, which drives a resonant, freestanding "Extra" coil. The main power source is a step-up transformer, which supplies power to a capacitor and primary coil, which together form a resonant tank circuit. A fast quenching spark gap is used to commutate the high voltage in the primary circuit. As the tank circuit oscillates, currents in the primary inductively drive the tightly coupled secondary coil. The secondary coil is a low impedance, high voltag e, high current source, which is used to base-drive the third, or "Extra" coil. The primary/secondary is designed to form a 1/8 wavelength helical driver pushing a ¼ wavelength resonator. The Extra coil has been carefully built so that its inductance, self-capacitance, and top load capacitance form a resonant circuit whose frequency is double that of the primary tank circuit. The "Extra" coil, which is not magnetically coupled to any other coil, forms a high-Q resonant circuit, whose peak voltages can reach hundreds of thousands or millions of volts potential. The electrical currents produced are high frequency, high voltage, and are relatively low current.


The components in a Tesla Magnifier


A Tesla Magnifier is NOT just a third coil being driven by a conventional Tesla coil! The coupling between the primary coil and secondary coil in a Magnifier generally starts at 0.4, whereas in a conventional Tesla coil the coupling factor is only about 0.1 to 0.2. This higher coupling in Magnifiers creates a high current spark in the gap, which can be very difficult to quench. A conventional Tesla coil has a tuned secondary, which achieves large voltages due to a "resonant rise" of voltage. Magnifiers do not have a tuned secondary at all, and all the voltage that appears at the top of the secondary is entirely due to pure transformer action. The Extra coil, on the other hand, is where resonant rise takes place in a Magnifier. The quenching in the spark gaps in a Magnifier is so fast that the gaps cannot operate a conventional two-coil system!


Figure 1.

Schematic diagram with all the required parts for a Tesla Magnifier. Many of the components used in a Magnifier are the same as used in a conventional coil, but much of the coil and spark gap design is different.


First, we will discuss each of the components used in a Tesla magnifier, then we will build and assemble a Magnifier system capable of producing sparks over ten feet in length.


Power Transformers

The first item we want to examine is the power transformer. Magnifier power transformers typically have much higher output voltages than transformers powering conventional Tesla coils. Conventional Tesla coil transformers are rated from about 5,000 volts up to about 14,400 volts, whereas magnifiers work best with transformers in the 20kV to 30 kv range. Also, conventional Tesla coils can be operated on neon transformers, something I do not recommend for Magnifiers. Neon transformers do not output high enough voltages, and are not tough enough to operate Magnifiers reliably. Potential or distribution transformers should be the only transformers used in Magnifier service. Unfortunately, high power transformers are more dangerous to operate than the small neon transformers, they come with a higher price tag, and they require ballasting and control equipment that neon transformers do not need.

How to use potential and distribution transformers

Potential transformers are used by 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 current self-limiting. For example, if you take an "unloaded" potential transformer (one that has no load connected to the secondary), and then connect its primary directly to a wall outlet, only a couple of amp s 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, is usually used to cont rol power to the transformer. Along with the variac, a variety of methods of controlling or ballasting current to the transformer are used, and we will cover these methods 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 secondary windings of potential transformers are generally designed to withstand voltage spikes of 110,000 volts! Potential transfor mers are far, far better than neon transformers when it comes to use in medium and 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. You will want to purchase a transformer with a ratin g of around 20,000 volts and two kVA power rating. You will not need more than a 5 kVA transformer for this project.


Figure 2.

Shown are a 2 kVA and a 5 kVA potential transformer. They are compared in size to a typical 360 VA neon transformer.


Again, you need to be aware that potential transformers require the use of large variacs and massive resistive and inductive ballasts to properly operate them. That $35 bargain transformer you buy 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’ll probably live to coil another day. A potential or distribution transformer will kill 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 used to be put into the oil to improve its dielectric properties, but PCB’s were proven to be both carcinogenic and mutagenic. Federal and state environmental agencies have banned PCB’s totally. It can cost you several hundred dollars to have a PCB capacitor disposed of in 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 3.

