Single-phase full-wave AC voltage controller using two thyristors For a single-phase full-wave AC regulator that uses two antiparallel thyristors only, how should the two gate drive circuits be arranged with respect to electrical isolation?

Introduction / Context:
Antiparallel thyristor AC regulators (two SCRs connected in opposite directions across the load) are widely used for single-phase voltage control. Proper gate drive isolation is critical for safety, reliable triggering, and avoiding unintended current paths through the control circuit.

Given Data / Assumptions:

• Two SCRs connected antiparallel across a single-phase source and load.
• Independent gate triggering required on positive and negative half cycles.
• Typical control uses pulse transformers or opto-isolated drivers.

Concept / Approach:

Each SCR anode/cathode swaps roles every half cycle. The gate emitter reference for one SCR sits at a different instantaneous potential than the other. Without isolation, a shared low-voltage controller can experience large common-mode voltages or create fault paths, risking mis-triggering or damage.

Step-by-Step Solution:

Verification / Alternative check:

Industrial regulator schematics consistently show isolated gate drives to avoid circulating currents and to meet insulation requirements.

Why Other Options Are Wrong:

• May or may not be isolated / preferably isolated: understates safety and EMC needs.
• Should not be isolated: unsafe and prone to mis-triggering.

Common Pitfalls:

Sharing a ground between both gates and the control PCB; ignoring common-mode dv/dt; insufficient isolation ratings of optocouplers.

Final Answer:

must be isolated