NEIEP Mechanics Practice Exam

Once triggered, a silicon controlled rectifier will stay on until when?

1. Power is removed

2. Current flow is interrupted

3. The gate is disconnected

4. Temperature falls below a threshold

The correct answer is: Current flow is interrupted

A silicon controlled rectifier (SCR) is a type of semiconductor device that acts as a switch. Once it has been triggered into the 'on' state by a gate signal, it will continue to conduct current as long as there is sufficient forward current flowing through the device. If this current is interrupted (for example, by an external circuit breaking the path for current flow), the SCR will turn off. This means that the switching capability of an SCR is fundamentally linked to the flow of current; if the current goes to zero, the SCR will cease to conduct. Power removal as a reason to turn off the SCR implies shutting down the entire circuit, but the SCR’s behavior is more closely related to current flow than power per se. Disconnecting the gate signal does not turn off the SCR; it only initiates the turn-on phase. Likewise, while temperature can affect the operation of an SCR, it is not the primary reason for it to turn off from a triggered state. Thus, the requirement for current flow to be interrupted for the SCR to shut off accurately reflects the device's operational characteristics.