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.