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Evolution of Electronics, Impact of Electronics
in industry and in society.
Resistors- types, specifications. Standard
values, marking, colour coding.
Capacitors: types, specifications. Standard
values, marking, colour coding.
Inductors- types, specifications, Principle of
working.
Transformers: types, specifications, Principle
of working.
Electro mechanical components: relays and
contactors.
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Branch of science that deals with
 study of flow & control of electrons
 study of their behavior & effects in vacuums,
gases, and semiconductors, and with devices
using such electrons.
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1752-Ben Franklin – Lightning
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1784- Charles Augustin Coulomb –
Electrical Charge
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1791-Luigi Galvani – Bio electricity
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1799- Alessandro Volta –Voltage
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1820- Hans Christian oersted – Electromagnetism
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1827- George Simon Ohm- Resistance
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1831- Michael Faraday - Electromagnetic induction
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1864- James Clerk Maxwell - Maxwell’s equation
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1876- Alexander Graham Bell-Telephone
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1879- Thomas Alva Edison – Electric Bulb
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1888- Heinrich Hertz – Radio Waves
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1895- Marconi-Radio
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1904- Ambrose Fleming – Vacuum Tube
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1906- Lee De Forest-Triode
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1925- John Logie Baird – Television
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1939- Russell Ohl – PN Junction Diode
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1948- William Schockley, John Bardeen and Watter BrattainTransistor
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1958- Jack Kilby – Integrated Circuit
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1971- Robert Noyce and Gordon Moore-Microprocessor
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Impact of Electronics in
industry and in society
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Besides electronic devices (radio & TV
receivers, audio & video players,
calculators, mobile phones, etc.,) electronics
has offered its services in different walks of
life.
Computer: major achievement
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All techniques and devices make use of electronics reliability & precision are key factors
 industrial operations,
 medical diagnostics and surgery, and
 in laboratory practice.
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development of communication facilities
 wireless communication
 Aircraft uses radio communication-weather & terminal traffic
information
 Satellite communication
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space voyages to moon or mars
in defence
 Radar
 electronic warfare
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In industrial applications
 counting, sorting, illumination control, welding control,
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liquid-gaseous flow control, automatically regulating
temperature and humidity and in early warning system
precision measurement made easier for electrical & non
electrical quantities
made measuring devices more reliable, accurate and
easy to handle – electronic devices & digital displays
Use of automatic control systems
Use of computers - reservations in railways & airways
power stations- controlled by tiny electronic devices and
circuits
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E-Commerce
 electronic commerce over the Internet
 it affects large sectors -communications, finance ,
retail trade, education, health services
 very effective at reducing the costs of attracting new
customers, because advertising is typically cheaper
than for other media and more targeted
 Electronics interface allows e-commerce merchants to
check that an order is internally consistent and that
the order, receipt, and invoice match
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electronic waste
 Exponential growth in the global market of electrical
and electronic equipment
 Rapid changes in technology, changes in media,
falling prices, and planned obsolescence have
resulted in a fast-growing surplus of electronic waste
around the globe
 comprises of a multitude of components, some
containing toxic substances that can have
an adverse impact on human health and
the environment if not handled properly
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Device which provides a force opposing the charge-flow
(or current) in a circuit. This opposing force is called
resistance (R).
measured in ohms (symbol is Ω).
power ratings.
It is the maximum power that can be dissipated without
raising the temperature too high.
Common standard power ratings are ¼ W, ½ W, 1 W and
2 W.
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Two basic types of resistors.
Linear Resistors
Non Linear Resistors
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Linear Resistors
values change with the applied voltage and temperature
which current value is directly proportional to the applied voltage
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Two types of linear resistors: a) Fixed Resistors b) Variable Resistors.
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Fixed Resistors
specific value and we can’t change the value.
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Types of Fixed resistors.
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Carbon Composition Resistors
Wire Wound Resistors
Thin Film Resistors
Thick Film Resistors
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Construction
made of carbon clay composition covered with a
plastic case. The lead of the resistor is made of
tinned copper.
 Available in wide range of values.
 available in as low as 1 Ω value and as high as 22
MΩ value.
 Tolerance range is of ± 5 to ± 20 %.
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Advantage
easily available in local market in very low cost and
they are very durable too.
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Disadvantage
very much temperature sensitive.
