Silicon controlled rectifier

A silicon controlled rectifier is a 4 layer solid state device that controls current flow. In the normal 'off' state the device restricts current flow to the leakage current. When the Gate to source current exceeds a certain point however the device turns 'on' and conducts current. The device will remain in the 'on' state even after gate current is removed so long as current through the device remains above the 'holding current'. Once current falls below the holding current the device will switch off. If the applied voltage increases rapidly, it may induce enough leakage current to trigger the device into the on state, but this is harmful to the device. SCR's are used in industry to produce variable DC voltages for motors (typically 5-100 HP). They can also be used in some electric vehicles to step the voltage down in a Jacobson circuit. Another common application is light dimmer switches used in domestic homes.