Crowbar (circuit)
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 output (Vo), much as if one were to drop 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 VREF 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 zener and cathode resistor. 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 VREF and the zener will begin to regulate voltage, drawing more current through it. The voltage at the gate terminal will be pulled down to VZ (the zener voltage), exceeding the gate trigger voltage of the TRIAC and latching it on.
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.
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