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Re: Mosfets survive
Greetings All,
I make progress. The electronics are now all housed in the old PC case,
the existing PSU gives me 5v and 12v supplies. I'll have to build a new box
underneath to hold my 20 war suplus 500uf 350v caps. I've pressed other
caps into service for the moment. The electronics are now being driven
by a CA1524 PWM chip supplying 170kHz to 240kHz on a 5% to 95%
duty cycle. Still some standard TTL in there to detect over current.
2 IRF740's at the business end, only slighty warm at 75v, 1.5A.
I need to used a grounded electrode though to pull a 1.5" spark out of the
toroid. Next step is to up the voltage to 150v, and add 2 or 4 more FETs.
I am driving directly a traditional primary coil, 9 turns of two core mains
flex using both wires in series, on a bucket 12" high, 12" diameter.
The secondary is 1600 turns on a 4" PVC pipe, 24swg (I need the turns
to keep the frequency under 300khz for the electronics). The bucket
was simply to hand when I grew frustrated with transformers and bottom
feeding. Pulsed DC means that transformers are not an option with this
circuit - they would just saturate.
Am I correct in saying that my coils should be closely coupled. And that
I should construct a primary which has a reduced width, more hieght
(I dont anticipate sparks bigger than 6" to 10"), but increase the number
of turns to keep the induction about the same. (too low an induction
would give fast rising currents overwhelming the current proctection).
Snubbers still seem too hot, I'm using 2 5W, 3.3K resistors in // plus a
1000pf 2kV cap (in series) . time constant about 1uS, I figured that
worst case if the full voltage was across them all the time then at 75v,
P =V*V/R about 2W each. But they still seem very hot, and wouldn't
cope at 150V. I'm planning to reduced the capacitor there to 200pF
leaving a little spike protection at the FETs and put a big snubber
across the coil, say 600ohm, 50w plus 10,000pf. - does this seem
reasonable?
I've just recieved Duane Bylands book, "modern tesla coil theory".
good material, though the print quality is poor. He includes the
suggestion of building 6 identical fet driven coils in a hexagonal
arrangement. Driving each one 60 degrees out of phase with
respect to its neigbours, giving a field which rotates at the coil
frequency, has anyone tried this. It should create a very attractive
display.
have fun
Alan Sharp (UK)
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