Recorders.ppt

• It is often necessary to have a permanent record of a
phenomenon being investigated, e.g., in industrial & research
processes, it is required to monitor continuously the condition,
state, or value of the process variables such as flow, pressure,
temperature, current, & voltage etc.
• A recorder, thus, records electrical/non-electrical quantities as
a function of time
• This record may be written/printed and later on can be
examined & analyzed to obtain a better understanding and
control of the process
• Currents/voltages can be recorded directly while non-electrical
quantities are recorded indirectly by first converting them to
equivalent currents/voltages with the help of sensors or
transducers
Introduction to Recorders
1

Classification of Recorders
• Depending on how the data acquired is recorded? The
recorders are classified as follow:
(A) Analog Recorders:
• When we are dealing with a wholly analog system
• Broadly, analog recorders are classified into:
(1) Graphic Recorders
(2) Oscillographic Recorders
(3) Magnetic-Tape Recorders
(B) Digital Recorder:
• For the system whose output is in digital form
2

(1) Graphic Recorders
• Generally, graphic recorders are devices which display &
store pen-and-ink record of the history of some physical
event on a chart
Basic Elements of Graphic Recorder:
• Chart for displaying & storing the recorded information
• A stylus or pen moving in proper relationship to
paper/chart
• A suitable means of Interconnection or Interface to
couple the stylus to the source of information
• Graphic recorders may be classified into:
(a) Strip chart recorders
(b) X-Y recorders
3

• A strip chart recorder records one or more variables w.r.t. time
• It is an X-t recorder, which consists of:
(i) A long roll of graph paper
(ii) A system for driving the paper at some selected speed (usually 1-100
mm/s) by means of speed selector switch
(iii) A stylus, moving horizontally in proportion to the quantity being recorded
(iv) A stylus driving mechanism which moves the stylus in nearly exact
replica of the quantity being recorded
• The mechanisms used for marking the marks on the paper are as
follow:
• Marking with ink filled stylus
• Marking with heated stylus
• Chopper Bar
• Electronic Stylus
• Electrostatic Stylus
• Optical Marking Method
Strip Chart Recorders (-contd.)
5

Marking with Ink Filled Stylus
• The stylus is filled with ink by gravity or capillary
actions
• The pointer supports an ink reservoir & a pen, or
contain a capillary connection between the pen &
pen reservoir
• This method is most commonly employed because
ordinary paper can be used and thus the cost is low
• Other advantages:
• Operation over a very wide range of
recording speeds is possible
• Little friction between the stylus tip & the
paper
• Disadvantages:
• Ink splatters at high speeds, batches at low
speeds, & clogs when the stylus is at rest
• The frequency limit of recorders incorporating
this method of writing is only few Hz
6

Marking with Heated Stylus
• Some recorders use a heated stylus
which writes on a special paper (a
thin white wax like coating on a black
paper)
• This overcomes the difficulties
encountered in ink writing system
• Quite reliable & offers high contrast
traces
• Sophisticated records, having a
frequency response upto 40 Hz, are
available
• Since the paper is a special one, the
cost is high
• This method can’t be used for
recording certain processes which
produce heat & indirectly affect the
recordings
7

Chopper Bar
• If a chart made of a pressure sensitive paper is used then a
V-shaped pointer is passed under a chopper bar which
presses the pen into the paper once per sec (or any other
selected interval) thus marking a series of marks on the
paper
• As this system is not purely continuous, thus it is suitable for
recording some slowly varying quantities (e.g. having a
variation of 1 cycle per hour)
8

Electronic Stylus
• This method uses a paper
with a special coating
which is sensitive to current
• When the current is
conducted from the stylus
to the paper, a trace
appears on the paper
• The electronic stylus
marking method has a wide
range of marking speeds,
low stylus friction, & a long
stylus life
9

Electrostatic Stylus
• This method uses a stylus which produces a high voltage
discharge, thereby, producing a permanent trace on an electro-
sensitive paper
• This arrangement has been incorporated in a recorder having a
50 mm wide chart with 9 voltage ranges (10 mV/mm – 5 V/mm),
8 chart speeds (300 – 600 mm/s), & a frequency response of 60
Hz at maximum amplification of 1 dB
10

