Relay

Relays are generally type of switches used to operate electrical ON and OFF connections electronically or electro-​mechanically. Relays are used to control a circuit by establishing and closing contacts with other circuits. When relay is not powered, it is in normally open (NO) condition, and when there is close contact it is normally closed (NC) again as it is not energized. After applying the electrical current, the switches will change their state.

Relays are mainly used to make flow of smaller currents in control circuits. They cannot control high-​power consuming devices except small motors and solenoids that control low amperes. By using amplifying concept, relays can also control large amperes and voltages. Protection feature also exists in relays including over-​currents, overloads, reverse currents and undercurrents to prevent equipment damage. Relays are also used to switch the starting coils, heating elements and audible alarms (Figure 9.8).

In Figure 9.9, relay schematic diagram is represented. As seen from the diagram, the contacts at the top are NC (normally closed). When flow of current occurs through this contact, magnetic field is created which makes the switch closed. Mainly a spring is used to pull the switch to open position when power is removed from the energized coil.

There are different configurations of relays like single pole single throw (SPST) and single pole double throw (SPDT). SPST has two contacts, but SPDT has three contacts.

These contacts are usually labeled as COM (common), NO (normally open) and NC (normally closed). When no power is applied, normally closed contact should be connected to common terminal, and similarly normally open contact should remain open when there is no power supply given to coil.

In another way, when coil is powered up, common terminal is connected to normally open and normally closed terminal is left open. Double and single pole versions are almost same, except for two switches that open and close together.