Relay

Relays are remote control electrical switches that are controlled by another switch. Relays allow a small current flow circuit to control a higher current circuit.several designs of relays are in use today, 3-pin, 4-pin, 5-pin,and 6-pin, single switches.

All relays operate using the same basic principle. Relays have a tow circuits,a control circuit and a load circuit.The control circuit has a small control coil while the load circuit has a switch. the coil controls the operation of the switch.

when relays is ON current flowing through the control circuit coil creates a small magnetic field which causes the switch to close the switch. which is part of the load circuit that may connect to it. Current now flows through switching pins.

Simply there is a three type of relays

Normally-open (NO) 

contacts connect the circuit when the relay is activated; the circuit is disconnected when the relay is inactive.

Normally-closed (NC) 

contacts disconnect the circuit when the relay is activated; the circuit is connected when the relay is inactive.

Change-over (CO), or double-throw (DT) 

contacts control two circuits: one normally-open contact and one normally-closed contact with a common terminal.