A control relay is a locally or remotely controlled electromagnetic switch that is widely used in all forms of equipment due to its ability to switch higher currents than would otherwise be impossible without it. Control relays, which may be controlled manually or automatically by logic circuits, also electrically isolate the output voltages from the control voltages. The control relay is an electromechanical device that relies on the force of an electromagnetic field. When the relay coil is energized by driving an electrical current above the holding current, the relay contacts move to the second of two positions. For instance, when the relay contact is open, energizing the relay coil will close the contact and vice versa.
The normal position of a relay contact is the default de-energized position of the relay. It is either normally open or closed. Using the normally open position is common for applications that power up a load. A normally closed position is usually used for applications that shut down the load. In this case, energizing the relay coil opens the relay contacts.
Switches are commonly used in electrical controls. When the load currents require bigger contacts for the switch, manually closing the switch becomes inconvenient and impractical. For instance, switching a fraction of an ampere is fine with simple switches. When bigger currents should be switched, the arching on the switch contacts will damage the switch too soon, more so with motors which are inductive loads. A relay is added to the circuit to build durable high-power switches.
Relay switches are used in many home appliances, such as refrigerators, air conditioners, and washing machines. These appliances need a good switch that can survive frequent switching of electrical currents with significant transient energies. Electrical relays assure that the switching will not cause excessive power dissipation on the switching device, which may cause dangerous high temperature due to the so-called transient plasma generated. Relays are designed to rapidly change conduction state to avoid excessive plasma heating. Plasma heating or electrical arcing also damages the relay contacts and may weld contacts together.
The master control relay is used in safety devices that will ultimately remove power from the load. Cutting and heavy pressing devices, for example, may need a master control relay to stop the device from causing injury to personnel. These master control devices will safely shut down the equipment.
There are semiconductor equivalents of the control relay. For direct current (DC) circuits, these include silicon-controlled rectifiers (SCR). In alternating current (AC) circuits, these include the SCR with diode bridge and the three-terminal AC switch.