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 usually 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 240 volts, controlled by v ariacs 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. Tesla Magnifiers require a potential or distribution transformer secondary rated at 20,000 to 25,000 volts. This higher voltage will necessitate the purchase of very good capacitors capable of withstanding the higher voltages.


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. Often, nichrome-he ating elements from electric heating furnaces or ovens are used. Heating coils out of clothes dryers are very good.


Figure 4.

Some heater resistive elements used as current limiting ballasts for large Tesla coils.


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 weld er, 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 you may use a larger welder.


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 millihenry down to about 2 millihenry. 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 resisti ve 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 voltage provided by the inductor.


Figure 5.

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


Coilers have found if they use pure inductance, with no resistive ballast, that 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 6.

Shown are several styles of variacs. Note the dual and triple transformers on one common shaft. Dual 120-volt transformers can be wired for 240 volts. The triple transformers are originally designed to be used with 3 phase, 240-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. I have run 50 amp variacs at 85 amps with no problems. You can find large variacs at salvage yards, theatrical equipment auctions, and in 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 240-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 7.

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, especially Magnifiers, can produce a lot of RFI, or Radio Frequency Interference. An Extra coil that has been tuned to produce maximum spark will radiate very little RFI through space. The secondary coil, on the other hand, can create an electromagnetic field that reaches out several yards and is intense enough to damage electronic equipment. 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 other elect ronic equipment. The old, long "Tesla trail" is littered with dozens of dead radios, computers, VCRs, garage door openers, and other gadgets ruined by severe RFI and voltage transients that 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 suitable capacitors and inductors, but there are many suitable 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. Keep your camcorder and other sensitive electronic equipment very far away from the primary/secondary driver circuitry of a Magnifier!


Figure 8.

Several line filters, ranging in size from 5 amps to 50 amps.


Protecting the transformer

There are three devices used to protect the transformer from 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 should a high voltage transient appear, the gap will conduct, shorting out the voltage and preventing it from damaging the insulation inside the transformer. The gaps are connected to ground, so that any transient voltages go straight to ground, bypassing the transformer windings. It is highly recommended that safety gaps, chokes, and bypass caps be used on potential transformers and pole pigs connected to Magnifiers, because of the additional RF fields and transients created by the driver circuitry.


Figure 9.

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 grounded.


The second protective device is a bypass capacitor. This is simply a small high voltage capacitor that goes across the high voltage windings of the transformer or from the high voltage winding to ground. Typical values are 250 to 500-pF. The capacitor will appear as an open circuit to the 60-hertz line frequency, but the cap will appear as a low resistance path to ground for RF noise, which might damage the transformer.


Figure 10.

Shown are typical bypass capacitors. These are small ceramic and mica "door knob" capacitors.


The third transformer protection is a choke. 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 careful construction is a must to prevent electrical arc-overs on the windings. Air core chokes must be physically large in order to have enough inductance to be effective. Be cause the chokes are not in the "high current" 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 $4 each.

Figure 11.

4 inch diameter, 6 inch long air core choke wound with 28 gauge enamel wire, and a 2 inch ferrite core choke, also wound with 28 gauge wire.


Figure 12.

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 resonant tank circuit of the Tesla coil. This is bad news. Should this happen, the choke no longer protects the transformer, indeed, the resonating choke can actually create high voltage transients that can damage the transformer! The 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 even 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 a form with a different diameter.

Spark gaps

Tesla Magnifier 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 io nized, the air across the spark gap becomes very conductive, and is for all intents a short. 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 components form the resonant tank circuit of the Tesla coil. The alternating current p assing 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 13.

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. Gaps used with Magnifiers require much faster quenching than conventional coils, due to the higher coupling between the primary and secondary coils.


