The Big List


1994, William Beaty

I've built a couple small (under 2ft) coils, neither over 500Watt, and I've read quite a bit, so now I'm an expert, right? :)


If you want to make long arcs, a sphere or oblate terminal is required for the top of your coil. You'll only get corona displays if using a bolt or a metal rod as the main terminal. No long arcs.

This occurs because whenever a large hollow sphere or toroidial electrode is used, then during each pulse from the spark gap, the voltage field in the space around the top of the secondary coil can build up to extreme levels before the plasma finally ignites and creates a spark. The large sphere causes delayed sparking and higher energy before the arc appears. A sphere has no sharp points, so it can support a huge voltage BEFORE the spark eventually starts. When corona finally does break out on the sphere surface, the e-field in the space around the terminal has already grown to such a high value that a branching spark-streamer will be driven by the ambient field to quite long lengths. Without the sphere, the spark starts early while voltage is still low, and since sparks eat electrical energy, the voltage will never build up. In some ways a Tesla Coil is like a capacitor: "charge" it up with RF oscillations until voltage rises high, and THEN trigger a spark. That gives the longest sparks.

The toroid terminal should be at least as large as the coil diameter, and preferably should be 2-3 times as large. Be aware that the toroid terminal acts as a capacitor to ground, and significantly reduces the secondary coil frequency, so you'll have to re-tune the primary when adding or removing the toroid or when switching to a different toroid size. Another hint: place a small needle-discharger on the surface of your toroid. This will allow an underpowered coil to create fairly long arcs. Experiment with the size of this "trigger" device (sometimes a thumbtack is best, sometimes a small nut works well.)


For tesla coil spark length, voltage is less important than wattage. I've heard it quoted that long lightning-like TC arcs require about 500 to 1000 Watt per foot of discharge when the coil is designed and tuned properly. Be aware of this when designing the power supply section of a big coil. Those huge coils that spew 10ft lightning bolts are usually being driven by 10,000W power pole transformers ("pole pigs"), with the transformer's primary/secondary wired backwards to provide voltage stepup. And to improve your coil's performance, try to increase any of the characteristics which lead to higher wattage. Examples: use heavier wire (especially in the whole primary coil/capacitor/sparkgap circuit), adjust optimum power-coupling between primary and secondary, use heavy gauge wire on the secondary, build a short and wide secondary coil, and use higher supply-voltage during operation (try a DC system with a rotary gap, or try using many neon transformers in parallel.)


The power supply transformer's current and the main resonator capacitor's capacitance must be chosen for best match. If the capacitor is too large, and the transformer too small, the capacitor will not fully charge on each half-cycle of the 60hz supply, and the total system wattage will be reduced. If the capacitor is too small, it will only draw a small current from the power supply, and the actual wattage of the system will be lower than the capability of the supply transformer. Refer to a good TC design program for choices of capacitance versus transformer current.


One problem you may run into: dead HV transformers, where the secondary shorts out internally. The RF from your coil's primary L/C section goes backwards through the circuit, adds to the HV transformer's secondary voltage and increases the peak voltage there. Also, the high frequency will tend to concentrate its voltage on fewer turns inside the HV transformer, rather than spreading it over the entire internal coil as the 60Hz energy does. This tends to exceed the transformer V-out rating and can create internal arcs which will wreck a neon-sign transfomer.

You can put a 'safety gap' sparkgap across the transformer output. Disconnect the coil circuit and adjust this gap's spacing so it is a bit larger than that needed to start an arc. The safety gap will then protect the transformer against large voltage pulses by arcing and safely shorting them out. But if RF and pulses are a problem in your design, this gap will continually fire.

A partial cure: use a design which places the main spark gap across the power supply and the main capacitor in series with the supply. In normal operation the voltage across the gap will then be lower than voltages elsewhere. A better cure: put a low-pass filter between power supply transformer and the TC primary circuit, with series inductor and parallel filter capacitor wired so that it filters out the RF energy coming FROM the primary. Use a high-inductance handwound aircore choke coil and a high voltage capacitor. Numerous TC project plans recommend an inductor which is far too small to stop the hundreds-KHZ energy from getting back to the transformer. For the filter coil I used a couple of pounds of #28 wire wound as a 6"dia, 1/4" thick pancake between two plexi disks, with the outer terminal going to the supply and inner terminal going to the tesla coil. For the filter cap I put a .001 capacitor directly across the power supply transformer output, and added a small "safety gap" across the cap for good measure. Adding this filter circuit let me completely remove the Variac I had been using to control the output, so I could then run the coil wide open at 120V with no firing of the safety gap.


Incorrect coupling between primary and secondary can be a source of low performance or failure. Even when tuned perfectly, a coil won't work well if over- or under-coupled. The typical way to adjust the coupling is to vastly overcouple the coil at the start, then reduce coupling until it works best. Do this by designing your primary to be wound quite close to (or over) the secondary, but prevent arcing between primary and secondary by using heavy plastic (etc.) as insulating shield between the coils. Place the primary against the base of the secondary (or over it, in the case of a cylindrical primary), then raise the secondary up out of the primary in stages while testing the coil for max output (and possibly retuning each time.) For spiral primary designs, the spiral should be fairly small with close turns, and initally positioned very close to the lower end of the secondary. A cone-shaped primary gives tighter coupling. If arcing between primary and secondary is a problem, use plastic insulator plates or tubes between the coils. If arcing between the turns of the primary is problem, space the turns wider, caulk the edges, or paint the edges with Urethane, remove burrs and sharp pointed splices from the wire, etc.


I've seen a number of old coil plans in magazines which direct you to wind incredibly fine #40 wire upon a long, narrow 2" form. This is a poor design, and is one of those infectious misconceptions that spread from book to book. The wire has too high a resistance, and it leads to energy loss via wire heating. A much more powerful coil can be had by using thicker wire on a wider form. A 5:1 ratio (or less) of coil length to dia is recommended. If your coil is 3ft. tall, it should be about 1ft. in diameter. Tesla's original coils were shaped like barrels! My last coil was 2' tall, 5" diameter, wound with #24 magnet wire. It made 14" sparks from an 8" carved foam sphere terminal covered with epoxy and AL foil. Its transformer was fairly tiny, so 14" was short for the size of coil.

If you want a lower frequency of coil, don't use finer wire for more turns. Instead, make the secondary bigger in diameter. Some people have explored the use of heavier wire at the ground end of the coil and fine wire above. The energy wave near the base is composed of high current, low voltage, while the wave at the top is high voltage, low current, so an ideal secondary would have tapered wire, with thicker wire at the grounded end.


Nikola Tesla's large coils were not the same as the "Tesla Coils" shown in most project plans. His coils were composed of three coils, not two. The first two coils form a large diameter air-core close-coupled stepdown transformer. The transformer primary is tuned with a large capacitor. The transformer secondary is a few turns and heavy wire on the same large coil form as the primary. The third coil is a large "extra coil" in place of the usual teslacoil secondary. The "extra coil" was self-resonant and, rather than being driven by magnetic coupling, it was driven by having its bottom wire directly connected to the heavy secondary winding of the 2-coil transformer. In typical project articles, the "transformer" section is entirely missing, and the "extra coil" is driven inductively. Tesla's 3-coil system is known by the name TESLA MAGNIFIER, and a big one can put out arcs approaching the megawatt level. Check out the "plastic bucket" coil project in Radio Electronics magazine for an example of a magnifier coil.

Are you aware of the Tesla Coil Builders Association? The back issues of their newsletter are an incredible library of Tesla material, and are available from the publisher (see elsewhere on my WWW page)


Created and maintained by Bill Beaty. Mail me at: .