Optical Marking Method
• This method uses a beam of light to write on a photosensitive
paper
• This method allows higher frequencies to be recorded and
permits a relatively large chart speed with good resolution
• Disadvantages:
• Paper cost is very high
• Photographic paper must be developed before a record
is available
• Not suitable for processes where instantaneous
monitoring is to be done
11

Strip Chart Recorders: Tracing Systems
• To produce graphical
representation of any quantity, two
types of tracing systems used
are:
• Curvilinear Tracing System
• Rectilinear Tracing System
Curvilinear Tracing System:
• In this system, the stylus is
mounted a central pivot & moves
through an arc which allows a full-
width chart marking
• If the stylus makes a full range
recording, the line drawn across
the chart will be curved & the time
intervals will be along these curved
segments 12

Curvilinear Tracing System
• This type of system is
used on many recorders
with PMMC
galvanometers actuating
the stylus filled with ink as
shown in fig. (28.50)
• The disadvantage of this
method of tracing is the
charts are difficult to
analyze because of
curved time base lines
13

Rectilinear Tracing System
• Fig. (28.51) shows rectilinear system of tracing
• This system produces a straight line across the width of the chart
• Here, the stylus is actuated by a drive cord over pulleys to produce
forward/reverse motion as determined by drive mechanism
• The stylus may be actuated by a self balancing potentiometer
system, a
photoelectric
deflection system,
a photoelectric
potentiometer
system, or a
bridge balance
system
• This is usually
used with thermal
or electric writing
14

Types of Strip Chart Recorders
• There are two different types of strip chart recorders:
• Galvanometer Type
• Null Type
Galvanometer Type Strip Chart Recorder:
• The deflection in this recorder is produced by a galvanometer
(d’Arsonval) which produces a torque on account of a current passing
through its coil
• The current is proportional to the quantity being measured
• When the pointer comes to rest because of controlling torque exerted
by springs, the stylus also comes to rest and thus the value of the
quantity is recorded
• This type of recorder is not useful for recording fast variations in
current/voltage/power and only records the average values
• The range of operation is from few mA/mV to several mA/mV
15

• These moving galvanometer type recorder is comparatively
inexpensive instrument having a narrow bandwidth of 0-10 Hz and
a sensitivity of 0.4 mV/mm
• For measurement of smaller voltage, linear amplifiers are used
Null Type Strip Chart Recorder:
• Many recorders operate on the principle whereby a change in its
input upsets the balance of the measuring circuit of the recorder
• As a result of this unbalance, an error signal is produced that
operates some device which restores balance or brings the system
to null condition
• The amount of movement of this balance restoring device is an
indication of the magnitude of the error signal and the direction of
the movement is an indication of the direction of the quantity being
measured has deviated from normal
• The signal from the transducer/sensor may take any of the forms: voltage
(ac or dc), current (ac or dc), resistance, inductance, or capacitance
Types of Strip Chart Recorders (-contd.)
16

Null Type Recorders
• There are a number of null type strip chart recorders:
(i) Potentiometric Recorders
(ii) Bridge Recorders
(iii) LVDT (Linear Variable Differential Transformer) Recorder
Note:
• The principle of operation for all these recorders is same, i.e.,
to obtain null condition
17

Potentiometric Recorders
• The basic disadvantage of a galvanometer type of recorder is
that it has a low input impedance
• A simple method of overcoming the input impedance problem
is to use an amplifier between the input terminals & the
display/indicating instrument
• However, this technique, while producing a high input
impedance (to reduce loading effect) & improved sensitivity
(4V/mm), results in an instrument having low accuracy
• The accuracy can be improved if the input signal is compared
with a reference voltage by using a potentiometer circuit
• The error signal (difference between the input signal & the
potentiometer voltage) is amplified and is used to energize
the field coil of a dc motor, which moves over the
potentiometer in the appropriate direction to reduce the
magnitude of the error signal and to obtain balance 18

