> What I'm after is something that will when the voltage droops to a set
> point basically short the cap and de-energise the system. Physically
> small is a constraint. An easy to use IC would be nice.

The circuit and I are separated so -

Two NPN transistors.
Ecap fed by diode from supply.
Resistor R1 from ecap to base of Q2. say 10k.
(Arguably unneeded: Resistor R2 say 100k from Q2 base to ground.)

Resistor R3 from supply to Q2 base.
Resistor R4 from Q2 base to ground.
R3/R4 proportioned to turn off Q1 at some rail voltage below nominal.

Q1 collector to Q2 base.
Q2 collector to rail, maybe via a small resistor R5.

While power is nominal Q1 is on and Q2 is off.

When rail fall to Q1 turn off point Q2 crowbars supply.
Capacitor is szied to allow Q2 to stay on long enough to do its job.
R1 sized to provide enough drive to Q2.

It works.
I built something like this long ago when ISD2590 ICs had latchup problems. 
They needed a hard low rail to stop a latchup.
ISD actually asked about the circuit as they had a customer who had the same 
problem but hadn't solved it.

E&OE - I may have swapped a wire or 3 there - the idea should be evident 
enough.

2Q, 3 or 4 R, 1 D, 1 C.
.
Or, as otherwhere suggested, use a commercial watchdog IC.


                 R



    Russell 

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist