K
kell
- Jan 1, 1970
- 0
Carrying your logic one step further, do we not come across a second
mode that might have destroyed the transistor:
Once the voltage produced by the inductive current collapse peaks, it
starts back down through zero on the negative swing of the ringing
wave form. At the point where the voltage at the collector drops 0.7
volts negative (referenced to its other end at B+), the collector-base
junction of the transistor becomes forward biased through the 10 Ohm
resistor. The collector-base junction becomes a 0.7 volt voltage clamp
for all the stored energy with only a 10 Ohm resistor for a load.
It would explain why this particular topology failed where the others
didn't.
I'd be interested to hear your response.
Rick
I think it's the initial spike that fried the base collector junction.
I put a cap across the coil as Jim suggested and the circuit operates
now, though I haven't put it through thousands of cycles. As someone
here mentioned already, I think the cap slowing down the dV/dt at the
collector gave it time to sweep the charge out of the base and turn off
quick enough so it would withstand the continued rise of the voltage.
Of course yes it rings, even more with the cap on it I guess. I
suppose it's possible the base-collector diode is getting a baker's
dozen millijoules pushed through it on the backswing. I hadn't thought
about that, but maybe it won't hurt. I don't have a scope. I'll just
have to see how things go out on the road!