or crowbar circuit
is an electrical circuit
used to prevent an overvoltage condition of a power supply
unit from damaging the circuits attached to the power supply. It operates by putting a short circuit
across the voltage
source, much as if one dropped a tool of the same name
across the output terminals of the power supply. Crowbar circuits are frequently implemented using a thyristor
(also called an SCR
) or a trisil
as the shorting device. Once triggered, they depend on the current
-limiting circuitry of the power supply or, if that fails, the blowing of the line fuse
or circuit breaker
A crowbar circuit
is distinct from a clamp
in that, once triggered, it pulls the voltage below the trigger level, usually close to ground
. A clamp prevents the voltage from exceeding a preset level. Thus, a crowbar will not automatically return to normal operation when the overvoltage condition is removed; power must be removed entirely to stop its conduction.
The advantage of a crowbar over a clamp is that the low holding voltage of the crowbar lets it carry higher fault current without dissipating much power (which could otherwise cause overheating). Also, a crowbar is more likely than a clamp to deactivate a device (by blowing a fuse or tripping a breaker), bringing attention to the faulty equipment.
The power supply is bad. I would not try to fix it.