Tendency of electric noise due to passage of
electrical current from one carbon particle to other
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Construction
Formed by wrapping a resistive wire around a nonconducting rod. The rod was usually made of some
form of ceramic that had the desired heat properties
since the wires could become quite hot during use.
End caps with leads attached were then placed over
the ends of the rod making contact to the resistive
wire, usually a nickel chromium alloy.
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available for wide range of ratings.
values varies from 1 Ω to 1 MΩ.
Tolerance limit varies from 0.01 % to 1 %.
Advantages
Different sizes and ratings can easily be achieved by
using different lengths and diameters of the wire.
They can be used for high power applications of 5 to
200 W dissipation ratings.
 Disadvantages
 The cost is much higher than carbon resistor. Normally
is used where carbon composition resistor cannot
meet the purpose because
of its limitations.
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Construction
Made of from high grid ceramic rod and a
resistive material. A very thin conducting
material layer overlaid on insulating rod,
plate or tube which is made from high
quality ceramic material or glass.
Types of thin film resistors.
Metal Film Resistor.
Carbon Film Resistor.
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Construction
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constructed by means of film deposition technique; deposition a thin film of
resistible material such as pure carbon or metal on to an insulating core.
Desired value of resistance of can easily be obtained by either trimming the
layer of thickness or by cutting helical grooves of suitable pitch along its
length. Metallic contact cap is fitted at both ends of the resistor. The caps
must be in contact with resistible film or helical grooves. The lead wires are
welded to these end caps.
Advantages
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can be made up to a value of 10,000 MΩ
size of this type of resistor is much smaller than wire wound resistor.
The accuracy level of metal film resistor can be of order ± 1 % and they are
suitable for high grade applications.
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Note: Carbon film resistor givers lower tolerances and smaller values of
electrical resistance than those available with metal film. But carbon film
posses a mildly negative temperature coefficient of resistance which is very
useful for certain electronic circuit.
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Construction
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same like thin film resistors, but the difference is
that there is a thick film instead of a thin film or
layer of resistive material around.
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Two types of thick film resistors.
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Metal Oxide Resistors
Cermet Oxide Resistors
Fusible Resistors
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Construction
constructed in a similar manner as the carbon film
resistor with the exception that the film is made of tin
chloride at temperatures as high as 5,000OC. Metal oxide
resistors are covered with epoxy or some similar plastic
coating.
Advantages
Available in a wide range of resistance with high
temperature stability.
Level of operating noise is very low and can be used at
high voltages.
Disadvantages
more costly than other types and therefore are only used
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when circuit constraints
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Construction
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The internal area contains on ceramic insulation
materials. And then a carbon or metal alloy film or
layer wrapped around the resistor and then fix it in a
ceramic metal (which is known as Cermet). They are
made in the square or rectangular shape and leads
and pins are under the resistors for easy installation
in printed circuit boards.
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Advantages
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Provide a stable operation in high temperature
because their values do not change with change in
temperature.
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Construction
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Same like a wire wound resistor.
When a circuit power rating increased than the
specified value, then this resistor is fused, i.e. it breaks
or open the circuit. That’s why it is called Fusible
resistors. Fusible resistors perform double jobs means
they limit the current as well as it can be used as a
fuse.
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They used widely in TV Sets, Amplifiers, and other
expensive electronic circuits. Generally, the ohmic
value of fusible resistors is less than 10 Ohms.
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value can be adjusted.
Construction
 Resistive material is deposited on a non-conducting base.
stationary contacts are connected to each end of the resistive
material. Finally, a moving contact or wiper is constructed to
move along the resistive material and tap off the desired
resistance.
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current flowing through it does not change according to Ohm’s Law
but, changes with change in temperature or applied voltage.
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flowing current through a resistor changes with change in body
temperature-Thermisters.
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flowing current through a resistor change with the applied voltagesVaristors or VDR (Voltage Dependent Resistors).
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Flowing current through a resistor change with the light falling on itPhoto Resistor
Different types of Non Linear Resistors.
1.Thermisters
2.Varisters(VDR)
3. Photo Resistor or Photo Conductive Cell or LDR
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Thermally sensitive resistors whose prime function is to
exhibit change in electrical resistance when subjected to
a corresponding change in body temperature.
 made from the cobalt, Nickel, Strontium and the metal
oxides of Manganese.
