Control relays are electromagnetic switches used to switch high currents and isolate output voltages. They rely on an electromagnetic field and have a normal position that can be open or closed. Relays are used in appliances to prevent excessive power dissipation and damage to switches. Master control relays are used in safety devices, and there are semiconductor equivalents for DC and AC circuits.
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 can 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 which relies on the strength of an electromagnetic field. When the relay coil is energized by driving an electric current above the holding current, the relay contacts move to the second of two positions. For example, 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 normally open or closed. Using the normally open position is common for applications that turn on a load. A normally closed position is usually used for applications that stop the load. In this case, energizing the relay coil opens the relay contacts.
Switches are commonly used in electrical controls. When load currents require larger switch contacts, manually closing the switch becomes cumbersome and impractical. For example, switching a fraction of an ampere is fine with simple switches. When larger currents are to be switched, arcing across the switch contacts will damage the switch too soon, especially with motors that 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 ensure that switching does not cause excessive power dissipation on the switching device, which can lead to dangerously high temperatures due to the so-called transient plasma generated. The relays are designed to rapidly change the conduction state to avoid excessive heating of the plasma. Plasma heating or an electric arc will also damage the relay contacts and can weld the contacts together.
The master control relay is used in safety devices that will ultimately remove power from the load. Heavy cutting and pressing devices, for example, may require a master control relay to prevent the device from causing injury to personnel. These master controllers will safely shut down the equipment.
There are semiconductor equivalents of the control relay. For direct current (DC) circuits, these include silicon controlled rectifiers (SCRs). In alternating current (AC) circuits, these include the diode bridge SCR and the three-terminal AC switch.
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