Different Types of Relays and their

Working Principles
Relays are the primary protection as well as switching devices in most of the control
processes or equipments. All the relays respond to one or more electrical quantities like
voltage or current such that they open or close the contacts or circuits. A relay is a
switching device as it works to isolate or change the state of an electric circuit from one
state to another.

Different Types of Relays

Classification or the types of relays depend on the function for which they are used.
Some of the categories include protective, reclosing, regulating, auxiliary and monitoring
relays.

Protective relays continuously monitor these parameters: voltage, current, and power;
and if these parameters violate from set limits they generate alarm or isolate that
particular circuit. These types of relays are used to protect equipments like motors,
generators, and transformers, and so on.
Reclosing relays are used to connect various components and devices within the
system network, such as synchronizing process, and to restore the various devices
soon after any electrical fault vanishes, and then to connect transformers and feeders to
line network. Regulating relays are the switches that contacts such that voltage boosts
up as in the case of tap changing transformers.
Auxiliary contacts are used in circuit breakers and other protective equipments for
contact multiplication. Monitoring relays monitors the system conditions such as
direction of power and accordingly generates the alarm. These are also called
directional relays.

This article’s main aim is to give a brief idea about various relays that are employed for
a wide variety of control applications. Some of these relays are described below.
Different Types of Relays
Depending on the operating principle and structural features relays are of different types
such as electromagnetic relays, thermal relays, power varied relays, multi-dimensional
relays, and so on, with varied ratings, sizes and applications.

1. Electromagnetic Relays
These relays are constructed with electrical, mechanical and magnetic components,
and have operating coil and mechanical contacts. Therefore, when the coil gets
activated by a supply system, these mechanical contacts gets opened or closed. The
type of supply can be AC or DC.
DC vs AC Relays
Both AC and DC relays work on the same principle as electromagnetic induction, but
the construction is somewhat differentiated and also depends on the application for
which these relays are selected. DC relays are employed with a freewheeling diode to
de-energize the coil, and the AC relays uses laminated cores to prevent eddy current
losses.
 DC vs AC Relays
The very interesting aspect of an AC is that for every half cycle, the direction of the
current supply changes; therefore, for every cycle the coil loses its magnetism since the
zero current in every half cycle makes the relay continuously make and break the
circuit. So, to prevent this – additionally one shaded coil or another electronic circuit is
placed in the AC relay to provide magnetism in the zero current position.

Attraction Type Electromagnetic Relays

Attraction Type Relays

These relays can work with both AC and DC supply and attract a metal bar or a piece of
metal when power is supplied to the coil. This can be a plunger being drawn towards
the solenoid or an armature being attracted towards the poles of an electromagnet as
shown in the figure. These relays don’t have any time delays so these are used for
instantaneous operation.

Induction Type Relays

These are used as protective relays in AC systems alone and are usable with DC
systems. The actuating force for contacts movement is developed by a moving
conductor that may be a disc or a cup, through the interaction of electromagnetic fluxes
due to fault currents.

Induction Type Relays

These are of several types like shaded pole, watt-hour and induction cup structures and
are mostly used as directional relays in power-system protection and also for high-
speed switching operation applications.

Magnetic Latching Relays

These relays use permanent magnet or parts with a high remittance to remain the
armature at the same point as the coil is electrified when the coil power source is taken
away.

2. Solid State Relays

Solid State uses solid state components to perform the switching operation without
moving any parts. Since the control energy required is much lower compared with the
output power to be controlled by this relay that results the power gain higher when
compared to the electromagnetic relays. These are of different types: reed relay
coupled SSR, transformer coupled SSR, photo-coupled SSR, and so on.
 Solid State Relays
The above figure shows a photo coupled SSR where the control signal is applied by
LED and it is detected by a photo-sensitive semiconductor device. The output form this
photo detector is used to trigger the gate of TRIAC or SCR that switches the load.

3. Hybrid Relay
These relays are composed of electromagnetic relays and electronic components.
Usually, the input part contains the electronic circuitry that performs rectification and the
other control functions, and the output part include electromagnetic relay.
4. Thermal Relay
These relays are based on the effects of heat, which means – the rise in the ambient
temperature from the limit, directs the contacts to switch from one position to other.
These are mainly used in motor protection and consist of bimetallic elements
like temperature sensors as well as control elements. Thermal overload relays are the best
examples of these relays.
5. Reed Relay

Reed Relay
Reed Relays consist of a pair of magnetic strips (also called as reed) that is sealed
within a glass tube. This reed acts as both an armature and a contact blade. The
magnetic field applied to the coil is wrapped around this tube that makes these reeds
move so that switching operation is performed.