 Negative Temperature Coefficient (NTC) thermistors
exhibit a decrease in electrical resistance when
subjected to an increase in body temperature
 Positive Temperature Coefficient (PTC)
thermistors exhibit an increase in
electrical resistance when subjected
to an increase in body temperature.
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flowing current through a resistor change
with the applied voltages-Varistors or VDR
(Voltage Dependent Resistors).
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used to protect circuits from destructive
voltage spikes.
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Resistance value changes with light intensity.
The material which is used to make these kinds of
resistors is called photo conductors, e.g. cadmium
sulfide, lead sulfide etc.
When light falls on the photoconductive cells (LDR
or Photo resistor), then there is an increase in the
free carriers (electron hole pairs) due to light
energy, which reduce the resistance of
semiconductor material (i.e. the quantity of light
energy is inversely proportional to the
semiconductor material). It means photo resistors
have a negative temperature coefficient.
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4-band resistors
 First two bands identify the first and second digits of the resistance value,
and the third band indicates the number of zeroes. The fourth determines
the tolerance.
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5-band resistors
 First three bands provide the first three digits of the resistor value. The
third band is only used when the tolerance of the resistor is less than 2%.
The fourth gives the multiplier.The fifth indicates the tolerance of the
resistor.
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6-band resistors
 First five bands have the same meaning as the 5-band resistors. The sixth
band is a temperature coefficient that indicates the change in electrical
conductivity with temperature.
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Colours
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Value
Black
Brown
Red
Orange
Yellow
Green
Blue
Violet
Grey
White
0
1
2
3
4
5
6
7
8
9
Tolerance:
Gold = ±5%
Silver = ±10 %
No colour means 20 %
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simple passive element that is used to ‘store
electricity’.
a component which has the ability or
‘capacity’ to store energy in the form of an
electrical charge producing a potential
difference across its plates, much like a
small rechargeable battery.
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consists of two or more parallel conductive
plates which are not connected or touching
each other, but are electrically separated
either by air or by some form of a good
insulating material such as waxed paper,
mica, ceramic, plastic or some form of a
liquid gel as used in electrolytic capacitors.
The insulating layer between capacitor
plates is commonly called the Dielectric.
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property of a capacitor to store charge on its plates in
the form of an electrostatic field is called
the Capacitance of the capacitor.
 Capacitance, C = ε0 εr A / d
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where A is the area of plates,
d is the plates separation,
ε0 is the permittivity of free space ( 8.84 x 10-12 F/m )
εr is the relative permittivity of the material being used as the
dielectric .
unit of capacitance being the Farad (abbreviated to F)
named after the British physicist Michael Faraday.
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Three main classes of capacitors:
 (i) Non electrolytic or normal capacitors
 (ii) electrolytic capacitors
 (iii) variable capacitors.
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Non electrolytic capacitors are mostly of
parallel plate type and can have mica, paper,
ceramic or polymer as dielectric.
 Mica Capacitors
 Ceramic Capacitors
 Paper Capacitors
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made from plates of Aluminium foil separated by sheets of
mica. The plates are connected to two electrodes. The mica
capacitors have excellent characteristics under stress of
temperature variations and high voltage applications
(~500 V). Available capacitances range from 5 to 10,000
pF.
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A ceramic disc is coated on two sides with a metal,
such as copper or silver. These coatings act as two plates.
After attaching tinned-wire leads, the entire unit is coated
with plastic .
Their working voltage ranges from 3 V up to 6000 V. The
capacitance value ranges from 3 pF to about 3 mF.
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consists of two metal foils separated by strips of paper.
This paper is impregnated with a dielectric material such
as wax, plastic or oil.
have capacitances ranging from 0.0005 mF to several mF,
and are rated from about 100 V to several thousand volts.
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Construction
 Consists of an aluminium-foil electrode which has an
aluminium-oxide film covering on one side. The
aluminium plate serves as the positive plate and the oxide
as the dielectric. The oxide is in contact with a paper or
gauze saturated with an electrolyte. The electrolyte forms
the second plate (negative) of the capacitor. Another layer
of aluminium without the oxide coating is also provided
for making electrical contact between one of the
terminals and the electrolyte. In most cases, the negative
plate is directly connected to the metallic container of the
capacitor. The container then serves as the negative
terminal for external connections.
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Disadvantage
 Relatively low voltage rating and due to the polarization of electrolytic
capacitors.
 They must not be used on AC supplies.
