An SCR latches on when gate is triggered and turns off when what occurs?

• A. Gate signal is removed
• B. Voltage is removed
• C. Temperature rises above limit
• D. Current falls below the holding current

Answer: (D)

The essential idea is that an SCR latches once it’s triggered by the gate, because its internal structure provides positive feedback that keeps the current flowing even after the gate pulse is gone. It will stay in the conducting state as long as the current through it remains above a certain value called the holding current. To turn it off, you must reduce the current through the SCR below that holding current. Gate removal doesn’t reliably turn it off because the device will continue to conduct until the current drops enough. Simply removing voltage isn’t guaranteed to accomplish this unless the circuit reduces the current below the holding level. Temperature limits are a failure mode, not the normal off mechanism. So the off condition is achieved when the current falls below the holding current.