What is a Relay?
A Relay is a simple electromechanical switch. The relay is the device
that open or closes the contacts to cause the operation of the other
electric control. It detects the intolerable or undesirable condition with
an assigned area and gives the commands to the circuit breaker to
disconnect the affected area. Thus protects the system from damage.
While we use normal switches to close or open a circuit manually, a
Relay is also a switch that connects or disconnects two circuits. But
instead of a manual operation, a relay uses an electrical signal to control
an electromagnet, which in turn connects or disconnects another
circuit.
Relays can be of different types like electromechanical, solid state.
Electromechanical relays are frequently used. Let us see the internal
parts of this relay before knowing about it working. Although many
different types of relay were present, their working is same.
Every electromechanical relay consists of an consists of an
1. Electromagnet
2. Mechanically movable contact
3. Switching points and
4. Spring
Electromagnet is constructed by wounding a copper coil on a metal
core. The two ends of the coil are connected to two pins of the relay as
shown. These two are used as DC supply pins.
Generally two more contacts will be present, called as switching points
to connect high ampere load. Another contact called common contact is
present in order to connect the switching points.
These contacts are named as normally open (NO),normally closed(NC)
and common(COM) contacts.
We can use a Relay either in a AC circuit or a DC Circuit. In case of AC
relays, for every current zero position, the relay coil gets demagnetized
and hence there would be a chance of continues breaking of the circuit.
So, AC relays are constructed with special mechanism such that
continuous magnetism is provided in order to avoid above problem.
Such mechanisms include electronic circuit arrangement or shaded coil
mechanism.
How a Relay Works?
The following animation shows a simplified working of a relay.
Relay works on the principle of electromagnetic induction.
When the electromagnet is applied with some current, it induces a
magnetic field around it.
Above image shows working of the relay. A switch is used to apply DC
current to the load.
In the relay, Copper coil and the iron core acts as electromagnet.
When the coil is applied with DC current, it starts attracting the contact
as shown. This is called energizing of relay.
When the supply is removed it retrieves back to the original position.
This is called De energizing of relay.
There are also such relays, whose contacts are initially closed and
opened when there is supply i.e. exactly to opposite to the above
shown relay.
Solid state relays will have sensing element to sense the input voltage
and switches the output using opto-coupling.
Working Principle of Relay
Now let's understand how a relay works in a normally closed condition
and normally open condition.
Relay in NORMALLY CLOSED condition:
When no voltage is applied to the core, it cannot generate any magnetic
field and it doesn’t act as a magnet. Therefore, it cannot attract the
movable armature. Thus, the initial position itself is the armature
connected in normally closed position (NC).
Relay in NORMALLY OPENED condition:
When sufficient voltage is applied to the core it starts to create a
magnetic field around it and acts as a magnet. Since the movable
armature is placed within its range, it gets attracted to that magnetic
field created by the core, thus the position of the armature is being
altered. It is now connected to the normally opened pin of the relay and
external circuit connected to it function in a different manner.
Note: The functionality of the external circuit depends upon the
connection made to the relay pins.
So finally, we can say that when a coil is energized the armature is
attracted and the switching action can be seen, if the coil is de-
energized it loses its magnetic property and the armature goes back to
its initial position.
Relay Contact Types
As we have seen that relay is a switch. The terminology “Poles and
throws” is also applicable for relay. Depending on the number of
contacts and number of circuits it switches relays can be classified.
Before we know about this classification of contacts we have to know
the poles and throws of a relay switch.
Poles and Throws
Relays can switch one or more circuits. Each switch in relay is referred
as pole. Number of circuits a relay connects is indicated by throws.
Depending on the poles and throws, relays are classified into
1. Single Pole Single Throw
2. Single Pole Double Throw
3. Double Pole Single Throw
4. Double Pole Double Throw
Single Pole Single Throw
A single pole single throw relay can control one circuit and can be
connected to one output. It is used for the applications which require
only ON or OFF state.
Single Pole Double Throw
A single pole double throw relay connects one input circuit to one of
the two outputs. This relay is also called as changeover relay.
Though the SPDT has two output positions, it may consist of more than
two throws depends on the configuration and requirement of the
application.
Double pole single throw
A double pole single throw relay has two poles and single throw and it
can be used to connect two terminals of a single circuit at a time. For
example, this relay is used for connecting both phase and neutral
terminals to the load at a time.
Double pole double throw
A DPDT (double pole double throw) relay has two poles and two throws
for each pole. In motor direction control, these are used for phase or
polarity reversal.
The switching action between contacts for all these relays is performed
when the coil get energized.
Types of Relays
Relays can be classified into different types depending on their
functionality, structure, application etc. We listed out some of the
common types of relays here.
1. Electromagnetic
2. Latching
3. Electronic
4. Non-Latching
5. Reed
6. High-Voltage
7. Small Signal
8. Time Delay
9. Multi-Dimensional
10. Thermal
11. Differential
13. Distance
14. Automotive
15. Frequency
16. Polarized
17. Rotary
18. Sequence
19. Moving Coil
20. Buchholz
21. Safety
22. Supervision
23. Ground Fault
We made a dedicated guide on different types of relays. To learn more
about different types of relays, read Classification of Relays.
Relay Applications
Relays are used to protect the electrical system and to minimize the
damage to the equipment connected in the system due to over
currents/voltages. The relay is used for the purpose of protection of the
equipment connected with it.
These are used to control the high voltage circuit with low voltage signal
in applications audio amplifiers and some types of modems.
These are used to control a high current circuit by a low current signal in
the applications like starter solenoid in automobile. These can detect
and isolate the faults that occurred in power transmission and
distribution system. Typical application areas of the relays include
1. Lighting control systems
2. Telecommunication
3. Industrial process controllers
4. Traffic control
5. Motor drives control
6. Protection systems of electrical power system
7. Computer interfaces
8. Automotive
9. Home appliances
How to Test a Relay?
Since they are electromechanical devices, relays can wear out
eventually and stop working over time. But there are few techniques to
test if a relay is working or not. These techniques include:
Testing a Relay with a Multimeter
Build a simple circuit to test the Relay
Use a DC Power Supply to see whether a relay is functioning properly
To learn more about testing a relay, read How to Test a Relay with
Multimeter.
