A control relay is the name given to an electromagnetic switch that is controlled either remotely or locally that is commonly used in almost all types of equipment because it is able to switch higher currents than would otherwise be the case.

Logic circuits can automatically or manually control the control relays, which are able to electrically isolate output voltages from control voltages. Control relays are electromechanical devices that rely on an electromagnetic field force.

Driving an electrical current over the holding current energises the relay field and the relay contacts shift to the second of two possible positions. For example, when the relay coil is energised when a relay contact is open, it will close it and the same applies in the other direction.

The relay contact’s normal position is the relay’s default de-energised position, which is either ordinarily open or closed. It is common to use the open position for applications that are responsible for powering up a load, with the closed position normally being used for applications that are responsible for closing down the load. In this instance the energising of the relay coil will open the relay contacts.


Switches are often made use on in electrical controls. When load currents need larger contacts for the switches, it becomes both impractical and inconvenient to have to manually close the switch. For example, it is easy to switch a fraction of an ampere when the switches are simple, but when larger currents need to be switched the arching on the switch contacts can quickly cause damage to the switch, and this is even more the case with motors that are inductive loads. Relays are added to circuits in order to build high-power switches that have a greater amount of durability.

Relay switches

Relay switches can be found in a large number of home appliances including the likes of air conditioners, washing machines and refrigerators. A good switch that is able to withstand the continual switching of electrical currents that have significant transient energies is required by such appliances.

The use of electrical relays ensures that switching will not result in excessive power dissipation on switching devices, which could cause a dangerously high temperature because of transient plasma that is generated. Relays are constructed to be able to quickly change conduction state in order to avoid any excess plasma heating, which can also cause damage to relay contacts and result in contacts being welded together, as can electrical arcing.

The master control relay

Master control relays are used within safety devices, the purpose of which is to remove power from the load. For instance, heavy pressing and cutting devices may require a master control relay to prevent the device from injuring personnel. A master control device can shut equipment done safely when necessary.

There are also semiconductors that are the equivalent of control relays. These include the likes of silicon-controlled rectifiers for DC circuits and the three-terminal AC switch and SCR with a diode bridge for AC circuits.

Control relays play a vital role in the electronic devices of today.

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