Based on dimensions, relays are differentiated as micro miniature, subminiature and

miniature relays. Also, based on the construction, these relays are classified as
hermetic, sealed and open type relays. Furthermore, depending on the load operating
range, relays are of micro, low, intermediate and high power types.

Relays are also available with different pin configurations like 3 pin, 4 pin and 5 pin
relays. The ways in which these relays are operated is shown in the below
figure. Switching contacts can be SPST, SPDT, DPST and DPDT types. Some of the
relays are normally open (NO) type and the other are normally closed (NC) types.

Relay pin configurations

Electromechanical Relay
Construction with Working
The electrical and electronics circuits are usually operated over a wide range of
voltage, current, and power ratings. For every circuit or equipment or electrical network
or power system protection system is desired to avoid the breakdown or temporary or
permanent damage. Such that, equipments or circuits used for protecting are called as
protecting equipment or circuit. In case of a small amount of voltage ratings, protection
of the circuit depends on the cost of the original circuit to be protected and cost of the
protection system essential to protect the circuit. But, in case of high cost circuits or
equipments, it is desired to adopt a protection system or protection circuit and
controlling device or controlling circuit to avoid economical loss and damage.
Electromechanical Relay

Relay
The relay is an electromechanical switch used as a protecting device and also as a
controlling device for various circuits, equipments, and electrical networks in a power
system. The electromechanical relay can be defined as an electrically operated switch
that completes or interrupts a circuit by physical movement of electrical contacts into
contact with each other.
Electromechanical Relay Construction
The flow of current through an electrical conductor causes a magnetic field at right angles
to the current flow direction. If this conductor is wrapped to form a coil, then the
magnetic field produced gets oriented along the length of the coil. If the current flowing
through the conductor increases, then the magnetic field strength also increases (and
vice-versa).
 Eelctromechanical Relay Coil – Magnetic Field
The magnetic field produced by passing current through coil can be used for various
purposes such as inductors, construction of transformer using two inductor coils with an
iron core. But, in electromechanical relay construction the magnetic field produced in coil
is used to exert mechanical force on magnetic objects. This is similar to permanent
magnets used to attract magnetic objects, but here the magnetic field can be turned on
or off by regulating current flow through the coil. Thus, we can say that the
electromechanical relay operation is dependent on the current flowing through the coil.
Electromechanical Relay Working
The electromechanical relay consists of various parts such as movable armature,
movable contact & stationary contact or fixed contact, spring, electromagnet (coil), the
wire wrapped as coil with its terminals represented as ‘C’ which are connected as
shown in the below figure to form electromechanical relay.

Electromechanical Relay Construction

If there is no supply given to the coil terminals, then the relay remains in the off
condition as shown in the below figure and the load connected to relay also remains
turned off as no power supply is given to load.
 Electromechanical Relay Working (OFF condition)
If the relay coil is energized by giving supply to the coil terminals at ‘C’, then the
movable contact of the relay is attracted towards the fixed contact. Thus, the relay turns
on and the supply is connected to the load as shown in the below figure.

Electromechanical Relay Working (ON condition)

There are various types of relays, the relays which are energized by electrical supply and
performs a mechanical action (on or off) to make or break a circuit are called as
electromechanical relays. There are various types of relays such as Buchholz relay,
latching relay, polarized relay, mercury relay, solid state relay, polarized relay, vacuum
relay, and so on.
Applications of Electromechanical Relay
There are numerous applications for electromechanical relays. Various types of relays
are used in various applications based on different criteria such as rating of contacts,
number & type of contacts, the voltage rating of contacts, operating lifetime, coil voltage
& current, package, and so on. Relays are frequently being used in power system
networks for controlling purpose, automation purpose, and protection purpose.
The typical applications of electromechanical relays include motor control, automotive
applications such as an electrical fuel pump, industrial applications where control of
high voltages and currents is intended, controlling large power loads, and so on.
Electromechanical Relay Logic
The method of using relays and contacts to control the industrial electronic circuits is
called as relay logic. The inputs and outputs of relay logic circuits are represented by a
series of lines in schematic diagrams and hence relay logic circuits are also called as
line diagrams. An electromechanical relay logic circuit can be represented as an
electrical network of lines or rungs where each line or rung have continuity for enabling
the output device.
Application of Electromehanical Relay Logic
The railways routing and signaling are controlled using relay logic and is considered as
a key application of relay logic. This safety critical application is used to reduce the
accidents and to avoid the selection of conflicting routes by using interlocking. The
human elevator operator was replaced by large relay logic circuits in elevators. The
relay logic circuits are used in electro-hydraulics and electro-pneumatics for controlling
and automation purpose.
Do you want to know the basic design of relay logic? Are you interested in
designing electronics projects? Then, post your queries, comments, suggestions, ideas in
the comments section below.
How Relays Work – Basics, Types &
Applications
Relays are electromechanical switches, used to control the several circuits by using
low-power signal or one signal. These are found in all sorts of devices. Relays allow one
circuit to switch a second circuit which can be completely separate from the first. There
is no electrical connection inside the relay between the two circuits; the link is magnetic
and mechanical only.