Cylindrical gap

While a small conventional Tesla coil can sometimes be operated with a very simple spark gap, Magnifiers require ultra-fast quenching. This is best accomplished by using series multi-gap rotary spark gaps connected to cylindrical series static gaps. This "series static gap" works very well if you use copper cylinders, 2.5 inches in diameter and 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 dissipati on. A copper cylinder gap, using 2.5-inch by 4-inch cylinders is good for power levels up to 8 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 with a Magnifier when combined with a series quenched rotary gap.


Rotary gap

A rotary gap is made of a 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 about 10,000-RPM. The electrodes 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 electrodes allows for excellent spark quenching. The "flying" electrodes stay very cool, and the air they stir up helps c ool the stationary electrodes a bit. The disks are usually made out of G-10 epoxy-glass composite material, polycarbonates such as Lexan, or even acrylic. The epoxy-glass material has the best thermal and tensile strength properties. Acrylic can shatter or melt. The polycarbonates are very tough, but can still melt.


The rotary spark gap used with a Magnifier has at least four stationary electrodes, and some designs use as many as eight. The gaps are connected in series, or a clever combination arrangement that allows sets of series gaps to be alternately connected together. The more gaps used, the better the quenching. Some series rotary gaps quench so well that they do not require the additional quenching provided by a cylindrical static gap.


Figure 14.

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


One problem with high power rotary gaps is the dissipation of heat from the stationary electrodes. This 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 blasts of compressed air, and even pumping cool oil t hrough a hollow electrode is a possibility.


Figure 15.

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 coil

The primary coil in a Tesla Magnifier system should meet several important criteria. It should be of low resistance. It should be "tappable" for tuning purposes. It must be well insulated from the secondary. Since Magnifiers require that the primary be tightly coupled to the secondary, this dictates that a helical, or cage type of primary be used. Pancake primaries do not provide the coupling that will be necessary for Magnifier work. This helical primary coil, with its tight coupl ing and tall physical stance will mean that there will be a very great tendency for the secondary to arc over to the primary. You will be required to provide some very, very good insulation between the two coils to prevent this arcing.


Figure 16.

The Helical primary. Note the polyethylene insulation between the primary and secondary coils.


Because of the "skin effect" which is present in currents at RF frequencies, copper tubing makes a superb primary coil conductor. Magnifier coil builders most often 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. The bare metal tubing must be space wound to prevent shorts between adjacent turns.


The primary coil support frame is made out of standard PVC pipe. Evenly spaced slots, the width of the copper tubing, are cut into the outside of the pipes making up the form. The slots hold the tubing in place, and maintain a precise spacing between turns.


Taps on turns are usually best accomplished by using a copper strap that wraps completely around the tubing 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 one a good tune has been achieved.


Secondary coil

The secondary coil is wound on a large diameter PVC pipe. Because of the phenomenal voltage stress on this coil, the wire used is high voltage test prod wire. This secondary coil will use four layers of wire connected in parallel. This arrangement provides relatively high inductance with low resistance.

Figure 17.

Secondary coil before insulating.


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


  • Secondary coils should be wound with large gauge, high voltage wire, or multiple layers of smaller gauge high voltage wire.
  • The larger size wire will necessitate a rather large diameter coil form to keep up inductance.
  • The inductance of the secondary should be much less than the extra coil – usually ½ the value.
  • Magnifier secondary coils generally have a small diameter-to-length ratio, which increases the coupling factor to the primary coil.
  • The secondary will have to have very good insulation between it and the primary.
  • A toroid with a large cross-section placed on top of the secondary will provide valuable field shaping and will help reduce primary strikes, and will lessen the chance of damage to the secondary insulation caused by such strikes.



Capacitors are one of the most important components of any Tesla coil, and especially so in a Magnifier. The only capacitors I recommend for Magnifier work are commercial impulse capacitors rated for Tesla coil service. You can use homemade capacitors, provided that you use a good combination of series/parallel connected units to provide adequate voltage protection.


Figure 18.

