A crowbar circuit is an electrical circuit used for preventing an overvoltage or surge 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 output (V_o), like dropping a crowbar across the output terminals of the power supply. 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.

An example crowbar circuit is shown to the right. This particular circuit uses an LM431 adjustable zener regulator to control the gate of the TRIAC. The resistor divider of R₁ and R₂ provide the reference voltage for the LM431. The divider is set so that during normal operating conditions, the voltage across R₂ is slightly lower than V_REF of the LM431. Since this voltage is below the minimum reference voltage of the LM431, it remains off and very little current is conducted through the LM431. If the cathode resistor is sized accordingly, very little voltage will be dropped across it and the TRIAC gate terminal will be essentially at the same potential as MT1, keeping the TRIAC off. If the supply voltage increases, the voltage across R₂ will exceed V_REF and the LM431 cathode will begin to draw current. The voltage at the gate terminal will be pulled down, exceeding the gate trigger voltage of the TRIAC and latching it on.