Question about transistor failure modes

You should have tried to wash the board and parts first. if you ask me, it sounds like you had a substance that may have collected on the surface over the years. you would be surprised and how high freq things work after washing down a board with parts on.

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Somewhat relevant.... Just yesterday I was just going through the achieves and read posts on how reverse biasing a BE transistor junction is damaging. I'm working on a design where, upon unit power down, a BE junction gets reversed.. So the more often the unit is turned off the more supposed transistor damage. D from BC

There was a tale told when I was youngish - around 1978 - about fixing HP counters by stubbing out a lighted cigarette on the metal can of one of the transistors.

Apparently the base-emitter junction of the affected part was regularly reverse biased into avalanche breakdown. The avalanche current was small, and the avalanche was brief, so the transistor wasn't destroyed, but the forward current gain was progressively degraded.

The cigarette was supposed to get the transistor junction hot enough to anneal out the damage.

Measuring forward current gain in-circuit could be a bit difficult.

-- Bill Sloman, Nijmegen

Yes ive had a few designs suffer from unforseen reverse vbe. some can be rather sensitive, especialy RF sections that burst into catastrophic oscillation.

IME 8/10 transistor failures are short circuit, shortly folowed by open circuit if the current is not limited, or transistor ejecting itself off the board.

Colin =^.^=

Be junctions have zener effects. commonly around the 6 volt region. the Vbe in reverse that is.

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"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
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I should add... The achieved discussions I read was about the damage in the BE reverse breakdown (zener ..also called avalanche IIRC) region. I'm guessing the transistor is ok with a BE junction reversed biased before that point. Any more and zener current starts kicking in and I suppose heat is the killer. IIRC somebody posted that the damage is accumulative and leads to eventual transistor failure.

I might have an example of this:

Figure 24. Change the R's for say 20V output. Put a cap on the output..Charge up the cct.. Then ground out Vin to imitate other loads present when the power is turned off. Doesn't it look like the transistor BE junction gets a reverse hit? If so, this could be a design flaw that can sneak through product testing.. Might be part of mysterious transistor failure.. D from BC

ooops spellcheck goof..."archived discussions" not achieved. D from BC

'Atlas'

(except

It's a pretty common input PSU circuit and a pretty common solution is to put a diode between be, reverse connected of course.

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Thanks,
Fred.