Two capacitors suitable for Tesla Magnifier coil use. They are shown in the preferred order, best to worst. Polypropylene commercial cap, homemade polyethylene capacitor


Let me tell you straight up front that you need to go buy yourself 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 40 kilovolts AC, preferably more. If you can find a good used capacitor, then good for you! They are very, very difficult to find. Search surplus yards and junkyards, and electronic surplus stores long eno ugh and you might find one, but it’s not likely. So, you’ll end up buying one. A 0.015 ufd, 45 kV capacitor is likely to cost you between $400 and $600. The larger the capacitance, the more it will cost you. The higher the voltage rating, the more it will cost you. But they 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 vo ltage rating three times higher than the RMS voltage that your transformer puts out. 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!


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. Connect a wire to each capacitor terminal, and placed the other end 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 on ly 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. Trust me, your beautiful hand-built capacitors, or expensive commercial capacitors are worth the expense and bother of installing a fifty-cent safety gap!


Figure 19.

Safety gap installed on a commercial capacitor.


All connections from the capacitor to the spark gaps and to the primary coil should be 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 gaug e 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 tight, neat layout of these components will result in better performance and longer sparks.


Figure 20.

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 Extra coil. Research over the last few years has shown that Tesla coils, and in particular Magnifier Extra coils, love large top load capacitances. The top capacitor appears to allow much higher voltages to form before the spark channel is initiated, and helps to sustain the spark channel once it is formed.


Figure 21.

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 possib le 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. For the Magnifier shown here, we will build two rather large toroids and stack them to achieve the required capacitance that we will need. Some coilers suggest th at if using a single toroid, the toroid diameter should be at least three times the extra coil length.


Figure 22.

Figure showing toroids mounted above top of Extra coil


Tesla coil toroids almost always have a center disk. This 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. This rod can also be used to support and separate multiple toroids, as we will do on this Magnifier coil.


For our toroids, we will use aluminum flexi-duct used for heating and air-conditioning. It is reasonably priced, but the ribbed aluminum is very soft. We will leave it compressed when making the toroids. Cost for each 36 inch by 8-inch toroid will be about $60, and we will require enough material to build three toroids, two for the Extra coil top-load, and one to act as an electrostatic field-shaper for the secondary.


Tesla coil grounding system

Magnifiers absolutely, positively, require a good external ground for safe operation.

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 23.

Do not connect the secondary to the wall outlet ground!




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 hav e 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?" One or more copper clad ground rods 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.


Start of lecture:

Before you begin digging or driving ground rods into your yard, 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 (ti me 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 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 inch es 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 supplies a very good ground connection for coil use.


Figure 24.


Good triangular grounding system


Whenever I operate my coil, I run a length of aluminum or copper strap from the bottom of the 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.



Constructing the Tesla Magnifier coil

NOTE: These Tesla Magnifier coil plans will produce a working Tesla Magnifier coil, capable of producing sparks in excess of ten feet in length, 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 Extra coil or the capacitor, or the toroids, 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 three 8-foot sections of 8-inch diameter aluminum flexi-duct.

240 volt, 50 ampere line filter

240 volt, 50 ampere variac

240 volt, 1 ampere DPDT on-off switch

Two 240 volt, SPDT 50 amp contactors or one 240-volt 50 amp DPDT contactor

Four 240 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

5000 feet of 14-gauge high voltage test prod wire

20 feet insulated, 8 gauge stranded wire

0.015 ufd, impulse capacitor, 45 kVAC voltage rating

100 foot roll of 0.5 inch diameter copper refrigeration tubing

20 feet of 2-inch diameter PVC pipe

18 pounds of 18 gauge enamel wire, approx. 3600 feet

One 4 foot by 8-foot sheet of 1-inch thick foil covered Styrofoam core insulating sheathing.

10 copper splice sleeves for 2.5 inch diameter copper pipe. These copper cylinders will be about four 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

42 inch length of 16 inch diameter, thin wall PVC drain 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

Decking screws, 1 and ¾ inches long, one box

Hot glue

PVC pipe, 8 inches in diameter and 26 inches long

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

240-volt AC meter

100-amp AC current meter.

