A 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 or low resistance path 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 or a trisil or thyratron 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 tripping the 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.
An active crowbar is a crowbar that can remove the short circuit when the transient is over thus allowing the device to resume normal operation. Active crowbars use a transistor, gate turn off (GTO) thyristor or forced commutated thyristor instead of a thyristor to short the circuit. Active crowbars are commonly used to protect the frequency converter in the rotor circuit of doubly fed generators against high voltage and current transients caused by the voltage dips in the power network. Thus the generator can ride through the fault and quickly continue the operation even during the voltage dip.
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 term is also used as a verb to describe the act of short-circuiting the output of a power supply.
High voltage crowbars are used for HV tube (Klystron and IOT) protection.
ON Semiconductor Manufacturer of thyristors
Paul Horowitz and Winfield Hill, The Art of Electronics, Second Edition, pages 318-319, Cambridge University Press, 1989.