To properly turn off an SCR once it is conducting, which action should be taken?

To turn off a Silicon Controlled Rectifier (SCR) that is conducting, it is essential to reverse bias the anode with respect to the cathode. This action effectively interrupts the flow of current through the SCR by creating a condition where the voltage on the anode is lower than that on the cathode. When reverse bias is applied, it prevents the SCR from conducting, thus turning it off.

This method takes advantage of the SCR's operating principles. When an SCR is forward-biased and conducting, it requires either the removal of the gate signal or achieving a zero crossing of the current to turn off. However, simply removing the gate signal doesn't stop the current flow; thus, reverse biasing is often necessary under certain conditions to ensure that the device is completely turned off.

The other potential actions, while relevant to SCR operation, do not effectively guarantee that the SCR will be turned off. For example, using a high-speed contactor in series with the anode may interrupt the circuit but is not a direct method for controlling the SCR itself. Additionally, reversing bias at the gate does not affect the conduction state of the SCR once it is latched on; it primarily influences gate-triggering rather than turning off a conducting SCR.