• The wiper comes to rest when the unknown signal voltage is
balanced against the voltage of the potentiometer
• The chart drive for most potentiometer recorders is obtained
from a motor synchronized to power line frequency
• Different speeds may be obtained by using a gear train that
uses different gear ratios
• The most common application of the potentiometric recorders
is for recording & control of process temperature
Potentiometric Recorders (-contd.)
19

Single Point & Multipoint Recorders
• Most of the graphic records show variation of one variable (e.g.
temperature) with time
• The instruments that record changes of only one measured variable
w.r.t. time are called single-point recorders and the trace of such
instruments is usually in the form of single continuous curve
• However, in process industry, it becomes necessary to record
simultaneously variables like pressure, temperature, flow rate, liquid
level etc.
• This feature may be performed either by having several pens
(maximum four) which overlap each other & record the inputs
simultaneously, or by replacing the pen by print wheel geared to a
selector switch so that when a particular input is connected to the
potentiometer balance circuit, a point plus its identifying character is
printed on the chart
• This form of recorder is called a multi-point recorder and may have
as many as 24 inputs, with traces displayed in 6 colors 20

• An X-Y recorder is an instrument which gives a graphic record of the
relationship between two variables, i.e., one emf is plotted as a
function of another emf
• Here, one self-balancing potentiometer circuit moves the recording
pen in x-direction while another self-balancing potentiometer circuit
moves the recording pen in y-direction and the paper remains
stationary
• An X-Y recorder consists of a pair of servo-systems, driving a stylus
in two axes through a proper sliding pen & moving arm arrangement,
with reference to a stationary paper chart
• Attenuators are used to bring the input signals to the levels (full scale
range: 0.5 mV) acceptable by the recorder
• As shown in fig (28.53), error signals (the difference between the
input signals & reference voltage) are fed to a choppers which
convert dc signals to ac signals
X-Y Recorders (-contd.)
22

• The signals are then amplified in order to actuate servomotors
which are used to balance the system & hold it in balance as
the value of the quantity being recorded changes
• Thus, we get a record of one variable w.r.t. another
• In some X-Y recorders, one self-balancing potentiometer
controls the position of the rolls (i.e. the paper) while another
self-balancing potentiometer controls the position of the
recording pen
• An X-Y recorder may have a sensitivity of 10 µV/mm, a
slewing speed of 1.5 m/s, a frequency response about 6 Hz for
both axes, chart size of 250 x 180 mm, and accuracy of about
± 0.3%
23

Uses:
(i) Speed-torque characteristics of motors
(ii) Plotting of characteristics of zener diodes, rectifiers, transistors
(iii) Regulation curve (output versus input voltage) of power
supplies
(iv) Electrical characteristics of materials such as resistance
versus temperature
(v) Plotting stress versus strain curves, hysteresis curves etc.
24

Oscillographic Recorders
Ultraviolet Recorders:
25

Ultraviolet Recorders (-contd.)
• These are basically electromechanical oscillographic recorders
consist of a number of galvanometer (moving coil) elements
mounted in a single magnet block as shown in figure
• The galvanometer uses a source of ultraviolet (u.v.) light in
place of white light and a paper sensitive to u.v. light is used
for producing a trace for recording
• The u.v. light is projected on the paper with the help of mirrors
attached to the moving coils (pencil galvanometers)
• As shown in figure, when a current is passed through the
galvanometer coil, it deflects under the influence of magnetic
field of permanent magnet
• The u.v. light falling on the mirrors is reflected & projected on
the u.v. light sensitive paper through a lens & mirror system
• The paper is driven past the moving light spot & thus a trace of
variation of current w.r.t. time is produced
26

• The u.v. sensitive paper may be processed & photo-developed,
permanized or photocopied
• The u.v. recorders are multichannel & have as many as 25
channels
• Since, each channel may produce a 10 cm wide p-p trace on a
30 cm wide paper, there will be considerable overlapping of
traces produced by different channels
• Therefore, it is essential to provide an identification mark for
each trace so as to avoid confusion
• A simplified identification process is to interrupt each trace
momentarily in turn & to coincide this interruption with a
character marked on the side of record by passing u.v. light
through cutouts of the character
27

