Half-wave controlled rectifier with continuous firing in positive half-cycle Assertion (A): In the shown circuit, if the SCR is continuously gated during the positive half-cycle, it will conduct for the full 180° of that half-cycle. Reason (R): A thyristor can conduct only when it is forward biased.

Introduction / Context:
Assertion–Reason questions test both conceptual correctness and causal linkage. In a half-wave controlled rectifier, continuous gating during the positive half-cycle can make the SCR conduct from near the start of that half-cycle until the natural current zero. The reason statement addresses a necessary condition for conduction (forward bias) but does not fully explain the sufficiency for full 180° conduction.

Given Data / Assumptions:

• Half-wave AC source, resistive or RL load with current naturally reaching zero at ωt = π.
• SCR is forward biased only during the positive half-cycle.
• Gate drive is applied continuously in that interval.

Concept / Approach:

With continuous gate drive, the SCR triggers as soon as forward bias appears (near ωt ≈ 0+), then remains on until the current naturally falls to zero at the end of the positive half-cycle. Thus, conduction spans approximately 180°. The reason statement is true—forward bias is required—but forward bias alone does not guarantee conduction without gate triggering (unless breakover occurs). Hence, R does not wholly explain why conduction lasts the entire half-cycle under continuous gating.

Step-by-Step Solution:

Verification / Alternative check:

Classic half-wave control waveforms show full half-cycle conduction when firing angle α → 0°, achievable with continuous gating.

Why Other Options Are Wrong:

Claiming R explains A overlooks the role of gate triggering; stating A is wrong contradicts standard operation; saying R is wrong denies the forward-bias prerequisite.

Common Pitfalls:

Assuming forward bias alone turns on an SCR; forgetting that without a gate pulse, the device may not conduct unless the voltage rises to breakover.

Final Answer:

Both A and R are correct but R is not correct explanation of A