What is the definition of a Silicon Controlled Rectifier (SCR)?

• A. A diode that blocks current until reverse-biased
• B. A transistor switch in linear mode
• C. A controlled rectifier that conducts when gate is triggered
• D. A device that stores energy magnetically

Answer: (C)

The main idea here is recognizing what an SCR is and how it behaves. An SCR is a controlled rectifier, a four-layer PNPN device (a thyristor) that remains off until its gate receives a trigger. Once triggered, it conducts current in the forward direction and stays on (latches) until the current drops below a certain holding level, at which point it turns off. This gate-triggered on-state is what makes it a “controlled rectifier,” meaning you can control when it conducts with a small gate current.

This differs from a simple diode, which requires forward bias to conduct and has no gate to trigger a turn-on; it cannot be switched on by a separate control signal. It also isn’t a transistor switch in linear mode, since an SCR operates as a latching switch controlled by a gate rather than a standard transistor amplifier or switch. And it isn’t a device that stores energy magnetically—that would describe inductors or transformers.

So the correct description is a controlled rectifier that conducts when gate is triggered.