This is an elaboration of an idea from Billings, combining a clamp
and SCR crowbar, in this case adapted to low-voltage clamping
(as low as 1.25V), with the additional feature of a shutdown of
the ac input when the SCR is triggered.  Al Sliski suggested the
latter feature, and Winfield Hill contributed lots of good ideas
to the design and implementation.  I've tested it with various
bench supplies, and with a robust 3.3V, 80A switcher, and it works
just wonderfully.  More details will follow, but here are some
photos of the beast itself, in the meantime.


Here you can see the PCB on the left, with npn clamp and SCR crowbar
mounted to a single heatsink.  The clamp/crowbar requires no power;
but for purposes of shutting down the ac input to the protected dc
supply there is an auxiliary supply whose PCB transformer can be seen.
The relay controls the ac input to the protected supply; it is a
latching type, good for 20A.  At right you can see the IEC power entry,
and the ac outlet for the protected supply.  This shot, and the next
one, were taken with a nifty Canon EOS 1Ds (11 Mpixel, full-frame
CMOS imager), using a 90mm "tilt-shift" lens to align the plane of
focus, view-camera style.

Another Photo

And from this view you can see the manual reset switch and the Dsub-9
connector for remote reset (via switch contact or logic level).

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