 Two basic forms; Aluminium Electrolytic Capacitors and Tantalum
Electrolytic Capacitors.
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Capacitance may be intentionally and repeatedly changed
mechanically. Variable capacitors are often used in L/C
circuits to set the resonance frequency, or as a variable
reactance for impedance matching in antenna tuners.
 The most common variable capacitor is the air-gang capacitor. The
dielectric for this capacitor is air. By rotating the shaft at one end,
we can change the common area between the movable and fixed
set of plates. The greater the common area, the larger the
capacitance.
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In some applications, the need for variation in the
capacitance is not frequent. One setting is sufficient
for all normal operations. In such situations, we use
a variable capacitor called a trimmer (sometimes
called padder). Both mica and ceramic are used as
the dielectric for trimmer capacitors.
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Electrolytic Capacitors
There are two designs of electrolytic
capacitors: (i) Axial where the leads are
attached to each end (220µF in picture) and
(ii) Radial where both leads are at the same
end (10µF in picture).
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Non-polarised capacitors ( < 1µF)
 Small value capacitors have their values printed but
without a multiplier. For example 0.1 means 0.1µF.
Sometimes the unit is placed in between 2 digits
indicating a decimal point. For example: 4n7
means 4.7nF.
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If the number written on the capacitor is greater than one, the value will be in
pF. Otherwise, it will be in μF. For example, 10 means 10 pF and 0.1 means 0.1
μF.
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If there are three digits in the number, the third number indicates the number
of zeros to be put after first two digits and the value will be in pF.104 means
10,0000 pF or 0.1 μF
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If the letter k follows the digits, the value will be in kpF (kilo picofarad). 10 k
means 10 kpF or 0.01 μF.
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If the letter is ‘n’ or ’M’ the value will be that much nano farads or micro farads
respectively. 47n means 47 nF and 47M means 47 μF.
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If the letter n, M or k is between two numerals, the value of the capacitor can be
obtained by putting a decimal in place of the letter and multiplying by the
factor nF, μF or kpF respectively.4k7 means 4.7 kpF and 2M2 means 2.2 μF.
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If the letters k or M follows the three digit number, it implies the tolerance
value 10% and 20% respectively.
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Sometimes capacitors just show bands like resistors when
printing is tough on them. The colours should be read like the
resistor code, the top three colour bands giving the value in pF.
The 4th band and 5th band are for tolerance and voltage rating
respectively. For example:
brown, black, orange means
10000pF = 10nF = 0.01µF.
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Passive components consisting of coils of insulated copper wire wound
around a former that will have some type of core at its centre. This core
might be a metal such as iron that can be easily magnetised; or in high
frequency inductors, it will more likely to be just air.
 A single wire with electricity flowing through it has a small magnetic field
around it. When the wire is coiled or looped with one or more turns the
magnetic field add together and reinforce the magnetic effect. This magnetic
field helps to store the electric current for a short time, even if the supply is
removed. When the magnetic field around the coil collapses, the electric
current also falls off.
 Working of an inductor is based on the Faraday’s Laws of Induction. The
magnetic field is referred to as magnetic flux. For most coils the current, ( i )
flowing through the coil produces a magnetic flux, ( Nφ ) around it that is
proportional to this flow of electrical current.
 But unlike a Capacitor which opposes a change of voltage across their plates,
an inductor opposes the rate of change of current flowing through it due to
the build up of self-induced energy within its magnetic field.
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Inductors resist or oppose changes of current but will
easily pass a steady state DC current. This ability of an
inductor
to
resist
changes
in
current
is
called Inductance which is given the symbol L .
 Inductance of a coil is measured in Henry’s. One Henry is the
amount of inductance required to produce an e.m.f. of 1 volt in
a conductor when the current in the conductor changes at the
rate of 1 Ampere per second.
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The amount of inductance in an inductor is dependent on:
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The number of turns of wire in the inductor.
The material of the core.
The shape and size of the core.
The shape, size and arrangement of the wire making up the
coils.
 Applications
 Inductors are used in many analog circuits and are also used
along with capacitors for forming filter circuits and thus signal
processing. They are also used in Switched Mode Power
Supplies (SMPS), oscillators, transmitters, receivers, voltage
regulators and also for over voltage protection.
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Inductors are normally made in the form of a coil. The reason for this is that the
magnetic field is linked between the windings and builds up. As the permeability of the
medium in which the coil is located has a major effect on the inductance.
