Module – 5

Classification of transducers – Selection

of transducers – Resistive, capacitive &
inductive transducers – Piezoelectric.
Working principle and specifications of
the Analog CRO and digital CRO, LED
and LCD
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 TRANSDUCER – INTRODUCTION
 A transducer is an electronic device that converts energy from
one form to another for various purposes like measurement or
information transfer.
 It provides a usable output in response to a specified input
measurand, which may be a physical or mechanical quantity, or a
property or conditions.
 Common examples include microphones, loudspeakers,
thermometers, position and pressure sensors, and antenna.

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CLASSIFICATION OF TRANSDUCERS
TRANSDUCERS

ELECTRICAL MECHANICAL

ACTIVE PASSIVE BASED ON TRANSDUCTION ELEMENTS

RESISTIVE PIEZOELECTRIC

INDUCTIVE CAPACITIVE

OPTICAL DIGITAL

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 TRANSDUCER
 Transducer is a device which is used to convert one form
of input into another form of output.
 The conversion can be to/from electrical, electro-
mechanical, electromagnetic, photonic, photovoltaic, or
any other form of energy.
 While the term transducer commonly implies use as a
sensor/detector, any device which converts energy can be
considered a transducer.

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 TYPES OF TRANSDUCER
 Resistive transducer
 Inductive
 Capacitive
 Electromagnetic
 Piezoelectric
 Photo-emissive
 Photo-resistive
 Potentiometric
 Thermo-electric

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 ELECTRICAL TRANSDUCER
 An electrical transducer is a sensing device by which the physical
,mechanical or optical quantity to be measured is transformed
directly by a suitable mechanism into an electrical signal
proportional to the input signal.
 They can be classified as
 Active
 Passive
 Based on transductional elements.

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An electrical transducer must have the following parameters:

 LINEARITY:
The relationship between the physical parameter and the resulting electrical
signal must be linear.
 SENSITIVITY:
This is defined as the electrical output per unit change in the physical
parameter.
 DYNAMIC RANGE:
The operating range of the transducer should be wide.
 REPEATABILITY:
The input/output relationship for a transducer should be predictable over a
long period of time. This ensures reliability of operation.
 PHYSICAL SIZE:
The transducer must have minimal weight and volume, so that its presence
does not disturb the existing conditions.

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ACTIVE & PASSIVE TRANSDUCER
 An active transducer are self generating devices, which operate
under energy conversion principle and generate an equivalent
output signal.
 Examples: Piezoelectric sensors , Photovoltaic cells.
 Passive transducers operate under energy controlling
principles which makes it necessary to use an external source.
They depend upon the change in an electrical parameter( R, L,
or C).
 Examples: Strain gauges, Thermistors

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 Based on transduction elements
 Transducer are classified into different categories depending
upon the phenomena employed to convert physical into
output electric signals.
 Resistive
 Inductive
 Capacitive
 Piezoelectric
 Optical
 Digital

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RESITIVE TRANSDUCER
 Resistance changes due to change in physical phenomenon, which,
when associated with change in length of the conductor can be used
to measure displacement.
RESISTIVE
TRANSDUCER

RESISTANCE RESISTIVE RESISTANCE

POTENTIOMETER PRESSURE POSITION THERMISTOR STRAIN GAUGES
THERMOMETER
TRANSDUCER TRANSDUCER

TRANSLATORY WIRE
TYPE

FOIL
ROTATIONAL TYPE

SEMICONDUCTOR
HELICAL POT

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Potentiometer
 Consist of a resistive element
provided with a sliding contact
called wiper.
 The motion of the sliding contact
may be translatory, rotational or
combination of both – helical
 Translatory Pots are linear or
straight devices. Rotational pots are
circular, are used for the
measurement of angular
displacement.
 Helical pots are multi turn
rotational devices which can be
used for the measurement of either
translatory or rotational motion.
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 ADVANTAGES AND DISADVANTAGES OF
POTENTIOMETER
 ADVANTAGES:
 They are inexpensive.
 They are simple to operate and useful for the measurement of large
amplitudes of displacement.
 Electrical efficiency is high, and they provide sufficient output to allow
control operations.

