Which option best describes a device that is used to control circuits with an electrical signal, acting as a switch?

• A. Relays
• B. Fuses
• C. Microswitch
• D. Rotary selector

Answer: (A)

Relays are the devices that act as controlled switches powered by a small electrical signal. When the control coil is energized, a magnetic field moves the relay’s contacts, closing or opening the load circuit. This lets a low-power signal control a higher-power circuit while keeping the two sides electrically isolated, which is a central reason relays are used in automation and SMA systems.

Fuses aren’t switches—they protect circuits by breaking the current if it’s too high, not by switching in response to a control signal. A microswitch is a mechanical switch activated by physical movement, not inherently driven by an electrical control signal to switch a separate circuit. A rotary selector is a switch for choosing among circuits, typically operated manually or by an actuator, and isn’t defined by being controlled by a signal to switch loads.