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Cores such as iron, ferrite and other magnetic materials are used. These all significantly
increase the level of inductance that can be obtained, but care has to be taken in the
choice of core to ensure its performance is suitable for the power level, frequency and
general application of the inductor.
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Air Core - Higher frequency operation due to no core losses but a lower inductance.
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Iron Core - Low resistance with high inductance. Core losses, eddy currents, magnetic
saturation limit the operating frequency and current.
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Ferrite Core - Non-conductive ceramic material for higher frequency operation.
Magnetic saturation limits the current capacity.
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Torroidal Core – it reduces radiated EMI and provides high inductance.
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Laminated Core - High inductance with lower eddy current losses.
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Inductors have a wide variety and important applications in electronics.
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Coupled Inductors
 Coupled inductors are types of inductors that share a magnetic path and
influence each other. Coupled inductors are often used as transformers to
step up or step down voltage, provide isolated feedback, and in applications
where mutual inductance is required.
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Multi-layer Inductors
 Multi-layer inductors get their name from the layers of coiled wire that is
wound around a central core. Adding additional layers of coiled wire to an
inductor increases the inductance but also increases the capacitance
between the wires.
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Molded Inductors
 Inductors that are molded in to a plastic or ceramic housing are known as
molded inductors. Generally these inductors have a cylindrical or bar form
factor and can be found with several types of winding options.
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RF Inductors
 High frequency types of inductors, also called radio
frequency or RF inductors, are designed to operate at
high frequencies. These inductors often have a higher
resistance and lower current rating. Most RF inductors
have an air core rather than use a ferrite or other
inductance boosting core material due to the increase
in losses when those core materials are used which
would reduce the operating frequency of the inductor.
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Chokes
 A choke is an inductor that is designed to block high frequency pulses
while letting lower frequency pulse through. Their names comes
from the choking off or blocking of high frequency signals. There are
two classes of chokes, power chokes and RF chokes. Power and audio
frequency chokes typically have a iron core to increase
their inductance and make them more effective filters. RF chokes use
iron powder or ferrite beads combined with complex winding
patterns to reduce parasitic capacitance and operate effectively at
high frequencies.
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Surface Mount Inductors
 The push for smaller and more mobile devices has led to the
explosion in options for surface mount types of inductors. Surface
mount inductors are often used in DC-DC converters, EMI filtering,
energy storage, and other applications. Their small size and footprint
make surface mount inductors an essential element in the mobile
and portable electronic designer's component toolbox.
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It is formed by moving the magnetic core in
and outside of the inductor windings. By
this magnetic core we can adjust the
inductance value.
 These types of inductors are used in radio and
high frequency applications where the tuning is
required. These inductors are typically ranged
from 10 μH to 100 μH and in present days these
are ranged from 10nH to 100 mH.
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Capasitors
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Blocks Direct Current
Passes Alternating Current
Voltage in Capacitor cannot change instantly
Quick Voltage change produces large Current
Stores Energy in Electric Field
Inductors
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Blocks Alternating Current
Passes Direct Current
Current in an Inductor cannot change instantly
Quick Current change produces large Voltage
Stores Energy in Magnetic Field
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Static device which transforms electrical
energy from one circuit to another without any
direct electrical connection and with the help
of mutual induction between two windings.
It transforms power from one circuit to
another without changing its frequency but
may be in different voltage level.
 Transformers are commonly used in applications
which require the conversion of AC voltage from
one voltage level to another.
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Two broad categories of transformers:
 Electronic transformers, which operate at very low power
levels, and power transformers, which process thousands of
watts of power.
 Electronic transformers are used in consumer electronic
equipment like television sets, CD players, personal
computers, and many other devices, to reduce the level of
voltage from 220V (available from the AC mains) to the
desired level at which the device operates.
 Power transformers are used in power generation,
transmission and distribution systems to raise or lower the
level of voltage to the desired levels. The basic principle of
operation of both types of transformers is the same.
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Need two coils having mutual inductance
and a laminated steel core. The two coils are
insulated from each other and from the
steel core. The device will also need some
suitable container for the assembled core
and windings, a medium with which the
core and its windings from its container can
be insulated.
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Works on the principle of mutual induction of
two coils or Faraday Law’s Of Electromagnetic
induction.
