What is required to turn on an SCR?

To turn on a Silicon Controlled Rectifier (SCR), it is essential to have both a forward bias across its main terminals (anode and cathode) and a gate trigger. The forward bias ensures that the SCR is in a conducting state, allowing current to flow through it, while the gate trigger provides the necessary control signal to initiate this conduction.

The gate trigger helps to inject a small controlling current into the gate terminal of the SCR, which reduces the required threshold for the device to conduct. Once the SCR is triggered, it continues to conduct until the current through it falls below a certain level, known as the holding current.

In the case of the other options, reverse bias would prevent the SCR from conducting, while negative voltage would also prevent it from turning on since SCRs are designed to operate with a positive forward bias. Input voltage alone does not ensure that the SCR will turn on without the additional input from the gate. Therefore, the combination of a gate trigger and forward bias is crucial for the operation of an SCR.