Why is a control transformer used in low-voltage control circuits?

• A. To provide a stable high-current, low-voltage supply with no isolation.
• B. To provide a safe, isolated lower voltage supply for control devices and protect control circuitry from line voltage.
• C. To convert DC to AC for control circuits.
• D. To increase the line voltage for control circuits.

Answer: (B)

Control transformers provide a safe, isolated lower-voltage supply for control devices and protect control circuitry from line voltage. They take the higher line voltage and step it down to a voltage suitable for relays, contactors, pushbuttons, timers, and PLC inputs, typically around 24 V AC. The key benefit is galvanic isolation between the power side and the control side, so faults, surges, or shocks on the line side don’t directly reach the control circuitry. This isolation reduces shock risk and helps prevent line-side disturbances from affecting control devices, while still delivering a stable voltage source sized to the total control load (in VA). They aren’t used to provide high-current with no isolation, nor to convert DC to AC, nor to raise line voltage, which is why this option is the best fit.