Relay Switch

RELAY
(ELECTROMAGNETIC
SWITCH)”
Name: Saqib Iqbal
Subject: Basic Electrical
Engineering
Roll no. : 14-CS-07
Submitted to:
Mr. GhulamMohammed

1
DECLARATION
Except where otherwise acknowledgement in the text,
this presents the original research of researcher. The
material contained here has not been submitted in either
in whole or in part at this or any other university.
Researcher
Saqib iqbal

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Abstract:
This research investigates about the Relay
(electromagnetic switch). It also includes the
criteria from where it is originate from.
History of relay. Also for construction,
working and its types. How we can design
relay. My project complete work. Application
of relay where uses. Factors affecting these
type of relay. Strategies to make good
performance of relay. Methods and functions
of relay. Costs expend to complete relay
project. Also some useful tips.

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S.No Contents Page no
01 Introduction 06
02 Definition 07
03 History 07
04 Design 07
05 Operation 08
06 Pole and throw 09
07 Application 11
08 De rating factor 12
09 Undesired arching 13
10 Types 13
11 Apparatus 15
12 Construction 16
13 Working 16
14 Expenditures 17
15 References 18

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INTRODUCTION:
An electronic switch is essentially just a switch that uses an electrical current, to turn on,
usually turning off when the current is turned off. Some applications of switches can be
quite inconvenient for someone to go and press a button to turn on or off, such as for
the starter motor in a car, or the "turn off nuclear meltdown" button inside a nuclear
reactor, or in an electronics project, a small low power device such as a receiver, must
somehow power a large energy guzzling component, like the motor in a garage door
opener. And others just want to control their houses with their computer's, which could
never possible supply the 240v/120v mains needed to run some appliances.

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What is relay (electromagnetic switch)?
There are 2 definitions of relay in English.
1: A group of people or animals engaged in a task or activity for a period of
time and then replaced by a similar group.
2: An electrical device, typically incorporating an electromagnet, which is
activated by a current or signal in one circuit to open or close another circuit
But I research about an electromagnet switch which basically complete an
open circuit
HISTORY:
The American scientist Joseph Henry invented a relay in 1835 in order to
improve his version of the electrical telegraph, developed earlier in 1831.
It is claimed that the English inventor Edward Davy "certainly invented the
electric relay" in his electric telegraph c.1835.
A simple device, which we now call a relay, was included in the original
1840 telegraph patentof Samuel Morse. The mechanism described acted as a
digital amplifier, repeating the telegraph signal, and thus allowing signals to
be propagated as far as desired. This overcame the problem of limited range
of earlier telegraphy schemes
BASIC DESIGN:
A simple electromagnetic relay consists of a coil of wire wrapped around
a soft iron core, an iron yoke which provides a low reluctance path for
magnetic flux, a movable iron armature, and one or more sets of contacts
Page 02

6
(there are two in the relay pictured). The armature is hinged to the yoke and
mechanically linked to one or more sets of moving contacts
CASE 01:It is held in place by a spring (automatic relay)so that when the
relay is de-energized there is an air gap in the magnetic circuit. In this
condition, one of the two sets of contacts in the relay pictured is closed, and
the other set is open
When coil is de-energized relay when no current is flowing in the
electromagnet coil the armature is pulled up by the spring and its COM
contact connects to the NC contact.
When coil is energized relay. When a voltage is applied to the electromagnet
coil the current flowing in the coil produces magnetic energy in the iron core
which pulls the armature down.
When the armature pulls down the COM contact switches from NC to NO.
CASE 02: when no spring is placed. (Manual relay) so it operate when
external switch is connected.
BASIC OPERATION OF RELAY:
Before extending to the various types of relays, i will first explain what and how the
basic relay operates. Each relay has two mechanical parts inside. The first one is the
contact(s) of the relay. The contacts operate similarly to the contacts of a simple switch
or pushbutton. You should consider the contacts as a pair of metals.

