For positive protection of electrical or electronic equipment, use this against excessive supply voltage. Because of incorrect switching, electrical wiring, short circuits, or failure of regulators, an electronic crow-bar circuit can quickly place a short circuit through the power lines, thereby decreasing the voltage across the protected device to close to zero and blowing a fuse. The triac and SBS both are bilateral components, the circuit will be equally useful on alternating current or direct current supply lines.
Using the values shown for R1, R2, and R3, the crow-bar operating stage can be altered over the range of 60 to 120 volts dc or 42 to 84 volts ac. The resistor values may be changed to cover a different range of supply voltages. The voltage rating of the triac should be greater than the highest operating-point as set by R2. I1 is a low power incandescent bulb with a voltage rating similar to the supply voltage. It may be used to check the set point and operation of the unit by opening the test switch and adjusting the input or set point to fire the SBS.
An alarm unit such as the Mallory Sonalert may be connected across the fuse to provide an audible indication of crowbar action. (This circuit may not act on short, infrequent power line transients).