The most common controller power supply is which of the following?

• A. 110 VAC to 9 VAC step-down transformer.
• B. 480 VAC to 24 VAC step-down transformer.
• C. 240 VAC to 12 VAC step-down transformer.
• D. 120 VAC to 24 VAC step-down transformer.

Answer: (D)

Controllers in building automation are typically powered by a low-voltage supply derived from the standard line voltage. A 120 VAC supply is stepped down to 24 VAC to feed the control circuitry. 24 VAC is a long-standing standard for HVAC controls because it provides enough voltage to reliably drive relays and sensors while remaining safe to wire and easy to source components for over long distances inside equipment and buildings. This arrangement balances safety, compatibility, and cost.

Other options use voltages less common for general control gear or would require heavier, more expensive equipment, so they aren’t as typical for everyday controller power.