7
The two terminals operate as a switch. When the contacts are 'in contact' then the
current flows from Terminal 1 to Terminal 2. There are two types of contacts: the NO
and the NC. NO stands for Normal Open contact, while NC stands for Normal Closed
contact. The Normal Open is a contact like the one showed in the previous illustration.
When the contact is still, then no current flows through it (because it is an OPEN circuit).
On the other hand, a Normal Closed contact allows the current to flow when the contact
is still. Bellow i illustrate both of these contacts:
POLE AND THROW:
Circuit symbols of relays. (C denotes the common terminal in SPDT and DPDT types.)
Since relays are switches, the terminology applied to switches is also applied to relays;
a relay switches one or more poles, each of whose contacts can be thrown by
energizing the coil in one of three ways:

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 Normally-open (NO) contacts connect the circuit when the relay is activated; the
circuit is disconnected when the relay is inactive. It is also called a Form A contact
or "make" contact. NO contacts may also be distinguished as "early-make" or
NOEM, which means that the contacts close before the button or switch is fully
engaged.
 Normally-closed (NC) contacts disconnect the circuit when the relay is activated; the
circuit is connected when the relay is inactive. It is also called a Form B contact or
"break" contact. NC contacts may also be distinguished as "late-break" or NCLB,
which means that the contacts stay closed until the button or switch is fully
disengaged.
 Change-over (CO), or double-throw (DT), contacts control two circuits: one
normally-open contact and one normally-closed contact with a common terminal. It
is also called a Form C contact or "transfer" contact ("break before make"). If this
type of contact utilizes “make before break" functionality, then it is called a Form D
contact.
The following designations are commonly encountered:
 SPST – Single Pole Single Throw. These have two terminals which can be
connected or disconnected. Including two for the coil, such a relay has four
terminals in total. It is ambiguous whether the pole is normally open or normally
closed. The terminology "SPNO" and "SPNC" is sometimes used to resolve the
ambiguity.
 SPDT – Single Pole Double Throw. A common terminal connects to either of two
others. Including two for the coil, such a relay has five terminals in total.
 DPST – Double Pole Single Throw. These have two pairs of terminals. Equivalent to
two SPST switches or relays actuated by a single coil. Including two for the coil,
such a relay has six terminals in total. The poles may be Form A or Form B (or one
of each).

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 DPDT – Double Pole Double Throw. These have two rows of change-over terminals.
Equivalent to two SPDT switches or relays actuated by a single coil. Such a relay
has eight terminals, including the coil.
The "S" or "D" may be replaced with a number, indicating multiple switches connected
to a single actuator. For example 4PDT indicates a four pole double throw relay (with 12
terminals).
EN 50005 are among applicable standards for relay terminal numbering; a typical EN
50005-compliant SPDT relay's terminals would be numbered 11, 12, 14, A1 and A2 for
the C, NC, NO, and coil connections, respectively.
APPLICATIONS OF RELAY:
 Coil voltage – machine-tool relays usually 24 VDC, 120 or 250 VAC, relays for
switchgear may have 125 V or 250 VDC coils, "sensitive" relays operate on a few
mill amperes.

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 Coil current - including minimum current required to operate reliably and
minimum current to hold. Also effects of power dissipation on coil temperature at
various duty cycles
 Package/enclosure – open, touch-safe, double-voltage for isolation between
circuits, explosion proof, outdoor, oil and splash resistant, washable for printed
circuit board assembly
 Switching time – where high speed is required
 "Dry" contacts – when switching very low level signals, special contact materials
contact protection – suppress arcing in very inductive circuits
 Coil protection – suppress the surge voltage produced when switching the coil
current
 Isolation between coil contacts
 May be needed such as gold-plated contacts
 Regulatory approvals
 Stray magnetic linkage between coils of adjacent relays on a printed circuit
board.
DERATINGFACTORS:
Control relays should not be operated above rated temperature
because of resulting increased degradation and fatigue. Common
practice is to derate 20 degrees Celsius from the maximum rated
temperature limit. Relays operating at rated load are also affected
by their environment. Oil vapors may greatly decrease the
contact tip life, and dust or dirt may cause the tips to burn before
their normal life expectancy. Control relay life cycle varies from
50,000 to over one million cycles depending on the electrical
loads of the contacts, duty cycle, application, and the extent to
which the relay is derated. When a control relay is operating at its derated value, it is controlling a
lower value of current than its maximum make and break ratings. This is often done to extend the
operating life of the control relay. The table lists the relay derating factors for typical industrial control
applications
Type of load % of rated value
Resistive 75
Inductive 35
Motor 20
Filament 10
Capacitive 75

