Weird transistor failures

Thanks Spehro, yes maybe you are right. Any easy way to protect them ? Tom

Thanks Tony, the LED's are at the end of quite a long run (about 12") with the transistor, resistor, caps and PIC all within about an inch of each other. Tom

The easiest (but not cleanest) way is to put a Zener diode across the power input. Put one of the appropriate voltage and it would drop anything above the given voltage. However, there is a risk of "burning" the power supply.

A cleaner way would be to use a voltage regulator, I guess.

Voltage suppresor diodes?

By the way, voltage can be as high as 14.4V in the car, possibly a bit more. Not that I can see how this could destroy your transistors.

A dodgy stock was suggested by someone, that's possible if you bought them from surplus.

The only similar effect I know is from the RF field where a power bipolar slowly builds a schottky diode across BE junction slowly deteriorating performance.

FWIW - I have seen this behaviour in cheap transistors used in bulk (e.g. C945, general purpose npn) - in any application it was necessary to provide a good ground potential to the base to prevent it from leaking. (it is amazing to see how little current is required to make a LED glow visibly) Sure the pic emits a solid "0V" in off position? Resistor directly from base to ground (~3K3) added to series resistor is std issue with these part ever since in my book.

I read in sci.electronics.design that Tom R wrote (in ) about 'Weird transistor failures', on Fri, 8 Apr 2005:

Lecher lines! Betcha they are oscillating!

Thanks for that John. My only question is how is oscillation on the collector (I presume) erradicated by a cap across the base/emitter ? Tom

I read in sci.electronics.design that Tom R wrote (in ) about 'Weird transistor failures', on Fri, 8 Apr 2005:

You don't get oscillation on one electrode; the whole device is involved. The point here is that you potentially have a VHF tuned circuit in the collector, formed by the long wires, and that, with the internal capacitances of the transistor, can make a quite efficient oscillator.

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Do you at any time TRIS the port(s) to high impedance in the code? How many ports are involved, are other bits of the ports inputs at any time, possibly causing an inadvertent unintended tristate on the active port bit. Check the code carefully, it may be doing things you don't intend especially if you write to the port in the wrong bank. When a transistor shows the mentioned leakage, what is the output voltage on the respective port bit? Is the leaky transistor actually bad? It's also possible that the PIC under certain conditions is driving the transistors with short unintended pulses and not really off. Again check the code. Interestingly the PIC's can putout 25mA so you may not even need the transistor(s). You might consider FETs with a

You should probably regulate the LED voltage to 8 volts or so and use 20Volt transorbs on the 12 Volt rail as has been mentioned. It's hard to believe that transistors may be oscillating as mentioned if they saturate properly. I guess it could happen but they would never oscillate if you wanted them to in that kind of a circuit.

Another possiblity is ESD damaging the transistors. They could have been damaged long ago and failures just showing now. How about heat? What is the maximum temp this circuit is subjected to? Just some ideas but I'm sure you've thought of them. Bob

Are you a University lecturer by any chance ? ;-)

Graham

If at all possible, track down a TransZorb to use instead of a Zener. A transzorb is really just a zener, but it's designed to take huge transient hits and dissipate them safely. And they're fast - I don't know how "ordinary" zeners compare for speed, but you won't get one that's as good as a transzorb for transients - that's where the name came from. :-)

And, of course, as others have mentioned, some ferret[sic] beads. ;-)

Cheers! Rich

Thanks for this Rich, will give them a try. Tom

Thanks for your time John, looks like I will belt and braces with a cap and transient supression. Tom

Thanks for your time Spehro, will try your suggestions and report back. Tom

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Thanks for your time Bob, I'm happy there are no firmware probs. The PIC shows, as near as dammit, 0v when I have investigated the problems and the tr's are u/s. PIC's have never failed and am currently considering a rare FET alternative (pin out issues - ain't many DGS or SGD fets about !) . Temperature is not a problem. My frustration is being unable to catch them failing. I am considering subjecting new tr's to some big spikes and 'iffy' base voltages to see if they damage in the same way i.e. partial failure rather than terminal. Tom