Question

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A) : Thyristors can be turned off by gate control. Reason (R) : Thyristors require current to fall below holding current to turn off. In the light of the above statements, choose the most appropriate answer from the options given below :

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A)
• C. (A) is correct but (R) is not correct
• D. (A) is not correct but (R) is correct

Hint:

SCR characteristics: \[ \text{Gate controls TURN ON only} \] Turn OFF occurs when: \[ I_A < I_H \]

Answer 1

The Correct Option is D

Concept: A thyristor (SCR) is a semiconductor switching device. Important characteristics:
• it can be turned ON using gate pulse,
• it cannot normally be turned OFF using gate signal,
• turn OFF occurs only when current falls below holding current.

Step 1: Analyzing Assertion (A). Assertion states: \[ \text{“Thyristors can be turned off by gate control.”} \] This statement is incorrect for a conventional SCR. Reason:
• gate terminal only initiates conduction,
• once SCR conducts, gate loses control,
• device continues conducting even after removing gate pulse. Thus Assertion (A) is false.

Step 2: Understanding turn OFF condition of SCR. An SCR turns OFF only when: \[ I_A < I_H \] where: \[ I_H = \text{holding current} \] Hence current through device must reduce below holding current.

Step 3: Analyzing Reason (R). Reason says: \[ \text{“Thyristors require current to fall below holding current to turn off.”} \] This is absolutely correct. This is the fundamental turn-off principle of SCR.

Step 4: Conclusion from both statements. Therefore:
• Assertion (A) is incorrect,
• Reason (R) is correct.

Step 5: Selecting the correct answer. Hence correct option is: \[ \boxed{(4)} \]