240 volt AC power indicator panel lamp

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,


Lincoln "cracker box" welder, 120 amp output rating, or similar adjustable inductor

Assortment of 1,000 watt or larger heating elements

Fan or blower to cool heater elements

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


                One set of hardwood handles for a wheelbarrow

Now that we’ve become familiar with all the parts of a Tesla Magnifier coil, let’s build one. The plans presented here will allow you to build a coil powered by a 2 kVA potential or distribution transformer, and make sparks over ten feet in length!

Figure 25.


The Tesla magnifier coil in operation, producing sparks in excess of 10 feet in length


Making the box frame

The Tesla magnifier driver is built on an open box framework, with a ½ inch thick piece of plywood on top. The primary, secondary, spark gaps mount on the plywood. If you use a commercial capacitor, it too can mount on 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 28). 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 s ensibilities, you can stain the wood first.


Figure 26.

The box frame and location of components


See Figure 28 for the location of the primary, secondary, gaps, and capacitor on the plywood.

This Magnifier design has handles and an axle and wheels attached to the box frame so that it may be rolled about. If you will be using homemade capacitors, you will want to add a capacitor shelf and straps to the side of the box frame for the capacitor – see Figure 28.


Making the primary coil


The primary coil form is made out of eight slotted lengths of 2-inch PVC pipe placed in a circle.


Cut a disk out of ¼ inch plywood, 19 inches in diameter. Set this disk aside for later use.


Using a 2-inch hole saw, drill eight evenly spaced holes in the plywood top of the box frame, as shown in Figure 29. Make sure that the inner distance between opposite holes is exactly 19 inches. Cut eight lengths of 2-inch diameter PVC pipe 28 inches long. Number each pipe, and mark one end of each pipe "TOP". Place the pipes in ascending number sequence in a simple square frame, aligning all the "TOPS" to one end. Mark the slots as illustrated in Figure 30. The idea is to have slots in the pipes into which we will wrap the copper tubing to form a smooth spiral. Use a radial table saw equipped with a dado blade, and cut twelve slots, each ½ inch wide, spaced 1 inch apart in each pipe, using the marks made in the previous step as a guide. Make the dado cuts just wide enough to snugly hold a test piece of copper tubing.


Place the 8 pipes in the 8 holes drilled in the plywood top. Make sure they are in number sequence, and that the ends of the pipes marked "TOP" are indeed at the top. Point the slots outwards on all the pipes. Hot glue the pipes into the holes in the plywood.


Put a five-gallon paint bucket in the middle of the primary form. Set the 19-inch plywood disk on top of the bucket. This disk will act as a temporary platform for the roll of copper tubing, and it also acts as an inner spacer for the tops of the pipes and it will prevent the pipes from bowing in when the copper tubing is wound around them. Set your coil of ½ inch copper tubing on the plywood disk on top of the paint bucket, taking care that you place the copper coil so that the OUTER end of the copper tubing will unwrap in a clockwise-direction. Attach the OUTER end of the copper tubing to the BOTTOM slot of pipe #1, using nylon tiewraps to hold it firmly in place. Start winding the primary using the copper tubing. Place the tubing into the slots, going around and around the form until you have wound 12 turns. You may have to use a rubber mallet to seat the tubing into the slots as you wind. Your primary will use over 50 feet of tubing, so partway through the winding you will have to use a copper sle eve to solder-splice another piece of tubing onto the primary. Be sure to setup the new coil of tubing to unwind in the proper direction! Finish-off the top-turn by tiewrapping the end of the tubing into the last slot.


Figure 27.

Bolt hole for secondary


See Figure 28. On the coil end of the top of the box frame, mark a spot in the center of the primary. Use a 1/2-inch diameter drill-bit and drill a hole at this spot. This hole is for the secondary coil support bolt.


Figure 28.

Two-by-four support and winding jig hole placement.


Figure 29.