 DISADVANTAGES:
 A large force is required to move the sliding contacts.
 The sliding contacts can wear out, become misaligned and generate noise.

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Resistance Pressure transducer
 Change in pressure results in a
resistance change in the sensing
elements.

 The element moved by the

pressure change is made to cause a
change in resistance.

 This resistance change can be taken

as either AC or DC signal to
determine the pressure indication.

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 RESISTIVE POSITION TRANSDUCER

 The physical variable under

measurement causes a
resistance change in the
sensing element.
 The output voltage depends
on the wiper position, and
hence depends on the
position of the shaft.

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 STRAIN GAUGES
 Uses the variation in the electrical resistance in wires to sense the
strain produced by a force on the wires.
 The change in resistance value of the conductor under strain is
enough for an increase in resistance due to dimensional changes.
This property is called PIEZO- RESISTIVE effect.
 Used in two basic forms:
 Unbonded resistance wire strain gauge.
 Bonded resistance wire strain gauge.

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 UNBONDED RESISTANCE WIRE STRAIN GAUGE.

 Consists of a wire stretch

between two points in a
insulating medium.
 The wires are kept under
tension so that there is no sag
and no free vibration.
 When an external load is
applied the resistance of the
strain gauge changes resulting in
an output voltage which is
proportional to the strain.

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 Bonded resistance wire strain gauge.

 A fine wire element is looped back and forth on a mounting plate.

 This carrier is then cemented to the object bieng studied.
 Due to the strain, the gauge resistance and the length of the wire get
changed.
 The Gauge Factor can be defined as the ratio of change in resistance to
change in length
o GF=(∆R/R)/(∆l/l)
o =1 + 2μ

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LVDT
Working
 DEFINITION:
• PIEZOELECTRIC TRANSDUCER is a
device which transforms one type of
energy to another by taking advantage
of the piezoelectric properties of certain
crystals or other materials.

• When a piezoelectric material is

subjected to stress or force, it generates
an electrical potential or voltage
proportional to the magnitude of the
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force.
  This makes the piezoelectric transducer ideal as a
converter of mechanical energy or force into
electric potential.

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 WORKING:
 Asymmetrical crystalline materials such as quartz , Rochelle
salt and Barium titanate produce an Emf when they are
placed under stress.
 This property is used in piezoelectric transducers , where a
crystal is placed between a solid base and the force summing
member.

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 PIEZOELECTRIC TRANSDUCER

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  An externally applied force, entering the transducer
through it’s pressure port, applies pressure to the
top of a crystal.
 This produces an Emf across the crystal proportional
to the magnitude of applied pressure.
 Since the transducer has a very good HF response,
it’s principle use is in HF accelerometers.
 In this application , it’s output voltage is typically of
the order of 1-30mV per gm of acceleration.

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 •The device needs no external power source
and is therefore self generating.
•The disadvantage is that it cannot measure
static conditions.
•The output voltage is also affected by
temperature variation of the crystal .
•The basic expression for output voltage E is
given by
E = Q/Cp
Where Q – generated charge.
Cp – shunt capacitance

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 This transducer is inherently a dynamic responding
sensor and does not readily measure static conditions.

 For a piezoelectric element under pressure , a part of

the energy is converted to an electric potential that
appears on opposite faces of the element ,analogous to a
charge on the plates of a capacitor.
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  The rest of the applied energy is converted to mechanical
energy ,analogous to a compressed string.
 When the pressure is removed ,it returns to it’s original
shape and loses it’s electric charge.
 An alternating voltage applied to a crystal causes it to vibrate
at it’s natural resonance frequency.
 Since the freq is a very stable qty , piezoelectric crystals are
principally used in HF accelerometers.
 The principal disadvantage is that vtg will be generated as
long as the pressure applied to the piezoelectric element
changes.

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 ADVANTAGES
 Piezoelectric devices are a very reliable.
 Inexpensive means of converting electrical energy into physical
motion and exhibit a high tolerance to environmental factors such as
electromagnetic fields and humidity.

APPLICATIONS:
 The high sensitivity of the piezoelectric transducer makes it useful in
microphones.

 Piezoelectric transducers are also used in non-destructive testing, in

the generation of high voltages, and in many other applications
requiring the precise sensing of motion or force.

