After firing a SCR, if the gate pulse is removed, then the current in the SCR

Explanation:

Thyristor or SCR (Silicon Controlled Rectifier) Behavior After Gate Pulse Removal

Definition: A thyristor, commonly known as an SCR (Silicon Controlled Rectifier), is a four-layered semiconductor device used in power electronics to control high-power applications. It operates as a bistable device, meaning it can switch between ON and OFF states based on the triggering of its gate terminal and the conditions of its anode current.

Working Principle: The SCR remains in its OFF state (blocking mode) until a gate pulse is applied. Once a sufficient gate pulse is provided to trigger the SCR, it enters the ON state (conducting mode). In the conducting mode, the SCR continues to conduct current even after the gate pulse is removed, as long as the anode current remains above a certain threshold, known as the latching current. If the anode current drops below a critical level called the holding current, the SCR turns OFF.

Correct Option Analysis:

The correct option is:

Option 1: Drops to zero

When the gate pulse is removed after firing the SCR, the current through the SCR does not immediately drop to zero. The SCR remains in the conducting state as long as the current flowing through it is above the holding current. However, if the anode current drops below the holding current level (e.g., due to circuit conditions such as a reduction in the load current or the AC supply reaching its zero-crossing point), the SCR will turn OFF, and its current will drop to zero. Hence, the option "Drops to zero" is correct but must be understood in the context of the current falling below the holding current.

Important Concepts Supporting This Behavior:

• Holding Current: This is the minimum current that must flow through the SCR to keep it in the ON state. If the current falls below this value, the SCR will turn OFF.
• Latching Current: This is the minimum current required to latch the SCR into the ON state immediately after it is triggered by the gate pulse. Once the SCR is latched, the gate pulse is no longer needed to sustain conduction.
• Gate Pulse Role: The gate pulse is only required to trigger the SCR into conduction. After firing, the gate signal has no effect on the SCR's conduction state.
• AC Circuit Operation: In AC circuits, the SCR naturally turns OFF at the zero-crossing point of the AC waveform because the current falls to zero, which is below the holding current.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 2: Increases

This option is incorrect. After the gate pulse is removed, the SCR's current does not increase. The current through the SCR is determined by the external circuit parameters, such as the load and supply voltage. The gate pulse only serves to trigger the SCR into conduction, and once the SCR is ON, the gate pulse has no impact on the current. An increase in current would require a change in the circuit's external conditions, not the removal of the gate pulse.

Option 3: Decreases

This option is partially correct but not entirely accurate in all scenarios. While the current through the SCR may decrease due to external circuit conditions (e.g., reduced load or the supply reaching a lower voltage), the mere removal of the gate pulse does not cause the current to decrease. The SCR remains in conduction as long as the anode current stays above the holding current. If the anode current falls below the holding current, the SCR turns OFF, and its current drops to zero rather than just decreasing.

Option 4: Remains the same

This option is incorrect in the long term. While the current through the SCR may remain the same immediately after the gate pulse is removed, it is not guaranteed to stay constant indefinitely. The current in the SCR depends on the external circuit conditions. In an AC circuit, for example, the SCR's current will naturally fall to zero during the AC waveform's zero-crossing point, causing the SCR to turn OFF.

Conclusion:

The behavior of an SCR after the gate pulse is removed is primarily governed by the anode current and its relationship to the holding current. Once the gate pulse has triggered the SCR into conduction, the gate signal has no further effect. The SCR will remain in the ON state as long as the anode current remains above the holding current. If the current drops below the holding current (e.g., due to external circuit conditions), the SCR turns OFF, and its current drops to zero. Therefore, the correct answer is "Drops to zero."