I have tried to figure out what exactly a Tesla Coil was for years. I finally occurred to me that the best way to think of a Tesla Coil was as a very high power rf transmitter simply with a terrible antenna. The secondary is really just an ultra shortened 1/4 wave antenna – so short it can barely radiate RF at all. Ultimately the energy boils off the top of this antenna as the discharge us mad scientists know so well.
To prove my conception of a Tesla Coil: I built a class E amplifier that is powered from a simple half wave rectifier and 120VAC straight from the wall. A class E amplifier is simply a power mosfet switched on and off by a square wave drive (highly non linear all on or all off). The trick is to load the output with a slightly out of tune resonant circuit with respect to the frequency of the pulsing. By doing this, the voltage and current waveforms become 90 degrees out of phase. Since power is current times voltage, if they do not overlap (or very little) as in the case of a class E amplifier; no power is dissipated across the semiconductor switch and efficiency can approach 90+ percent. The draw back of such an amplifier is that it is narrow band and must be critically tuned.
A Tesla Coil is ideal as a load for such an amplifier. If you check out the schematic below you will see it is shockingly simple (pun intended). The schematic is incomplete in that I do not show the driver circuit. The driver is nothing special – just a mosfet driver IC (there are a million of them out there) and a couple of 555 timers to create a modulated (30Hz 20% duty cycle) 300KHz pulse source. I can also just use my bench sine/square generator. The reason I modulate it at 30 Hz it is twofold, one, to give the crackly long streamers and two, if I run in continuous mode the resonant caps will explode from over heating, also the secondary overheats and starts melting, catching on fire ( both of these events happened to me).
While the design is simple, tuning is challenging – if the current and voltage are ever in phase… forget it. Get ready to blow some FETS!