Single-phase half-wave controlled rectifier with R–L load and a freewheeling diode (FWD) When the freewheeling diode is conducting in this circuit, what is the bias state of the SCR (thyristor)?

Introduction / Context:
In controlled rectifiers with inductive (R–L) loads, a freewheeling diode (FWD) is often added across the load to provide a path for the inductor current when the source voltage reverses or the thyristor (SCR) turns off. Understanding which device conducts at different parts of the input cycle is essential for analyzing current paths, device stresses, and average output.

Given Data / Assumptions:

• Single-phase half-wave controlled rectifier using one SCR feeding an R–L load.
• A freewheeling diode is placed across the load with correct polarity.
• Supply is sinusoidal; the load inductance causes current to persist beyond source zero crossings.

Concept / Approach:
When the AC source goes negative (or whenever the source voltage is less than the load’s induced back EMF), the FWD provides a low-impedance path for the decaying inductor current. With the FWD clamping the load voltage close to zero (approximately the diode drop), the supply appears across the SCR in reverse, thereby reverse-biasing it and preventing conduction through the SCR.

Step-by-Step Solution:
Assume SCR had previously been triggered and load current is flowing.As the source voltage falls and crosses zero, the inductor attempts to maintain current.The FWD becomes forward biased and takes over the current path across the load.With FWD conducting, the load terminals are clamped near diode drop; the line (anode) is negative with respect to the cathode of the SCR during negative half-cycle.Hence, the SCR anode is at lower potential than its cathode: the SCR is reverse biased.

Verification / Alternative check:
Using polarities: during negative input, source anode at a lower potential than cathode; FWD conducts and provides local loop for the inductor, so the SCR sees reverse voltage and cannot conduct until next appropriate positive half-cycle and gate trigger.

Why Other Options Are Wrong:
Forward biased (A): Not when FWD is conducting; the anode is negative relative to cathode.
May be forward or reverse (C): In this standard topology it is reverse biased during FWD conduction.
Forward then reverse (D): The described condition is specifically the FWD conduction interval, during which the SCR remains reverse biased.

Common Pitfalls:
Confusing the moment of SCR turn-off with FWD conduction. Once FWD is on, the SCR is not carrying current and is reverse biased until the next positive cycle and proper gating.

Final Answer:
is reverse biased