Royer oscillator

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Examples

Some circuits that use the Royer oscillator.

Flyback driver

The capacitor should be a polypropylene type - other types can get somewhat hot. It will actually oscillate without it, but you get some pretty nasty spikes which are likely to kill the transistors - you should have at least 10 nF. The input inductor is essential - some experimenting with values may be needed, but something around 330 µH with a suitable current rating is a good start.

When oscillating correctly, the waveforms on the transistor collectors should look like a half-wave rectified sinewave - the transistors are switched either hard on, or off so they don't get particularly hot. An advantage of a self-resonant supply is that when the output gets abused, the drive level tends to reduce, so it is reasonably robust.

The main limiting factor (apart from the Flyback transformer insulation...) is the voltage across the transistors, which reaches about 60 V for a 24 V input. I cranked this up to about 38 V before it melted, by which time my EHT meter was well off the 30 kV end-of scale, and it was pulling about 4 A.

There is plenty of scope for tweaking, which I will leave to others with more time on their hands..

I found that the waveform starts getting a bit spiky once the negative excursions of the base drive voltage hit about -10 V. Adding a 100 Ω resistor in series with the base winding, and a 1N4004 + 6.8 V, 5 W zener series combination from base to emitter seemed to improve this quite a lot (Emitter - [A 1N4001 K] - [K Zener A] - Base}.

A fun effect is to increase the capacitor to 1-2 µF to pull the resonant frequency into the audio range, then make a little jacob's ladder on the output - the pitch changes as the arc rises, giving a 'twittering' effect.