Relays are simply switches that open and close the circuit electromechanically or electronically.
It consists of an electromagnet and a set of contacts.
The switching takes place by means of electromagnet.
WHAT IS THE DIFFERENCE BETWEEN RELAY AND SWITCH?
A Switch is just ON or OFF type device. But Relays are used to allow large current on other circuit when small current activates it in one circuit.
Example: To ignite lights the relay uses switch at one end to activate the magnetic field by small current and to allow large current at other end by short circuit the load.
HOW RELAY WORKS?
The power source is given to the electromagnet through the control switch and to the load through contacts.
When the source is switched ON the current starts flowing and the electromagnet energizes.
Due to the magnetic field from the electromagnet the upper arm of the contact attracted towards the lower arm, thus makes short circuit (closes) to the load.
When the electromagnet de-energizes the contacts moves in the opposite direction to make a open circuit.
4-PIN RELAY WORKING:
The green line indicates control circuit and the red line indicates load circuit.
The switch in the load circuit is controlled by the coil in the control circuit.
ENERGIZED RELAY (ON)
When the current flows through the coil (between pin1 and 3) the magnetic field induces around the coil, this makes the switch to close.
Thus, when relay is energized the current flow through pin2 and 4.
DE-ENERGIZED RELAY (OFF)
When the current flows through the coil stops then the magnetic field reduces and also the switch open.
Thus, when relay is de-energized the current flow through pin2 and 4 also stops.
CONTACTS IN RELAY:
NORMALLY OPEN CONTACT (NO): It is called as Make Contact. The circuit connects when the relay is activated.
NORMALLY CLOSED CONTACT (NC): It is called as Break Contact. The circuit disconnects when the relay is activated.
CHANGED OVER (CO) / DOUBLE THROW (DT) CONTACTS: It is called as Make Before Break and Break Before Make. It is used to control NO and NC with a common terminal.