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PIEZOELECTRIC BUZZER

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31
 CRO – INTRODUCTION
• A cathode ray oscilloscope is a very useful and versatile laboratory
instrument.
• It is used for measurement, display and analysis of waveform and
other phenomena in Electric and Electronic circuits.
• The main parts of CRO are:
i) CRT-cathode ray tube
ii) Vertical amplifier
iii) Delay line
iv) Trigger circuit
v) Time base generator
vi) Horizontal amplifier
vii) Power Supply
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 CATHODE RAY TUBE
• This is the heart of CRO
• It is used to emit the electrons required to strike the phosphor
screen to produce a spot for visual display of the signals.
VERTICAL AMPLIFIER
• The vertical amplifier stage is used to amplify the signals.
• They are generally wide band amplifiers so as to pass the entire
band of frequency to be measured faithfully.
TRIGGER CIRCUIT
• It is necessary that horizontal deflection starts at the same point of
the input vertical signal, each time it sweeps
• Hence to synchronize horizontal deflection with vertical
deflection, a synchronizing or triggering circuit is used.
• It converts the incoming signal into the triggering pulses, which
35 are used for the synchronization.
 TIME BASE GENERATOR
• The time base generator is used to generate the saw-tooth voltage,
required to deflect the beam in the horizontal section.
• This voltage deflects the spot at a constant time dependant rate.
• This the X-axis on the screen can be represented as time, which helps to
display and analyze the time varying signals.

HORIZONTAL AMPLIFIER
• The saw-tooth voltage produced by the time base generator may not
be of sufficient strength. Hence before giving it to the horizontal
deflection plates, it is amplified using the horizontal amplifier.
POWER SUPPLY
The power supply block provides the voltages required by CRT to
generate and accelerate an electron beam and voltages required by
other circuits of the oscilloscope like horizontal, vertical amplifier,
etc.
There are 2 sections: High voltage section & Low voltage section
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 CRT
• A Cathode Ray Tube(CRT) is the heart of the CRO

• It is used to emit the electrons required to strike the phosphor

screen to produce the spot for the visual display of the signals

• The main parts of the CRT are:

i) Electron gun assembly
ii) Deflection plate assembly
iii) Fluorescent screen
iv) Glass envelope
v) A base through which connections are made to various parts

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38
 ELECTRON GUN
• The “Electron Gun Assembly” produces a
sharply focused beam of electrons which are
accelerated to a high velocity

• This focused beam of electrons strikes the

fluorescent screen with sufficient energy to
cause a luminous spot on the screen

• The electrons are emitted from the cathode

and then they pass through the hole grid and
are accelerated by the high positive potential
which is applied to the “pre-accelerating” and
“accelerating anodes”.
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 DEFLECTION PLATES
• The electrons, after leaving the electron gun, passes through two
pairs of deflection plates

• One pair of plates is mounted horizontally an produces an

electric field an electric field in the vertical plane. This pair
produces a vertical deflection and is called “Y-plates”

• The other pair of plates is mounted vertically and produces a

horizontal deflection. It is known as “X-plates”

• These plates are flared so as to allow the beam to pass through

them without striking the plates.
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 FLUORESCENT SCREEN
• The front of the CRT is called the face plate

• The inner surface of the face plate is coated with phosphor. It consists of
very pure inorganic crystalline phosphor crystals, about 2-3 microns in
diameter, known as ACTIVATORS

• When an electron beam strikes the phosphor crystal, it raises their

energy level. This is known as CATHODOLUMINESCENCE

• When the cathode beam is switched off, the phosphor crystals return to
their initial state, and release a quantum of light energy. This is known as
PHOSPHORESCENCE or PERSISTENCE

• There are colour CRT’s available which make use of multicomponent

phosphor giving peak outputs of orange and cyan light

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 GLASS ENVELOPE
• The working parts of a CRT are enclosed in an evacuated glass
envelope so that the emitted electrons are able to move freely from
one end of the tube to the other end
• A typical glass envelope of a CRT would look like this

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 BASE
• The base is the important part of the
CRT which has many pins attached to
it.

• These pins are connected to the

power supply or some other wires
which are used for displaying the
output.

