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Re: [TCML] Frequencies
Paul Brodie wrote:
Rhys,
That is a very old and very well tested ciruit that works fine. The
diode won't hurt but is not necessary at all. I have used this circuit
and modifications of it quite a bit. I would like to pass on to you that
the 3055 transistors are way over rated and will not stand this for very
long before they "blow" and let all of their smoke out. They claim to be
rated at 15 amps but I have yet to see one take much more than 5 amps
for long, even with a massive heat sink.
Funny, I've got 3055s in power supply duty doing 10A day in and day out.
I suspect you're killing them with overvoltage. I've killed many a
3055 in PWM switcher duty, before I got smart and chose a different
transistor. The 3055 has a BVce of 60V, and, more to the point a pretty
low reverse voltage limit on the base emitter junction.
The other thing to watch out for on the 3055 is that the hFE (large
signal current gain) is really, really low (like 10 or less).. so you
need a LOT of base drive, or it drops out of saturation, and the
dissipation goes up, or worse, with an inductive load, the decrease in
current causes a increase in Vce, killing it with overvoltage.
As you note, it needs a decent heatsink, and probably forced air
cooling, if you'rre going to get close to the 150W rating.
By the way, be sure to use a
large heat sink for the 3055 transistor to have it last more than a few
seconds.Also, the circuit works much better with a separate power supply
across the ignition coil from the one powering the 555 IC. This way, you
are able to use much higher voltages across the ignition coil without
the 15 volt limitation of the 555.
You will be much better served by a simple modification and use a MosFet
instead of the 3055. There are many MosFet's available with much higher
voltage and amp ratings than the 3055 and they are just as inexpensive.
Indeed.. Or a IGBT or a decent darlington. There are also power
transistors MADE for driving ignition coils that have a 400V max Vceo,
and a good breakdown characteristic (almost like they have a 400V zener
built in). I can't recall the number off hand.. If you hunt down the
data sheet for the old motorola electronic ignition IC, there's a wealth
of useful information.
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