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Jinx wrote...

>A couple of days ago I had a 7805 regulator fail and stick
>14V up a circuit, taking out an F877 and an LCD. To quote
>my old French boss " I em 'ow you say ? urn'appy "
>
>What can I use as a shunt circuit to stop this 'appening
>again ? I've seen various semis used (SCR, FET, BJT)
>with dividers, zeners etc but know that this list always
>comes up trumps. I've tried the rest, now I want to try the
>best
>
>Generally I'd want this to protect a circuit that needs 5V @
>100mA or less, which is probably not enough to blow a
>common fuse, but should be sinkable by a modest semi.
>Failure indication by LED would be nice

Since you've got plenty of voltage up front of your regulator,
you might consider a series current limiter instead of an
overvoltage crowbar circuit.  The attached example is of a
"precision current limiter" circuit from National Semiconductor's
LM117 datasheet.

The advantage of a current limiter, in my view, is that it
protects against a wider variety of mishaps than a crowbar, which
can only protect against overvoltage.

I concur with Olin; the type of failure you experienced isn't
very common.  Far more common is when some fumblebumpkin (like
me) accidently shorts something out with a screwdriver or probe.

DD


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