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From: Kevin McMurtrie
Subject: Re: My transistor blew up...
User-Agent: MT-NewsWatcher/3.2 (PPC Mac OS X)
Date: Tue, 26 Nov 2002 21:42:45 GMT
NNTP-Posting-Date: Tue, 26 Nov 2002 13:42:45 PST
In article ,
> I had a flyback transformer driven by a single transistor circuit, which
>was working fine, but then out of nowhere my transistor blew up. I was using
>an MJE13009, NPN power transistor rated for 400V at 12A. It was heatsinked
>and all, wasn't even getting hot. So I thought to myself... if I took two
>transistors that were exactly the same and wired them up in parallel, would
>that split the current to both transistors to switch separately and equally?
>BTW the circuit I was using came from here:
>(http://www.angelfire.com/80s/sixmhz/flyback.html) Sorry for my ignorance, I
>don't know much about electronics in general Thanks for all your help!!
Driving a transformer with feedback isn't a great idea if you're going
to make sparks with it. The sparks will cause the feedback to misfire
and blow the whole thing out.
It would be better to use a fixed pulse width from a 555 timer chip.
Keep a 5 to 10 Ohm power resistor in series with the coil while you look
for the proper frequency range and duty cycle. Now you can make sparks
without blowing the whole thing out.
Overvoltage protection will help too. Once the coil is charged the
power must go somehwere. With no load, the power dissipates itself
inside the circuit and causes wear. Protection can be a simple zener or
surge protecting diode rated for high power. You can get fancy by
building a feedback loop to control the duty cycle. Remember that the
voltage is highest while the transistor is already off. You would need
to filter the feedback with capacitor so that it's still there to
control the next 'on' pulse.
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