The circuit turn-off time of an SCR (Silicon Controlled Rectifier), often denoted as \(t_q\), is a crucial parameter in understanding how quickly an SCR can be turned off after it has been conducting current. Unlike turning off a simple switch, an SCR requires special conditions to switch from the conducting state to the blocking state, especially when used in AC circuits where the current naturally goes to zero or in DC circuits where forced commutation is applied.

Defining Circuit Turn-Off Time of an SCR

Once an SCR is turned on and conducting current above the holding current, it remains in the ON state even if the gate signal is removed. To turn off a conducting SCR, the anode current must be reduced below the holding current for a sufficient amount of time, and importantly, a reverse voltage must be applied across the SCR for a specific duration to allow the stored charge carriers (electrons and holes) within the device junctions to recombine.

The circuit turn-off time (\(t_q\)) is defined as the time interval during which the SCR is subjected to a reverse voltage by the external commutation circuit. This reverse biasing is essential for the SCR to regain its forward blocking capability. If a forward voltage is reapplied across the SCR before the stored charge is removed (i.e., before \(t_q\) has elapsed), the SCR may turn on again even without a gate signal. The commutation circuit provides the necessary conditions, including the reverse voltage period, to facilitate the turn-off process.

Let's look at the given options:

• Option 1: taken by the SCR turn on and them turn off. This describes the entire cycle of operation, not just the turn-off time definition.
• Option 2: required for the SCR current to become constant. When an SCR is conducting, the current is typically flowing, and becoming constant is not related to the process of turning it off or the definition of turn-off time.
• Option 3: for which the SCR is reverse biased by the commutation circuit. This aligns perfectly with the definition of circuit turn-off time (\(t_q\)). The commutation circuit forces the reverse bias across the SCR for a period allowing it to turn off.
• Option 4: for which the SCR is reverse biased to reduce its voltage below the holding current. The holding current is related to turning off by reducing forward current to zero. While reverse bias is involved in forced commutation turn-off, the definition of the *time* is the duration of the reverse bias applied by the circuit, not tied to a specific voltage level relative to holding current during reverse bias. The key aspect is the duration of reverse bias provided by the commutation circuit.

Therefore, the circuit turn-off time of an SCR is defined by the period the commutation circuit applies reverse bias across the SCR terminals.