Tesla Coil

Tesla Coil Classic

Tesla coils produce dramatic high-frequency, high-voltage electric sparks. They utilize the principles of transformers, ac driven circuits, and RLC transients.

A Tesla coil circuit uses two transformers. The first consists of a step-up iron core transformer at 120 V and 60 Hz, which drives the secondary to 25 KV and 60 Hz. See Figure:

Tesla coil. The 60 Hz power source on the left drives an LC circuit that, when the voltage is high enough, causes sparking across the spark gap. This drives the primary on the right into oscillation at its high resonance frequency (on the order of a MHz). The Tesla coil, coupled weakly to this oscillation, draws off energy, at a high voltage and at the frequency of the resonance.

This charges up a low C capacitor and a spark gap (which, when it is not sparking, may be thought of as a capacitor), in series with an inductorL. Including the resistance in the wires, this is an RLC circuit. Because 60 Hz is a relatively low frequency, the voltage associated with the inductor L is not very high, so the voltages across the capacitor and the spark gap are both nearly 25 KV. A glass sheath keeps the capacitor from sparking when subjected to such high voltage, but nothing prevents the spark gap from sparking. When that occurs, charge rushes across the spark gap, thus taking this RLC circuit out of equilibrium. It then undergoes transients, as discussed, which consist of a damped RLC resonance at a frequency on the order of a MHz. In contrast to what happens for the low-frequency 60Hz voltage from the power transformer, for the high-frequency voltage associated with the LC circuit, the inductor couples to the step-up air transformer. This raises the high-frequency voltage of the latter to on the order of a MV (See Figure). The many-turn secondary coil of this second transformer, which can be thought of as a parasite off the main RLC circuit, is what is meant by the Tesla coil.

Resonant Transformer Construction

Here you can see how I build my resonant transformer.

Resonant Transformer for Cold Electricity Experiments

Here is a very detailed explanation of my home build resonant transformer. I hope this helps fellow experimenters to build their own.

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