• It is through the base that the

connections are made to various parts
of the CRT.

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 ADVANTAGES OF CATHODE RAY TUBES:
i. They offer better response time than LCD’s.
ii. They have a higher contrast than LCD’s.
iii. They can be viewed form any angle unlike LCD monitors which can
be viewed only upto a certain angle, beyond which they seem
ambiguous.
iv. Colour saturation is excellent because they make use of light beams
and phosphors, which are good colour projectors.

DISADVANTAGES OF CATHODE RAY TUBES:

i. The CRT monitors are comparatively heavier than the modern
display systems.
ii. The resolution of CRT is lesser.
iii. If they are operated for long hours, it may result in the burning of
phosphors.
iv. The electron beams that make up the image are distorted when the
CRT TV or CRT monitor is placed near an object with strong
44 magnetic effect.
 APPLICATIONS OF CATHODE RAY TUBES:

1. Although LCD’s and LED’s are the latest technology

available, CRT’s are still favoured by millions of people for
either personal or professional work.
2. They are very useful in the field of medicine, graphic
designing, video editing and printing.
3. The CRT’s are also used in oscilloscopes which have a variety
of uses in physical and electrical engineering applications.

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 NEED FOR CRO IN ELECTRONIC PRACTICALS

• Measurement of A.C. and D.C. voltages.

• Measurement of current.
• Measurement of frequency and phase shift between input and
output waveforms.
• Testing of various components.
• Measurement of distortion in a waveform.

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 Digital Storage Oscilloscope

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 Modes of Operation
 Roll Mode:
This mode is used to display very fast varying signals on the
screen. The signal is displayed as if it is changing slowly on
the screen
 Store mode:
In this mode, the input initiates the trigger circuit, memory
write cycle. Digital data transferred to memory. When
memory is full write cycle stops. Next trigger occurs when
memory is refreshed
 Hold or Save mode:
This is automatic refresh mode. Sweep signal is generated by
time base generator, old contents gets over written. Storage
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time is infinite.
 LIGHT EMITTING DIODE

The LED is based on the semiconductor diode.

When a diode is forward biased (switched

on), electrons are able to recombine with holes
within the device, releasing energy in the form
of photons. This effect is called
electroluminescence and the color of the light
(corresponding to the energy of the photon) is
determined by the energy gap of the
semiconductor.

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50
 HOW DOES IT WORK?
•A typical LED
needs a p-n
junction.
•There are a lot of
electrons and holes Electrical
P-n
at the junction due junction Contacts
to excitations.
•Electrons from n need to be
injected to p to promote
recombination.
•Junction is biased to produce even more e-h and to
inject electrons from n to p for recombination to
happen.
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Recombination produces light!!
 LED’S of different colors are available based upon the doping
materials:
i. Gallium Arsenide (GaAs)-Red
ii. Gallium Arsenide phosphide (GaAsP)-red or yellow
iii. Gallium Phosphide (GaP)-Red or green
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ADVANTAGES:
A) They are very small in size and it can be considered as an point source . Thus they
can be stacked in a high density matrix to serve as a numeric and alpha numeric
display .

B) The output light obtained from LED is a function of current flowing through it.
Therefore it can be used as a displaying device under different ambient conditions.

C) LEDs emits light in different color viz.., red, amber, green and yellow are most
commonly available.

D) LED are very fast devices , having a turn ON-OFF time less than 1ns.

E) The low supply voltage and current requirements of LEDs make them more
compatible.

F) The light emitted by LED travel only in one direction.

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54
 Bargraph 7-segment Starburst Dot matrix

LED Displays
LED displays are packages of many LEDs arranged in a pattern, the
most familiar pattern being the 7-segment displays for showing
numbers (digits 0-9). The pictures below illustrate some of the
popular designs

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 LIQUID CRYSTAL DISPLAY
 DEFINITION
 TYPES OF LC(LIQUID CRYSTAL)
 CONSTRUCTION
 WORKING
 POLARISATION
 LCD TYPES
 ADVANTAGES
 DISADVANTAGES

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Liquid crystal:
 Liquid crystals (LCs) are a state of matter that has properties
between those of a conventional liquid and those of a solid
crystal. For instance, an LC may flow like a liquid, but its
molecules may be oriented in a crystal-like way.

