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 (for example ) 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 instead of a thyristor a transistor, gate turn off (GTO) thyristor or forced commutated thyristor to short the circuit. Active crowbars are commonly used to protect the frequency converter in the rotor circuit of the 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.