Solid State coil driver theory

Inverters normally supply power to loads that are a combination of resistance and inductance, (e.g. Electric motors, transformers, etc.) This combination of inductance and resistance gives rise to a load current which has a triangular wave shape. Most inverters produce some sort of square-wave output voltage that is applied across the load. The inductance of the load 'integrates' the applied voltage and the current rises and falls during the high and low portions of the output voltage waveform. See diagram opposite.

Classic power electronics theory is based around this idea of applying a square voltage waveform to an inductive load resulting in a triangular current flowing from the inverter through the load. (Note that the load current is at its maximum when the voltage changes polarity, so the active components in the inverter have to switch a significant current.)

In contrast, the inverter used to drive a Tesla coil is supplying power to a load that contains resistive, inductive and capacitive components. (The Tesla coil resonator.)

This combination of R, L and C results in a resonant condition which responds most favourably to a particular frequency, (its natural resonant frequency.) If this load is driven with a square voltage waveform at its resonant frequency, it will result in a sinusoidal current flow.

The square voltage waveform contains a fundamental frequency and all of the odd harmonics of this frequency, but the resonant load only 'sees' the fundamental frequency. The load current contains only the fundamental frequency from the square wave that was applied, so it is sinusoidal in shape.

(Note that the sinusoidal current passes through zero at the same time as the voltage changes polarity, so the inverter does not have to switch an appreciable current in this case.)