 Faraday’s law states that a voltage appears across
the terminals of an electric coil when the flux
linkages associated with the same changes. This emf
is proportional to the rate of change of flux linkages.
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e=dφ/dt
Where, e is the induced emf in volt and φ is the
flux linkages in Weber turn.
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Consists of two inductive coils; primary winding and
secondary winding. The coils are electrically separated but
magnetically linked to each other. When, primary winding
is connected to a source of alternating voltage, alternating
magnetic flux is produced around the winding.
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The core provides magnetic path for the flux, to
get linked with the secondary winding. Most of the
flux gets linked with the secondary winding. As the
flux produced is alternating, emf gets induced in the
secondary winding according to Faraday's law of
electromagnetic induction. This emf is called
'mutually induced emf', and the frequency of
mutually induced emf is same as that of supplied
emf. If the secondary winding is closed circuit, then
mutually induced current flows through it, and
hence the electrical energy is transferred from one
circuit (primary) to another circuit (secondary).
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As the transformer is basically a linear device, a ratio
exists between the numbers of turns of the primary coil
divided by the number of turns of the secondary coil.
This ratio, called the ratio of transformation, more
commonly known as a transformers “turns ratio”, (TR).
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It is necessary to know the ratio of the number of turns
of wire on the primary winding compared to the
secondary winding. The turns ratio, which has no units,
compares the two windings in order and is written with
a colon, such as 4:1 (4-to-1). This means in this example,
that if there are 4 volts on the primary winding there
will be 1 volt on the secondary winding, 4 volts-to-1
volt.
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N1 = Number of turns in primary
N2 = Number of turns in secondary
Ømax = Maximum flux in the core in webers = Bmax X A
f = Frequency of alternating current input in hertz (HZ)
As shown in figure above, the core flux increases from its zero value to
maximum value Ømax in one quarter of the cycle , that is in ¼ frequency
second.
Therefore, average rate of change of flux = Ømax/ ¼ f = 4f ØmaxWb/s
Now, rate of change of flux per turn means induced electro motive force in
volts.
Therefore, average electro-motive force induced/turn = 4f Ømaxvolt
If flux Ø varies sinusoidally, then r.m.s value of induced e.m.f is obtained by
multiplying the average value with form factor.
Form Factor = r.m.s. value/average value = 1.11
Therefore, r.m.s value of e.m.f/turn = 1.11 X 4f Ømax = 4.44f Ømax
Now, r.m.s value of induced e.m.f in the whole of primary winding =
(induced e.m.f./turn) X Number of primary turns
Therefore, V1 = 4.44f N1Ømax = 4.44fN1BmA
Similarly, r.m.s value of induced e.m.f in secondary is
V2 = 4.44f N2 Ømax
= 4.44fN2BmA
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V2/ V1 = N2/N1 = K
This constant K is known as voltage transformation
ratio.
(1) If N2>N1 , that is K>1 , then transformer is
called step-up transformer.
(2) If N2<N1, that is K<1 , then transformer is
known as step-down transformer.
For an ideal transformer,
Input V1 = output V2
V1I1 = V2I2
Or, I2/I1 = V1/V2 = 1/K
Hence, currents are in the inverse ratio of the
(voltage) transformation ratio.
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Based on voltage levels, design, Core medium
used, winding arrangements and the type of
cooling employed.
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Classified as step-up and step-down
transformers as the voltage ratios from
primary to secondary.
 Important thing to remember that there will not
be any difference in primary power and
secondary power. That means if the voltage is
high at secondary side then the current drawn
from the secondary will low so that the power
will be same.
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 The secondary voltage is stepped up with a ratio
compared to primary voltage. This is achieved
by increasing the number of coil turns in the
secondary as shown in figure.
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Voltage is stepped down at the secondary
from high voltage primary so that it is called
as step-down transformer.
 The winding turns will be high at primary side
where as it will less at secondary side.
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Devices which carry out electrical
operations by using moving parts are
known as electromechanical devices.
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Devices which involve an electrical signal to
create
mechanical
movement,
or
mechanical movement to create an electric
signal.
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A relay is an electrically operated switch.
 Many relays use an electromagnet to
mechanically operate a switch.
 Relays are used where it is necessary to control
a circuit by a low-power signal or where several
circuits must be controlled by one signal.
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Electromechanical relays are electrically operated
switches that rely on mechanical contacts as the switch
mechanism.
