1. Relay operation
Water level Indicator
Relays are a remote control electrical switch that can be switched using low current to control a high current load.

2. RELAY A relay is an electrically operated switch.
Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts.
The coil current can be on or off so relay have two switch positions and most have double throw (changeover) switch contacts.

3. A relay is an electrically operated switch
A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts.
The coil current can be on or off so relays have two switch positions and they are double throw (changeover) switches.

6. Electromagnetic Induction

7. SYMBOL
The relay’s switch connections are usually labeled COM, NC and NO:
COM =Common, always connect to this, it is the moving part of a switch.
NC= Normally Closed,COM is connected to this when the relay coil is off.
NO= Normally open,COM is connected to this when the relay coil is on.

8. Relay operation based on Load
Load side. (red)
Control side. (green)

9. Relay operation (off) Relay operation (on) Drawn in natural state.
Where the voltage present when load side is open.
Relay operation (on)
When there is current through the coil of wire on the control side a magnetic field is produced.
This magnetic field closed the contacts on the load side.

10. Normally open Normally closed
In this relay’s natural state it is in the normally open state.
When the coil is non-energized the contacts are open.
Normally closed
In this relay’s natural state it is in the normally closed state.
When the coil is energized the contacts closed.

11. Five pin Relay Four pin Relay
Most common contains normally open and normally open contacts.
Four pin Relay
4(Four) pin relay.
Contain either normally open or normally closed.

12. Typical 5 Pin Relay Pin Identification

13. Types of Relay switching contacts

14. Switch ratings (voltage and current)
The relay’s switch must be suitable for the circuit they are to control. You will need to check the voltage and current ratings. Note that the voltage rating is usually higher for AC,
for example: “5A at 24V DC or 125V AC”.

15. RELAYS

16. Advantage vs. Disadvantage

17. Advantages of Relays Relays can switch AC and DC.
Relays can switch higher voltages.
Relays are often a better choice for switching large currents (>5 A).
Relays can switch many contacts at once.

18. Disadvantages of Relays
The contacts becoming worn, resulting in a limited lifespan.
That they don't perform that well when it comes to switching high inrush currents.

19. Applications
The Electromagnetic relays are employed for the  protection of various AC and DC equipments.
In the over/under current and voltage protection of various AC and DC equipments.
For differential protection.
Used as auxiliary relays in the contact systems of protective relay schemes.