crowbarred

A crowbarred power supply or circuit contains a crowbar circuit: a protection device that shorts the output to ground the instant voltage exceeds a safe threshold. The name is apt. When a regulator fails or an inductive load causes a voltage spike, the crowbar fires (typically via a thyristor or silicon controlled rectifier), clamping the supply to near zero volts. It is a destructive, sacrificial act, meant to kill the circuit before the voltage spike kills what it feeds.

Crowbar circuits are most common in industrial power supplies feeding sensitive loads: programmable logic controllers, analog data acquisition systems, or distributed motor controllers. They work by monitoring the output voltage with a comparator. When that voltage drifts above a preset trip point, usually 5 to 10 percent above nominal, the comparator triggers a gate drive that fires the crowbar element (typically a thyristor rated for the fault current). The crowbar then holds until a fuse or series current-limiting resistor melts, or until an upstream breaker opens.

Failure modes and limits

Crowbarred designs assume the crowbar will fire once per decades of service, if at all. The thyristor itself must survive the surge current during its first milliseconds of conduction; inadequate surge ratings lead to gate-oxide rupture or junction melt. The fuse or series current limiter must blow before the thyristor junction temperature exceeds absolute maximum, typically 150 to 175 degrees Celsius. If the crowbar triggers repeatedly, it signals a deeper fault in regulation or the load circuit itself; repeated nuisance trips are a sign to investigate the root cause, not to raise the threshold.

One pitfall is parasitic inductance in the crowbar circuit path. If the trace or wire from output to the thyristor cathode has significant inductance, the voltage spike can propagate past the thyristor before it can conduct, reducing the protective margin. Good crowbar design routes the thyristor as close as possible to the output capacitors and uses wide, short PCB traces or bus bars. Another common mistake is setting the trip threshold too close to the normal load regulation band, which causes the crowbar to fire on transients that a slower, softer clamp circuit could handle without nuisance shutdown.

Crowbar protection is crude but reliable for its cost and simplicity. It offers no tuning or soft response; it is an on-off safety valve, not a linear regulator. For that reason, it sits alongside other hard-limit protection schemes like zener diodes and transient-suppression diodes, and is often used alongside them. Modern switched-mode supplies often pair a crowbar with active voltage clamp circuits that engage before the crowbar fires, reducing fuse wear and collateral damage when a transient occurs.