In 30 years I have never had a 78xx go full throttle. It's pretty rare
unless maybe subject to a backfeed voltage to the output. An SCR, zener,
and resistor should suffice on the output. Zener cathode to the output,
anode to resistor to ground and the gate of the SCR to the junction of
the Z and R. Cathode of SCR obviously to ground and anode to the output.
To protect the supply, maybe a 1/4 watt 3R3 or less in series with the
source and kept a distance from any components or circuit board. It will
burn up before anything else does and will let you know it.
Rick

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
>
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