Power Supply Overvoltage Protection

A circuit on a power supply designed to protect the connected equipment from receiving too much voltage if the supply should malfunction.

The typical power supply a control voltage is fed into a series regulating device such as a transistor known as a voltage regulator. Typically the input voltage to this may be well in excess of the output voltage. If the series regulator transistor in the power supply fails in a mode where it becomes a short circuit, the pre-regulated smoothed voltage level will be applied to the circuit being powered.

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silicon controlled rectifier

An SCR overvoltage crowbar circuit is an over voltage crowbar protection circuit using a silicon controlled rectifier or SCR. The SCR over voltage crowbar or protection circuit is connected between the output of the power supply and ground. The Zener diode voltage is chosen to be slightly above that of the output rail. Typically a 5 volt rail may run with a 6.2 volt Zener diode. When the Zener diode voltage is reached, current will flow through the Zener and trigger the silicon controlled rectifier or thyristor. This will then provide a short circuit to ground, thereby protecting the circuitry that is being supplied form any damage and also blowing the fuse that will then remove the voltage from the series regulator.