Basically a Relay consists of an electromagnet, an armature, a spring and a series of

electrical contacts. The electromagnet coil gets power through a switch or a relay driver
and causes the armature to get connected such that the load gets the power supply.
The armature movement is done using a spring. Thus the relay consists of two separate
electrical circuits which are connected to each other only through magnetic connection
and the relay is controlled by controlling the switching of the electromagnet.

Relay 3Co
Current moving through the coil of the relay makes a magnetic field which attracts a
lever and changes the switch contacts. The loop or coil current can be on or off so
relays have two switch positions and generally have double throw (changeover) switch
contacts. Relays are usually SPDT or DPDT however they can have numerous sets of
switch contacts.
The contacts are usually common (COM), normally open (NO) and normally closed
(NC). The normally closed contact will be connected to the common contact when no
power is applied to the coil. The normally open contact will be open when the no power
is applied to the coil. When the coil is energized the common is connected to the
normally open contact and the normally closed contact is left floating. The double pole
versions are the same as the single pole version except there are two switches that
open and close together.

Relay 3Co Circuit

Applications of Relays:
 Control a high-voltage circuit with a low-voltage signal, as in some types of modems or audio
amplifiers
 Control a high-current circuit with a low-current signal, as in the starter solenoid of an
automobile
 Detect and isolate faults on transmission and distribution lines by opening and closing circuit
breakers
 Time delay functions. Relays can be modified to delay opening or delay closing a set of
contacts. A very short delay would use a copper disk between the armature and moving blade
assembly
Current flowing in the disk maintains magnetic field for a short time. For a slightly longer
delay, a dashpot is used. A dashpot is a piston filled with fluid that is allowed to escape
slowly. The time period can be varied by increasing or decreasing the flow rate. For
longer time periods, a mechanical clockwork timer is installed.
Working of Relay with 3coil:
From the circuit, relay-1 and relay-2 the contacts of which are connected in series with
relay-3 coils, to the first dc supply. Relay-3 switches ON only if relay 1 and 2 are ON
meaning supply at R, Y and B are available. The output contacts of relay-3 are fed to
relay-4 Q1, NC contacts both of which are 3-Co relays. Thus the R, Y, B fed to relay-3
reaches NO contacts of relay-4. All the NO contacts of relay-4 are joined together to
develop a star-mode configuration to the motor connection coil U1-U2, V1-V2, W 1-W 2.
While relay-4 is switched ON by the timer IC; after main supply switch ON taking a time
delay, the contacts of relay-4 bring the motor connections to delta mode by the NC
contacts duly wired. Single phasing meaning any one or two phases Y and B missing
bring either relay-1 or relay-2 to off condition that results in relay-3 to switch OFF. Thus
relay-3switch off prevents the input 3-phase to reach the motor supply to protect the
same for single phasing.

3Co-Circuit
Working of Relay with 2coil:
Relay with latching construction composed of 2 coils: set coil and reset coil. The relay is
set or reset by alternately applying pulse signals of the same polarity.

From the circuit, relay is used which is driven by a transistor from port pin number10.
The contacts of the relay are interfaced to a land line telephone connection. The output
of which is super imposed on the telephone lines through only if the relay1 is ON. The
relay operates (with an led indication L2) from pin number 10 through Q2 transistor
before dialing data reaches the encoder from the MC. The dialing continues till dialed
number party lifts the receiver or otherwise it automatically switches of the relay after 3
minutes to force hand set to virtual “On the hook” condition.

