TRIAC Crowbar For AC Or DC Lines

A TRIAC Crowbar circuit which works for both AC and DC voltage is shown in the schematic diagram below. This circuit provide overvoltage protection for electronic or electrical equipment against excessive supply voltage.  Crowbar circuit provide two level of protection. First, this protection circuit will drop the voltage across the protected equipment to near zero by shorting the voltage, and second, the circuit will blow the fuse since it short the current. Here is the schematic diagram of the circuit:

Using the values for R2, R3 and R1 as shown in the schematic diagram, this circuit can operate within range of 42 to 84 VAC or 60 to 120VDC. R2 set operating point where the crowbar will be triggered. To be safe, the TRIAC voltage rating should be greater than that triggering point, actually it should handle the highest possible voltage exposure. To accommodate testing, when we need to check if the crowbar action is triggered without blowing the fuse, a testing switch  is provided (normally closed push  button). An alarm unit can be added  in parallel with the lamp for audible indication in testing crowbar action. [Circuit’s schematic diagram source:]