Single-phase half-wave converter into a resistive load Just before triggering (gating) the thyristor in a half-wave converter supplying a pure R-load, the voltage across the thyristor is:

Introduction / Context:
In a single-phase half-wave controlled rectifier with a resistive load, the thyristor is reverse-biased or forward-blocked until a firing pulse is applied at angle α. The device then turns on and conducts for the remainder of that half-cycle.

Given Data / Assumptions:

• Purely resistive load (no energy storage).
• Sinusoidal input voltage.
• Thyristor is ideal and off before triggering.

Concept / Approach:
When the SCR is off, it behaves as an open switch. The full instantaneous source voltage therefore appears across the device. At the firing instant, the device turns on and the anode-to-cathode voltage collapses to a small conduction drop while current flows.

Step-by-Step Solution:
Before gate pulse at angle α: SCR open → v_SCR ≈ v_source (instantaneous).At firing: SCR conducts → v_SCR drops to a small on-state value.Since load is resistive, current is in phase with applied segment.

Verification / Alternative check:
Textbook waveforms for half-wave controlled rectifiers show device voltage tracking the supply until firing, then clamping to low on-state voltage during conduction.

Why Other Options Are Wrong:

• Zero or ~1 V: only true after turn-on.
• Twice input: not applicable here.
• Average output: unrelated to instantaneous device blocking voltage.

Common Pitfalls:

• Confusing average DC values with instantaneous AC waveforms.
• Forgetting that an open device sees nearly the full source voltage.

Final Answer:
Equal to the instantaneous input (supply) voltage