 Liquid crystals are more heat sensitive than usual liquids. A little
amount of heat can easily turn the liquid crystal into a liquid.

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 Liquid Crystal Display (LCD)
 A liquid crystal display (LCD) is a thin, flat electronic visual
display that uses the light modulating properties of liquid
crystals(LCs). LCs do not emit light directly.
 It is an electronically modulated optical device made up of any
number of pixels filled with liquid crystals and arrayed in front of
a light source (backlight) or reflectors to produce images in
colour or monochrome.
 We get the definition of LCD from the name “Liquid Crystal”
itself. Solids usually maintain their state unlike liquids who
change their orientation and move everywhere in the particular
liquid
 Liquid crystal materials show more of a liquid state than that of a
solid
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 Types of Crystal

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 WORKING

60
 WORKING
 The main principle behind liquid crystal molecules is that when
an electric current is applied to them, they tend to untwist.

 This causes a change in the light angle passing through them.

 This causes a change in the angle of the top polarizing filter with
respect to it.

 So little light is allowed to pass through that particular area of

LCD. Thus that area becomes darker comparing to others.
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  For making an LCD screen, a reflective mirror has to be setup in
the back.

 An electrode plane made of indium-tin oxide is kept on top and

a glass with a polarizing film is also added on the bottom side.

 The entire area of the LCD has to be covered by a common

electrode and above it should be the liquid crystal substance.

 Next comes another piece of glass with an electrode in the shape

of the rectangle on the bottom and, on top, another polarizing
film.

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  It must be noted that both of them are kept at right angles.

 When there is no current, the light passes through the front of

the LCD it will be reflected by the mirror and bounced back.

 As the electrode is connected to a temporary battery the current

from it will cause the liquid crystals between the common-plane
electrode and the electrode shaped like a rectangle to untwist

 Thus the light is blocked from passing through. Thus that

particular rectangular area appears blank

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 POLARISATION

Linear polarizing filter allows only one axis to flow

 Two linear polarizing filter placed perpendicular blocks all light

 The only way to get this light axis through the filters is to twist it
90⁰

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 LCD TYPES
There are two types

 Dynamic scattering type

 Field effect type

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 DYNAMIC SCATTERING TYPE
 The liquid crystal material may be one of the several organic
compounds which exhibit optical properties of a crystal though they
remain in liquid form.

 LC is layered between glass sheets with transparent electrodes

deposited on the inside faces.

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  When a potential is applied across the cell charge carriers
flowing through the liquid disrupt the molecular alignment and
produce turbulence.

 When the liquid is not activated, it is transparent.

 When the liquid is activated the molecular turbulence causes

light to be scattered in all directions & the cell appears to be
bright.

 The phenomenon is called DYNAMIC SCATTERING.

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 FIELD EFFECT TYPE
 The construction of a field effect LCD is similar to that of the dynamic
scattering type with the exception that two thin polarizing optical are
placed at the inside of each glass sheet.

 The LC material in the field effect is also of different type from that
employed in the DYNAMIC SCATTERING CELL.
 The material used is twisted nematic type and actually twists the light
passing through the cell when the latter is not energized.

 This allows the light to pass through the optical filters and the cell
appears bright.

 When the cell is energized no twisting of light takes place and the cell
appears dull.
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69
 ADVANTAGES
 Low power dissipation
 Low cost
 Better than LED in power consumption
 LCD TVs can perform much better in low light conditions
because the picture they produce has a much stronger vibrancy.
 Thin, with a small footprint.
 Consume little electricity and produce little heat
 screen that is easier on the eyes, reducing strain and headaches
that sometimes accompanied long hours on a CRT
 flicker free, less weight

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 DISADVANTAGE
 Slow devices-turn on in few milli seconds and turn off in ten
milli seconds

 They are used in a.c supply having frequency less than 500Hz.

 Limited viewing angle. Brightness, contrast, gamma and color

mixtures vary with the viewing angle. Can lead to contrast and
color reversal at large angles. Need to be viewed as close to
straight ahead as possible

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 GAUGE FACTOR

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