Location of secondary ground hole.


A 1-inch wide, 2 inch long hole now needs to be drilled through the plywood top. Use Figure 31 as a guide for placing this hole. It is near the end of the box, centered exactly between two of the primary support posts, and 1 inch inside.


Building the secondary

Cut a length of 16-inch PVC pipe, exactly 42 inches long. The best way to mark the cut line around the circumference of the pipe is to wrap a flexible plastic ruler completely around the pipe and mark along one edge. Use a handsaw or fine toothed skill saw and cut the pipe squarely. Use soap, water, and a piece of Scotchbrite and scrub the pipe clean. Do not use steel wool, as it will leave tiny bits of metal embedded in the plastic. Use a moderately warm source of hot air (hair dryer, room heater, etc.) and bake the tube dry. PVC is somewhat hydroscopic, so we want to get out as much moisture as we can. After the tube has been baked dry, coat it inside and out with a good polyurethane coating. Do not use water-based polyurethane.


Figure 30.


Two-by-four support and winding jig hole placement.


Cut two lengths of two-by-four about 16 inches long, and shape the ends so that they fit exactly inside the ends of the 16-inch PVC pipe. See Figure 31. Carefully measure and drill a ½ inch hole in the center of each support. Mount these two-by-fours to the pipe with epoxy and several ¼ inch diameter plastic or wooden pegs. DO NOT USE METAL SCREWS!


Figure 31.

Hand-crank operated coil winding jig.


It is recommended that you construct a coil-winding jig something like the one illustrated in Figure 32, 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 accidentally drop the wire. Be sure there are no gaps between any of the turns. Gaps will propagate from layer to layer and will plague you with continuing problems. Wind the coil to within one inch of the top end of the tube, and within 2 inches of the bottom end of the tube. When you have finished winding the coil, tape the ends of the wire to the tube. Run a bead of hot glue al the way around the ends of the windings, securing the wire to the tube.

Start the second layer of wire at the same end of the tube where the first layer began. Wind the second and following layers in the same direction as the first layer. You should be able to lay the second layer of wire right on top of the first layer, using the turns of the first layer as a winding aid. Finish each layer of wire with a bead of hot glue around the ends of the winding to hold the windings securely in place.


After the four layers of high-voltage test-prod wire have been wound, connect the bottom ends of the four wires together and solder them to a ground plate mounted as described below.


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 seconda ry coil to this copper ground plate.


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. Do not attach the top end of the secondary wires to anything at this point of construction.


Cut a 25-foot length of 120-mil thick polyethylene, 42.5 inches wide, which should be long enough to wrap completely around your secondary coil four times, forming an insulating barrier four layers thick. If you have to use more than one piece of polyethylene, be sure to overlap the two pieces of poly about four inches. You can use three or four short pieces of clear Scotch box tape to tape the overlap together. Do not use a single long piece of box tape from the top of the coil to the bottom! Doing so will produce an arc-path, which can short out the coil. This important tip was learned the hard way!


Figure 32.

Ground plate details



Building the Extra coil

Cut a length of 10-inch diameter PVC pipe, exactly 26 inches long. The best way to mark the cut line around the circumference of the pipe is to wrap a flexible plastic ruler completely around the pipe and mark along one edge. Use a handsaw or fine toothed skill saw and cut the pipe squarely. Use soap, water, and a piece of Scotchbrite and scrub the pipe clean. Do not use steel wool, as it will leave tiny bits of metal embedded in the plastic! Use a moderately warm source of hot air (hair dryer, room h eater, etc.) and bake the tube dry. PVC is somewhat hydroscopic, so we want to get out as much moisture as we can.


Figure 33.

Details of the Extra coil, with the three toroid centering pins.


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.


Wind the 10-inch diameter Extra 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.


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.


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.

Special note: Do not coat the Extra coil with polyurethane or any other coating. You may place a bead of hot glue around the top and bottom of the windings to hold them in place. Coatings of any kind over your windings can cause your Extra coil to fail.



