Which device is used as an electrically operated switch to open or close circuits?

The device used as an electrically operated switch to open or close circuits is a relay. A relay functions by using an electromagnetic coil to control the connectivity of contacts within the switch. When electrical current passes through the coil, it generates a magnetic field that moves an armature and subsequently opens or closes the contacts, allowing or interrupting the flow of electricity in a circuit.

This functionality makes relays ideal for applications where it is necessary to control a high-power circuit with a low-power signal, providing isolation and protection for sensitive components. Relays are commonly used in various applications, including automation systems, motor control, and circuit protection, due to their capability to handle different voltage and current levels effectively.

In contrast, the other devices listed have different primary functions: a fuse protects circuits by breaking the connection in case of overload or short-circuit, a voltage regulator maintains a consistent output voltage despite variations in input voltage or load conditions, and a transformer is primarily used to increase or decrease voltage levels in AC circuits without changing frequency.