Relay with 2Coil Circuit

Working of Relay with 1coil:
Relay with latching construction that can maintain on or off state with a pulse input. With
one coil, the relay is set or reset by applying signals of opposite polarities. In this we are
going to see a relay with 1 coil using ULN2003.

ULN2003 is an IC which is used to interface relay with the microcontroller since the
output of the micro controller is maximum 5V with too little current delivery and is not
practicable to operate a relay with that voltage. ULN2003 is a relay driver IC consisting
of a set of Darlington transistors. If logic high is given to the IC as input then its output
will be logic low but not the vice versa. Here in ULN2003 pins 1 to 7 are IC inputs and
10 to 16 are IC outputs. If logic 1 is given to its pin1 the corresponding pin 16 goes low.
If a relay coil is connected from positive to the output pin of the IC then the relay
contacts change their position from normally open (NO) to normally close (NC) then the
light will glow. If logic 0 is given at the input the relay switches off. Similarly up to seven
relays can be used for seven different loads to be switched on by the normally open
(NO) contact or switched off by the normally closed contact (NC) but in this we used
only one relay for operation.
 Load On and Off
2 Ways to Control Relays
Using a Table Clock

One of the simplest ways is using a timer to control the switching of the relay.Here a
simple circuit is developed, which can switch on / off a load when the set time arrives. It
can be used to switch on AC loads such as TV, Radio, Music system etc. Its triggering
pulse is obtained from a small table clock. The clock alarm timing is set to manually
control the switch on/off. Basic idea is to control the relay switching by controlling the
triggering of SCR through the Optocoupler which is in turn triggered by the clock alarm.

A few Components Used in the Circuit:

The circuit consists of the following:

 A low cost table clock

 An optocoupler IC MCT2E
 An SCR to trigger the relay.
 A diode connected across the relay
 A 9V battery and a capacitor
 A resistor
System Working:

The clock output is given to the circuit using an Optocoupler IC MCT2E. The alarm
buzzer gets around 3 volts when the alarm rings. The Optocoupler is triggered with this
voltage. The Optocoupler has an LED and phototransistor inside. When the LED inside
the Optocoupler lights by receiving an external voltage, the phototransistor conducts.

When the Phototransistor conducts, the SCR BT169 fires and latches. This actuates the
relay and the load will switch on / off. If the load is connected through the common and
NO contacts, the load switches on. The load switches off if it is connected through the
common and NC contacts.

Relay
Control using Clock Circuit Diagram
The SCR starts conducting when a triggering pulse is applied to the gate terminal. The
SCR continues the conduction even if the gate pulse is removed. It can be switched off
only by removing the anode current. So a Push to off switch S1 is used to reset the
SCR. Capacitor C1 has a buffering action at the gate of SCR for its smooth working.
Diode IN4007 protects the SCR from back emf.

The table clock used is the low cost one. Open its back cover and solder two thin wires
at the buzzer terminals and connect to the pins 1 and 2 of the Optocoupler observing
polarity. Enclose the circuit with the power supply in a case and fix the clock above it
using glue. To connect the load, an AC socket can be fixed on the box.
Using Relay Driver IC ULN 2003
A relay can also be controlled using a relay driver IC ULN2003 which is interfaced to a
microcontroller and drives the relay based on the signals from the Microcontroller. It is a
high voltage IC consisting of 7 Darlington pairs of transistors. It is basically a 16 pin IC.
It consists of 7 input pins and 7 corresponding output pins.

Working of the System

The relay driver can drive upto 7 relays with each relay connected to each of the 7
outputs. The input pins of the relay are connected to the I/O pins of the Microcontroller.
Here only one relay is shown for demonstration purpose. The relay as well as the relay
driver requires a power supply of 12 V at pin 9. The operation is similar to an inverter
where a logic low input results in a logic high output. The load is connected to the
normally open contact. When a logic zero is applied to one of the input pin of the relay
driver, a logic high output is developed across the corresponding output pin. Since the
relay is connected to almost same voltage at both of the end points, no current flows
and the relay is not energized. Incase of a high logic at the input pin, the output pin gets
a low logic signal and due to a potential difference, a current flows and the relay coil
gets energized such that it causes the armature to move from the normally closed
position to the normally open position , thus completing the circuit and causing the lamp
to glow.