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UNDESIRED ARCING:
Switching while "wet" (under load) causes undesired arcing between the contacts,
eventually leading to contacts that weld shut or contacts that fail due to a buildup of
contact surface damage caused by the destructive arc energy
TYPES OF RELAY:
1. LATCHING RELAY:
A latching relay has two relaxed states (bitable). These are also called "impulse", "keep",
or "stay" relays. When the current is switched off, the relay remains in its last state.
2. Reed relay:
A reed relay is a reed switch enclosed in a solenoid. The switch has a set of contacts
inside an evacuated or inert gas-filled glass tube which protects the contacts against
atmospheric corrosion; the contacts are made of magnetic material that makes them
move under the influence of the field of the enclosing solenoid or an external magnet.
3. Polarized relay:
A polarized relay places the armature between the poles of a permanent magnet to
increase sensitivity. Polarized relays were used in middle 20th Century telephone
exchanges to detect faint pulses and correct telegraphic distortion. The poles were on
screws, so a technician could first adjust them for maximum sensitivity and then apply a
bias spring to set the critical current that would operate the relay.
4. Ratchet relay:
This is again a clapper type relay which does not need continuous current through its
coil to retain its operation. A ratchet holds the contacts closed after the coil is

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momentarily energized. A second impulse, in the same or a separate coil, releases the
contacts.
5. Coaxial relay:
Where radio transmitters and receivers share a common antenna, often a coaxial relay
is used as a TR (transmit-receive) relay, which switches the antenna from the receiver
to the transmitter. This protects the receiver from the high power of the transmitter.
APPRATUS:
1 A wooden sheet (36/12) inches
2 Two 9 volts d.c batteries
3 Armature (of stainless steel)
4 Soft wire (use for winding)
5 Iron nail (core)
CONSTRUCTION:
Relay:
I used a soft type copper wire of 15m long and make turn on a yoke (iron nail). Turns of
copper winding are about 550 turns.
Armature:
The armature is of L-shaped which is exactly placed above the relay so that when relay
is energized it pulled the metal strip.
Circuitof load:
The circuit is design so that it remain open at a certain place when armature is pulled
down it again start to flow current and stop or re-open when coil is de-energized.

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Supply:
I used two D.C supplies of 9volts. One is for magnetizing coil and other for load circuit.
WORKING:
Normally open:
1 CURRENT IN COIL:
When 9volts D.C is applied to coil, the magnetic flux will produce is according to the
formula:
2 Contactof armature:
The flux tend to attract armature toward itself now when armature is pulled down so
it sort the circuit of load (in other words the current in load from will find the way to
flow)
3 Working of load:
When load is shorted so the current start to flow and finally working of relay starts.
Normally closed:
1 Currentin coil:
When current stop flowing in coil the coil will demagnetize and armature will go back to
its original position. Now the formula is given by
B=0(I=0)

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2 Contactof armature:
Because of inertia armature will gain back its original position.(in other words the circuit
of load will opened again and it stop working)
3 Working of load:
When load circuit is open again it stops working simultaneously.
EXPENDITURE OR COST OF MATERIALS:
Materials: Cost:
Copper wire (15m) 100rs
Wooden sheet 30rs
DC batteries 80rs
soft iron wire 50rs
REFERENCE:
 Wikipediafree encyclopedia
 www.sparkfun.com
 http://electronics.howstuffworks.com/
 http://www.allaboutcircuits.com/
 http://www.made-in-china.com/
 http://www.thefreedictionary.com/

Relay Switch

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