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Diodes used as fuses

I'm aware that in the days of overwind transformers for EHT in 'scopes very low current and high voltage rectifier diodes were used rather than cheaper higher current ones as they would fail open circuit to protect the fine windings on the mains transformer. Is anyone familiar with using diodes for more general applications. Particularly what generic type or named series of diode always fails open rather than closed circuit and rough idea of rupture current with respect to normal use maximum as of course never in the data sheets ?

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Reply to
N Cook
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Don't diodes tend to fail shorted?

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Reply to
Sam Goldwasser

Only in cases where shorting causes more damage than open. (ref: Murphy, first law)

--
Kind regards,
Gerard Bok
Reply to
Gerard Bok

Not unless they're blown to bits...in my experience a diode will usually short when it fails.

Tom

Reply to
Tom MacIntyre

I would think of a diode used that way as a fire hazard...

Reply to
Someone

rather

I can't think of one offhand that didn't, except in cases that it was reduced to a charred crater in the board.

Reply to
James Sweet

I seem to recall that the old selenium rectifiers tended to fail open (after making a really bad smell, as I recall). It's been quite a while since I saw one of those, though. These were the ones that looked kinda like a stack of square bits of sheet metal with a bolt through the middle.

Reply to
Ned Konz

Oh yuck, I had an experience with one of those when I was about 8 years old, flipped the switch and there was a buzz and clouds of colored (orange?) absolutely putrid smelling smoke, I still gag at the thought.

Reply to
James Sweet

I'm thinking of 2 to 6KV but only 1 to 10mA diodes. The older Selenium ones perhaps 3 inches long

1/8 inch diameter often paxolin? brown casing and about 60 V forward drop. Or the small Si ceramic beaded ones about 6V forward drop. Its just a few times coming across non functioning ones that were o/c but not coming across shorted ones - was it just luck that I never came across a shorted one or was it by design choice of diode type?
Reply to
N Cook

The entire purpose of fuses is to have something guaranteed to fail in a certain way. Diodes are not guaranteed to fail one way or the other and I can only tell you that it's not a great idea to use a diode as a fuse.

- NRen2k5

Reply to
NRen2k5

I think you're confused by the fact that early HV rectifiers had low current ratings because the technology was in its infancy at that time. Modern HV rectifiers have more robust ratings because the manufacturers understand the processes a lot better now, and the manufacturing process is much more refined. Diodes/rectifiers have never, in my experience, been used as fuses. A fuse is a device that has a predictable mode of failure in an overload condition; a diode does not. In fact, most diodes that I have seen fail almost always fail to a shorted condition. The PN junction fuses and becomes a conductor rather than a semiconductor.

--
Dave M
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the address)

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Reply to
DaveM

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