3. Relays are a remote control electrical switch
that can be switched using low current to control
a high current load.
Water level Indicator
4. Relays were invented in 1835 by American
electromagnetism pioneer Joseph Henry, in
a demonstration at the College of New
Jersey.
R E L A Y
5. 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.
6. • 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.
7.
8.
9.
10. o The relay’s switch connections are usually labeled
COM, NC and NO:
oCOM =Common, always connect to this, it is the
moving part of a switch.
oNC= Normally Closed,COM is connected to this
when the relay coil is off.
oNO= Normally open,COM is connected to this when
the relay coil is on.
SYMBOL
11. Relay operation based on LoadRelay operation based on Load
Load side. (red)
Control side. (green)
12. Drawn in natural state.
Where the voltage present
when load side is open.
Relay operation (off)
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.
Relay operation (on)
13. In this relay’s natural state it is in the
normally open state.
When the coil is non-energized the
contacts are open.
Normally open
Normally closed
In this relay’s natural state it is in the
normally closed state.
When the coil is energized the contacts
closed.
14. • 5(Five) pin relay.
• Most common contains normally open and
normally open contacts.
• 4(Four) pin relay.
• Contain either normally open or
normally closed.
17. 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”.
20. 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.
21. The contacts becoming worn, resulting in a
limited lifespan.
That they don't perform that well when it comes
to switching high inrush currents.
22. 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.