Building the toroids

Figure 34.

Toroid details


On the three toroids, we will use 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.


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 the ends 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 drill 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 Extra coil. The toroid disk will slide down over the PVC support pins, which stick out of the top of the Extra 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 Extra coil.

The third toroid you will build mounts on top of the secondary coil. You may use some hot glue to hold it in place. Attach the four wires from the top of the secondary to the toroid, using a small piece of aluminum tape.


Building the transmission line and Extra coil stand


The transmission line is very easy to build. Cut a length of 10-inch to 15-inch wide aluminum flashing 12-feet in length. Staple two 11-foot lengths of ½ inch by ½ inch pine strip down the sides of the aluminum to act as stiffeners.

The Extra coil stand is made from a 5-foot length of 10-inch PVC pipe securely fastened to a large square base made from a 4-foot by 4-foot sheet of ½ inch thick plywood. The goal is to make a sturdy stand on which you can set the Extra coil and the toroids. Remember that you have large diameter toroids balanced on top of a small-diameter coil, and this configuration is by nature very unbalanced, and the whole mess will want to topple over!

At the base of the PVC pipe stand, you can cut a 10-inch diameter plywood disk and screw it inside the end of the pipe and bolt it to the center of the Plywood Square. The top of the pipe has a 12-inch diameter disk attached to the top of the pipe, using nylon screws or other non-conducting attachments. The disk has three short 1-inch PVC pipes which will stick up inside the Extra coil form and which will prevent it from falling over.


The capacitor


Hopefully you followed my advice, and are using a commercial capacitor. If so, it will probably fit on the sheet of plywood with no problem. If you are using a homemade cap, or a very large commercial cap, then place the cap on a plastic or wooden stand beside the box frame, as close to the designated spot as possible. Refer to Figure 28 for parts placement. The goal is to place the capacitor and spark gap close to the primary, where the connecting wires are as short as possible. Note: To preve nt accidental arcs, do not place either the capacitor or spark gap assembly closer than about four inches to the primary.


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 35.

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 36.

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 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 stil l 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 an d 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 nice 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 37.

Layout of rotary disk


Use Figure 38 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 toothed 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. B est 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 mo tor speed was controlled with a light dimmer control, but gosh was it loud!


Figure 38.

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. They were often used in the old IBM mainframe tape drives. 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 39 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 gonna have to make or pay someone to make a suitable shaft-adapter that will allow you to bolt your disk to the shaft. Once you have the motor with some kind of disk mounting arrangement, then you can start to make the disk.

Start making the disk by using a compass and marking off a 9-inch circle on your disk material. Use a jig- saw or saber saw and 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 correct blade, adjust the blade speed and cut at just the right forward sp eed 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 39.

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 40. Mount a bolt in the center hole of the d isk, 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 wo bble 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 8 marks are made, drill holes at these marks, using a drill bit suitable for #10 hardware.


Figure 40.

Detail of flying electrode

Figure 40.

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 41.

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 42 for clarification.

The Magnifier rotary spark gap uses two sets of stationary electrodes, forming a total of four air gaps. The stationary electrode pairs are mounted across the disk from one another, so that each of the two sets of gaps fires at the same time. The two sets of gaps are wired in series. See Figure 42 for clarification on the construction and connection of the electrodes.

Connect the first set of stationary electrodes to the second set of electrodes using a short, curved piece of copper tubing as shown in Figure 42 and Figure 43. The remaining two electrode connections on the rotary gap are used to connect the rotary to the primary coil and to the copper static gap.


Figure 42.

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 43 for a sample rotary design.


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 rotary platform. There will only be an air gap of a fraction 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 melt. The tungsten-tipped steel electrodes are mounted in brass blocks to improve heat dissipation. The brass is attached to thick, solid phenolic or G-10 material.
  • The stationary electrode mountings have to be very, very rigid, because if they move, and the flying electrodes hit the stationary electrodes, all hell will break loose! Lock the electrodes in place 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-in 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 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 to protect you from flying shrapnel should the rotary disk ever fail.


