In an SCR, which electrode is used to control conduction?

• A. Anode
• B. Cathode
• C. Gate
• D. Anode and Gate

Answer: (C)

The gate is the control port. An SCR (silicon controlled rectifier) has a four-layer PNPN structure, and it conducts from the anode to the cathode only after a trigger current is applied to the gate. That small gate current kick-starts the regenerative action inside the device, turning it on so the main current can flow from anode to cathode. Once turned on, the device latches on and stays conducting as long as the current stays above the holding value, even if the gate current is removed. The anode and cathode handle the main current path, not the control trigger, so they don’t serve to control conduction by themselves. The option that combines anode and gate isn’t necessary because the gate alone provides the control signal to initiate conduction.