 Relay consists a inductor coil, a spring, Swing terminal
(armature), and two high power contacts named as normally
closed (N/C) and normally opened (N/O). Relay uses an
Electromagnet to move swing terminal between two contacts
(N/O and N/C). When there is no power applied to the inductor
coil (Relay is OFF), the spring holds the swing terminal is
attached to NC contact.
 Whenever required power is applied to the inductor coil, the
current flowing through the coil generates a magnetic field
which is helpful to move the swing terminal and attached it to
the normally open (NO) contact. Again when power is OFF, the
spring restores the swing terminal position to NC.
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A switch has at least two terminals: one for the signal to go in and one for the
signal to go out.
 The poles of a switch are the input terminals; these define how many separate
circuits the switch can control.
 The throws of a switch are the output terminals; these define the number of different
output connections each switch pole can connect its input to.
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A relay is said to switch one or more poles. Each pole has contacts that can be
thrown in mainly three ways. They are
 Normally Open Contact (NO) – NO contact is also called a make contact. It closes
the circuit when the relay is activated. It disconnects the circuit when the relay is
inactive.
 Normally Closed Contact (NC) – NC contact is also known as break contact. This is
opposite to the NO contact. When the relay is activated, the circuit disconnects.
When the relay is deactivated, the circuit connects.
 Change-over (CO) / Double-throw (DT) Contacts – This type of contacts are used
to control two types of circuits. They are used to control a NO contact and also a NC
contact with a common terminal. According to their type they are called by the
names break before make and make before break contacts.
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There are two types of electromechanical
relays: latching and non-latching.
 A latching relay maintains its state after being
actuated. It has no default position and remains
in its last position when the drive current stops
flowing. A latching relay has internal magnets
that hold the relay once current starts flowing;
this reduces energy because once actuated, it
requires no current flow to maintain its
position.
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A relay can also be classified by its number of throws and poles.
 Single Pole Single Throw (SPST) – This type of relay has a total of
four terminals. Out of these two terminals can be connected or
disconnected. The other two terminals are needed for the coil.
 Single Pole Double Throw (SPDT) – This type of a relay has a total
of five terminals. Out f these two are the coil terminals. A common
terminal is also included which connects to either of two others.
 Double Pole Single Throw (DPST) – This relay has a total of six
terminals. These terminals are further divided into two pairs. Thus
they can act as two SPST’s which are actuated by a single coil. Out of
the six terminals two of them are coil terminals.
 Double Pole Double Throw (DPDT) – This is the biggest of all. It
has mainly eight relay terminals. Out of these two rows are
designed to be change over terminals. They are designed to act as
two SPDT relays which are actuated by a single coil.
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Advantages of relay:
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Contacts can switch AC or DC
Low initial cost
Very low contact voltage drop, thus no heat sink is required
High resistance to voltage transients
No Off-State leakage current through open contacts
Limitations of Electromechanical relay
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Low speed of operation.
Change in characteristics over a period due to ageing effect.
Shorter mechanical lifetime.
Component failure leading to relay failure.
Relay is Bulky: Because there are internal mechanical components
with physical dimension restraints, the package size of an
electromechanical Relay can limit the size of a PCB design Excessive
power consumption.
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 Relays are used to realize logic functions. They play a
very important role in providing safety critical logic.
 Relays are used to provide time delay functions. They
are used to time the delay open and delay close of
contacts.
 Relays are used to control high voltage circuits with the
help of low voltage signals. Similarly they are used to
control high current circuits with the help of low
current signals.
 They are also used as protective relays. By this function
all the faults during transmission and reception can be
detected and isolated.
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When a relay is used to switch a large
amount of electrical power through its
contacts, it is designated by a special
name: contactor.
 Contactors typically have multiple contacts, and
those contacts are usually (but not always)
normally-open, so that power to the load is shut
off when the coil is de-energized. Perhaps the
most common industrial use for contactors is
the control of electric motors.
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The top three contacts switch the respective
phases of the incoming 3-phase AC power,
typically at least 480 Volts for motors 1
horsepower or greater. The lowest contact is
an “auxiliary” contact which has a current
rating much lower than that of the large motor
power contacts, but is actuated by the same
armature as the power contacts. The auxiliary
contact is often used in a relay logic circuit, or
for some other part of the motor control
scheme, typically switching 120 Volt AC power
instead of the motor voltage.
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