Solid state tesla coils - general notes

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6. Pulse generators

Thoughts on PWM chips

A large number of SSTC circuits seem to use some expensive PWM IC (pulse-width modulator IC) like L494, LM3524, or UC3846, whereas a driver running at a precise 50% duty cycle would be optimal for SSTC use. PWM ICs usually add a minimum dead time to the output pulses, so the duty cycle is less than 45% - of course this will work too, but it will slightly increase transistor switching losses.
As another note, the pulse width modulation function is not often used in SSTCs, except as a very versatile low-freq <1kHz interrupter signal generator ("staccato controller"). The second most common use is to modulate the streamers with audio input and generate a "plasma loudspeaker". As PWM is quite harsh on the transistor switches (hard switching), a better way of modulating is to run at a constant 50% duty cycle and vary the driver frequency, e.g. wobbling or detuning according to the audio input voltage. This smoothly modulates the powerdraw, directly and instantly, instead of chopping a "constant-high-power" input to average-out to a lower power draw.

Simple, basic pulse generators

A simple NE555 timer or preferrably the faster cmos versions like TLC555 work quite well to replace these expensive PWM ICs.

Another alternative is the inexpensive and well available CD4046. It is a simple digital phase locked loop IC, and has a voltage controlled oscillator (VCO) on board. The oscillator generates a precision 50% duty cycle signal at all frequencies. The frequency can be adjusted with the VCO input voltage.

Even a ring buffer oscillator or a simple schmitt-trigger oscillator will work, as long as the frequency is adjustable with a potentiometer or by other means (t.ex. a microcontroller and external DAC).

Microcontroller output clock generators like the one in AT89C52 or some PICs and AVRs are not so good. They do give a constant 50% duty cycle, but the frequency is only adjustable in crude steps, and the step size grows linearly when moving from low to high frequencies. Usually
outFreq ~= clockFreq / (65536-counterValue)

A SSTC can also be run without any fixed pulse/frequency generator. Just take a feedback signal from the secondary coil (base current probe, antenna). Add some weak noise to the feedback signal. Overamplify (ultrafast analog comparator etc) the result such that it clips into a square wave - the drive signal for the SSTC. The TC secondary sort of acts as its own drive frequency generator, and the system is always instantly in-tune.

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