Crowbar (circuit)

Crowbar circuits are frequently implemented using a thyristor, TRIAC, 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.

The name is derived from having the same effect as throwing a crowbar over exposed power supply terminals to short the output.

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, or the malfunction of a CMOS circuit -- the PMOS half of a pair lingering in a near on-state when only its corresponding NMOS is supposed to be on (or the NMOS when the PMOS is supposed to be on) -- resulting in a near short-circuit current between supply rails.