Diodes used as fuses

[N Cook]

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 ?

 

[Sam Goldwasser]

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 ?

Don't diodes tend to fail shorted?

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[Gerard Bok]

Don't diodes tend to fail shorted?

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

 

[Tom MacIntyre]

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

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

Tom

 

[Someone]

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

 

[James Sweet]

Don't diodes tend to fail shorted?

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

 

[Ned Konz]

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

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.

 

[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.

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.

 

[N Cook]

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.

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?

 

[NRen2k5]

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 ?

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

 

[DaveM]

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 ?

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
MasonDG44 at comcast dot net (Just subsitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!