Common supply

[Barry L Klein]

Another approach you may want to consider is one I mentioned before -
using TVSD's, which are transient protection diodes, much like big zeners.
 They just solder across the supply in question and will clamp
overvoltages beyond the part's spec'd clamp voltage.  They are very fast
and probably would do everything the crowbar circuit would, maybe faster. 
The crowbar detects an overvoltage, fires an SCR to short out the supply
to ground.  If you don't have protection fuses the supply may fry as a
result, depending on its design.  The TVSD would clamp overvoltages to a
fixed voltage level.  Momentary spikes get clamped and your circuit
continues functioning - no fuse gets blown and all is well.  But if your
supply dies and results in overvoltage, the TVSD will clamp and hopefully
the fuse opens.  If you don't have a fuse the TVSD will probably get hot
enough to fry your pcb or melt its solder connections, fall off, and then
all your remaining parts would fry too.  We use them on drive designs here
at WDC and it has resulted in a much reduced failure rate of the various
IC's.  

You can get TVSD's from Digikey and Mouser I think.  A 5V part will clamp
slightly over 7 volts, a 12V part at 17 volts or so.  Much cheaper - no
heatsink required for an SCR for example.

These are not MOV's and don't degrade with time, BTW.

Barry


>
The idea here is that if some fault causes the power supply unit to 
put out too much voltage (say a resistor failure somwhere) then a SCR
produces a short circuit across the output, hopefully triggering
another current limiting ckt to shut down the PSU.. it doesn't have to
be destructive.
When the SCR triggers, it looks like a crowbar, resistance wise.