Which device is used to control a high-current circuit with a low-current control signal?

• A. Transformer
• B. Relay
• C. Circuit Breaker
• D. Contactor

Answer: (B)

Electrically operated switching where a small control current drives a larger load current is the idea here. A relay does exactly that: its coil is energized by a low-current control signal, which then moves internal contacts to open or close the high-current path. This provides isolation between the control side and the load while allowing a tiny signal (from a microcontroller, switch, or sensor) to control a much heavier circuit, such as a motor or heater.

Transformers aren’t switches; they change voltage or current levels but don’t sequence or interrupt power. A circuit breaker protects the circuit by interrupting current during faults, not by controlled operation from a low-current signal. A contactor is a larger, rugged version of a relay used for heavy loads like motors; it performs the same basic function, but the classic device described for a general low-current control signal is the relay.