At 03:42 PM 7/17/2011, Tom wrote:
That sounds like a crowbar circuit. If these are
linear power supplies, it's more likely to have a
crowbar, but I've seen some switchmode power
supply units with it. There's a simple circuit
somewhere that senses a failure that's in some
way dangerous to the equipment operated by that
power supply, maybe just a zener, resistor, and
maybe a capacitor too, to sense overvoltage.
Trips the crowbar by firing an SCR to short the
output so it will shut down abruptly. Called a
crowbar because it's like dropping a very large
conductor, like a crowbar, across the output terminals.
I doubt a crowbar circuit is involved. A crowbar tends
to be used to blow a fuse or trip a mechanical circuit
breaker, not to trip an electronic overcurrent protection.
A single fault could both result in an overvoltage and a
failure to respond to an overcurrent condition. Activating
a crowbar circuit and relying on the overcurrent protection to
cut the power in that situation could result in a big mess.
Since over voltage can fry a lot of expensive
kit, they use a crowbar to pop a fuse or breaker,
rather than risk more damage.
Yes. However, there is no fuse blowing or breaker
being tripped in this case, just the power supply
shutting down.
The crowbar is possibly adjustable, but I'd start
by checking the specs on output voltage and make
sure it's adjusted correctly on all output busses.
I'm not sure what all the voltages are supposed to
be but the main ones appear to be correct. I've also
tried a substitute power supply with identical results.
It would be very unlikely that the two power supplies
are both producing overvoltage conditions intermittently.
Regards,
Peter Coghlan.