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 "

Ouch!

> 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

Hi Jinx, try a series resistor before the
7805, drop 5v on the resistor at 100mA, giving
14-5 = 7v for the regulator and dropout.

Try 56 ohms, 1/2 watt as it will get hot over 50mA
or so. This alone gives pretty good protection,
but adding a 1W 5.1v zener across the PIC is the
finishing touch.

The series resistor goes a long way by taking the
heat burden off the 7805, and you probably won't
blow another one. :o)
-Roman

PS. I like to split the cap before/after the
series resistor too, gives much cleaner power to
the 7805.

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