A rotary spark gap of this design, using a 9-inch disk, with acrylic motor stand and high-temp phenolic electrode mounts has operated 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-inch diameter PVC pipe, at least 8 inches long. Close-wind it 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 yo u 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-shri nk 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 43.

Controller parts layout


Figure 44.

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 44 and 45. The power controller is built into a box, 24 inches tall, 24 inches wid e, and 18 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 44 & 45. You absolutely must use a grounded, three-wire power cable to connect this panel to a 240-volt receptacle!

Your power controller 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.

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!

Figure 40.

Figure 45.

An 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 clamp. Your two electrical connections will be on one end of the wires. You can change the resistance of the load by changing the location of the slideable cl amp. 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.


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 s upply 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.


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 four-layer secondary coil with high-voltage test-prod wire, and built a nice toroid to place on top. You have wound a helical copper tubing primary coil, which is separated from the secondary with heavy polyethylene insulation. 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 high-power copper cylindrical static gap. You have also built a series four-gap rotary spark gap.


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

Connect the TOP connection of the primary to the dedicated GROUND. This is the same connection that the BOTTOM of the secondary will be connected. The top winding of the primary should be the ground connection, so that any secondary strikes to the top turn of the primary will go straight to ground and not into the capacitor or transformer.

Step 4.

Mount the Extra coil on top of the 5-foot tall PVC pipe pedestal. Insert the four locking pins into the base of the coil so it cannot fall. Place the two toroids on top of the Extra coil, separating them with a 10-inch long PVC pipe spacer.

Step 5.

Run a 12-foot length of 10-inch wide aluminum flashing from the toroid on top of the secondary to the BOTTOM of the Extra coil.

Step 6.

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, and then clamp the alligator clamp onto the 12th turn of the primary.

Step 7.

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 8.

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 11.

On the "transformer protector", connect the binding post marked "POWER-OUT" to the one remaining spark gap connection. Connect the "GROUND-OUT" binding post to the dedicated ground (the buried ground connection that connects to bottom of the secondary.)

On the "transformer protector", connect a length of high voltage wire from the binding post marked "POWER-IN" to the output terminal of the potential transformer. Connect the "GROUND-IN" on the protection circuit to the ground connection of the potential transformer.

Step 12.

Connect the transformer primary extension cable to the main power receptacle on the back of the power controller box. 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 40.

Figure 46.

Controller, Tesla coil, and the cable locations


Step 13.

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

Step 14.

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 240-volt wall outlet.

Step 15.

On the controller box, turn on the power switch. Verify the panel light is "on."

Step 16.

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 17.

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 safety gap setting to be such that the safety gaps on th e 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 18.

We will now test the main power circuits. Turn on the rotary spark gap and bring it up to about 80% speed.

Step 19.

Slowly bring up power on the main power variac. At some point, usually at about 80% 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 Extra 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.

Also, it is a good idea to discharge the capacitor prior to changing the primary taps or working on the coil.

Step 20.

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 to roid.

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 Magnifier coil plans will produce a working Tesla Magnifier coil, capable of producing sparks in excess of ten feet in length, 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 Extra coil or the capacitor, or the toroids, th en 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:


    1. Try using only one toroid on the Extra coil instead of two toroids. Re-tune the primary after doing this.
    2. Verify that you have wired the circuit, exactly as shown in Figure 1. There are several opportunities where you could have miswired the circuit.
    3. 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.
    4. 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.
    5. 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.
    6. 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 coil er 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 b lowing your tank capacitor, as low rotary speeds cause kickback voltages that will fire your safety gaps.
  • If you are using a potential transformer and 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.
  • A particular Extra coil may perform better with more or less top capacitance. Try reducing or increasing top capacitance by adding or removing toroids or using toroids of various sizes. Changing top capacitance, will, of course, require re-tuning the primary coil.


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


End of Conventional Tesla Coil Construction File