Which device is used to switch a large circuit under the control of a small electrical signal?

Which device is used to switch a large circuit under the control of a small electrical signal?

• A. Transformer
• B. Circuit breaker
• C. Relay
• D. Fuse

Answer: (C)

The key idea is using a small control signal to operate a switch that handles a much larger current. A relay does exactly this: a low-power electrical input (the coil) creates a magnetic field that pulls a set of contacts to open or close the high-current circuit. This lets a microcontroller or control circuit switch big loads—motors, heaters, or power supplies—without having to pass the large current through the control side. It also provides electrical isolation between the control circuitry and the load, which is a big safety and design advantage. The other devices don’t fit this role. A transformer changes voltage and current levels but doesn’t switch circuits on or off. A circuit breaker protects the circuit and interrupts current when something goes wrong, not for regular control by a small signal. A fuse similarly opens the circuit when overcurrent occurs and isn’t something you actively switch with a control signal.

The key idea is using a small control signal to operate a switch that handles a much larger current. A relay does exactly this: a low-power electrical input (the coil) creates a magnetic field that pulls a set of contacts to open or close the high-current circuit. This lets a microcontroller or control circuit switch big loads—motors, heaters, or power supplies—without having to pass the large current through the control side. It also provides electrical isolation between the control circuitry and the load, which is a big safety and design advantage.

The other devices don’t fit this role. A transformer changes voltage and current levels but doesn’t switch circuits on or off. A circuit breaker protects the circuit and interrupts current when something goes wrong, not for regular control by a small signal. A fuse similarly opens the circuit when overcurrent occurs and isn’t something you actively switch with a control signal.