A relay is a remote switch controlled by current, magnetism, or temperature. The relay was invented in 1835 by Joseph Henry (1979-1878), an American scientist. Relays are a common component in many devices, and there are many different types of relays.

Relay Types

The two main types of relay are electromechanical and solid-state. Electromechanical relays have a moving plate with contacts on it, while solid-state relays work similar to transistors and have no moving parts.

Power relays use an electromagnet to move a set of contacts. The contacts are pulled towards the electromagnet while current is flowing it. The contacts are connected to a spring that pulls them back when the current stops flowing.

Latching relays acts like a two-pole switch in that the contacts stay in position when when the current stops flowing. This is achieved by using a solenoid to move a ratchet and cam, or by using an electromagnet on either side.

Reed relays have only a set of ferro-metallic contacts inside a glass tube. The contacts close when an external magnetic field is applied, and open again when it is removed. Reed relays are commonly used as door sensors for alarm systems.

Overload relays are similar to reed relays but use a bimetallic strip to move the contacts. High temperature causes the strip to bend, bringing the contacts together.

Relay Applications

Relays are used to control the flow of large currents using a small current. In the past, they were widely used in telephone exchanges to switch calls, but have been largely replaced by digital equipment. They are used to switch motors on and off, and to protect them from overheating. Thermostats use an overload relay to shut off the current when the temperature rises above the required setting.

Solenoids and Relays

A solenoid is a special type of relay that has a moving core, while the electromagnet core in a relay is fixed. Solenoids are mostly used as mechanical actuators but can also be used for switching large currents, while relays are only used to switch currents.