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Subject: Re: My transistor blew up...
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Tue, 26 Nov 2002 11:16:46 GMT
NNTP-Posting-Date: Tue, 26 Nov 2002 12:16:46 MET
> Hello everyone,
> 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!!
That guy of the webpage doesn't have any idea how this circuit functions,
impressing the list of various transistors he "fried" already.
It is more of a miracle this circuit works at all. There are 3 basic design
flaws, each leading to either immediate or gradual destruction of the
first and worst:
you just need to miss one single base current pulse and the primary current
will be only limited by the lead resistance to the battery. If there is a
short on the high voltage side is enough.
The transformer saturates...
just put a 0.47R/10W resistor in line with a 2A fastblow fuse into the +
line to the 12V battery. If you have a higher battery voltage, the resistor
should be (Ubat - 9V)/5A
2. The collector voltage of the transistor rises until a current flows on
the high voltage side.
Across the primary coil has to be one or two zener diodes 150V/5W in series
with a normal diode (fast 400V/2A). All rings have to point away from the
collector side. The zener voltage should be below the max. repetetive
Another fast diode across the emitter and collector with the ring to the
3. also the base voltage can become too negative, this probably has damaged
a 1n4148 from gnd to base. Ring to base.
A 0.1uF from the 12V coil-in to gnd.
Better would be a current steered design with a comparator instead of that
extra winding on the transformer.
electronic hardware designer
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