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godfrey 1st January 2012 01:47 PM

Interesting BJT failure mode...
It was a low voltage, high current PNP transistor used as the output device in the dimmer circuit for my son's car's dashboard lights. Testing with a multimeter suggests the base is now shorted to the collector and the emitter is totally open circuit.

Some previous owner had screwed up the wiring and connected the output directly to ground, so cause of death will have been overcurrent or overheating. There's no external signs of damage though (like a burned/exploded case).

Is this in any way normal? Seems weird to me!

Elvee 1st January 2012 02:56 PM


Originally Posted by godfrey (

Is this in any way normal? Seems weird to me!

Hi Godfrey,

It looks normal to me: first, the junction was melted into a short by the overload, and then the emitter bonding wire acted as a fuse, leaving just B and C shorted

godfrey 1st January 2012 09:55 PM

Thanks, that makes sense.:)

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