NEIEP Mechanics Practice Exam

Once an SCR is gated, how long does it continue to conduct?

1. Until the load is disconnected

2. Until the AC sine wave passes through zero

3. Indefinitely until it is gated off

4. For a fixed time interval

The correct answer is: Until the AC sine wave passes through zero

When an SCR (Silicon Controlled Rectifier) is gated, it enters a state of conduction, allowing current to flow through it. This conduction state will persist as long as the current through the SCR remains above a certain threshold called the holding current. In an AC circuit, the current alternates in direction and periodically returns to zero. Once the AC sine wave passes through zero, the current drops below this holding current level, causing the SCR to turn off. This is a fundamental characteristic of SCRs in AC applications, where they can only turn off when the current goes to zero. In contrast, in a DC application, the SCR would continue to conduct until the load is disconnected or the circuit is opened, demonstrating that the context of the power supply (AC vs. DC) is crucial in how the SCR operates. For AC applications, the correct understanding is that the SCR stops conducting when the sine wave crosses zero.