Over voltage protector

Crowbar Over Voltage Protection Circuit Diagram

A crowbar over-voltage protection circuit is an electrical circuit that prevents a power supply unit from damaging due to over-voltage. Crowbar protection is a fail-safe protection mechanism that shorts circuits the output of a power supply under failure conditions such as overvoltage. Crowbar protection can also refer to a circuit that has its sole purpose to cause a fuse to blow by subjecting it to a high current.

Quite a few electronic components, particularly active ones, cannot withstand too high voltages. Preventing costly circuits dying a premature death, because the supply voltage has been raised too high, makes over-voltage protector no luxury. Such a protector must, of course, act swiftly; otherwise the deed has been done before it had a chance to act. Therefore (slow-acting) relays are not suitable for this purpose. In this article, the crowbar protection circuit is explained.

Crowbar Over Voltage Protection Circuit Diagram:

crowbar over voltage protection circuit

The circuit shown here, a so-called crowbar, contains several fast-acting components, it is intended to be connected between the mains and the appliance to be protected.


The circuit depends, as it were, on brute force: when the supply voltage raises too high, a thyristor short-circuits the output. This means that the too high voltage is immediately removed from the connected appliance and also that fuse F1 blows. Brute force, indeed!

Setting up of Crowbar Over Voltage Protection Circuit:

The voltage at which the crowbar comes into action is set between 5 V and 25 V with P1.

Set the P1 to its maximum value.

Replace the fuse temporarily by a wire bridge and connect the crowbar to a variable power supply. Set the current limit of that supply to 1A and output voltage to the value at which the crowbar is desired to act.

Turn P1 back slowly until the thyristor comes on, that is, when the current limiter of the power supply comes into action.

The crowbar is now set. Replace the wire bridge by the fuse (maximum rating to 5 A). In quiescent operation the crowbar circuit draws a current of only about 1 mA.


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