• The u.v. recorder may be used for dc & ac signals having a
fundamental frequency upto 400 to 500 Hz
• The frequency range depends on the recorder being used and
paper driving speeds
• The recording of high frequency inputs is possible if recorders
with high paper speeds of about 10 m/s are available
• These recorders are also used for recording the magnitude of
low frequency signals which can’t be measured with analog
(pointer) type instruments
Applications:
• Typical applications of u.v. recorders are in recording:
(i) output of transducers, (ii) control system performance, &
(iii) regulation transients of generators
28

Magnetic Tape Recorders
Take up
Real
29

• It is frequently desirable & in many cases necessary to record
data in such a way that they can be retrieved/reproduced in
electrical form again
• The most common & useful way of achieving this is through
the use of magnetic tape recording
• Generally, the magnetic tape recorders are used at high
frequency
• A magnetic tape recorder consists of the following basic
components:
(i) Recording Head, (ii) Magnetic Tape, (iii) Reproducing Head,
(iv) Tape Transport Mechanism, & (v) Conditioning Devices
Magnetic Tape Recorders (-contd.)
30

(i) Recording Head:
• This device responds to an electrical signal in such a manner
that a magnetic pattern is created in a magnetizable medium
• A fine air gap of length 5-15 µm is shunted by passing the
magnetic tape
• A current in the coil causes a flux of the same shape to
bridge the air gap & hence to pass through the magnetic
tape, thereby, magnetizing the iron oxide (Fe2O3) particles as
they pass the air gap
• The state of magnetization of the oxide as it leaves the air
gap is retained, thus the actual recording takes place at the
trailing edge of the gap
• Any signal recorded on the tape appears as a magnetic
pattern dispersed in space along the tape similar to the
original coil current variation with time
31

(ii) Magnetic Tape:
• Magnetic tape (12.7 mm wide & 25.4 µm thick) is composed
of a coating of fine magnetic iron oxide particles (Fe2O3) on a
plastic ribbon
• The magnetic particles conform to the magnetic pattern
induced in them & retain it
(iii) Reproducing Head:
• The reproducing head detects the magnetic pattern stored in
them & converts it back to original electrical signal
• Reproducing is similar in appearance to that of a recording
head
(iv) Tape Transport Mechanism:
• This mechanism moves the tape along the recording of the
reproducing heads at constant speed
32

• The tape mechanism must be capable of handling the tape
during various modes of operation without straining,
distortion, or wearing out the tape
• This requires that the mechanism must use arrangements to
guide the tape past the magnetic heads with great precision,
maintain proper tension, & obtain sufficient tape to magnetic
head contact
• Arrangement for fast winding & reversing are also provided
• A capstan & pinch roller are used to drive the tape
(v) Conditioning Devices:
• These devices consist of amplifiers & filters required for
modifying the signal to a format that can be properly recorded
on a tape
33

Working Principle:
• When a magnetic tape is
passed through a recording
head, any signal recorded on
the tape appears as magnetic
pattern dispersed in space
along the tape, similar to the
original coil current variation
with time
• The same tape when passed
through reproduce/playback
head produces variations in the
reluctance of the winding,
thereby, inducing a voltage in
the winding dependent upon
the direction of magnetization &
its magnitude on the magnetic
tape
34

• The induced voltage is proportional to rate of change of flux
linkages
• Therefore, the emf induced in the winding of reproducing head is
proportional to rate of change of the level of magnetization on the
rate , where N is number of turns of the winding
put on the reproduce head
• Since, the voltage in the reproduce head is proportional to , the
reproduce head acts as a differentiator
• For example, let the original signal be A sin(ωt)
• The current in the record head winding & the flux produced will be
proportional to this voltage, i.e., Ф = K1 A sin(ωt) … (1)
where K1 is constant
• Assuming that the tape retains this flux pattern & regenerates it in
the reproduce head, the voltage induced in the reproduce head
winding is ... (2)





 

dt
d
N
e
.,
e
.
i rep
dt
d
)
t
cos(
NA
K
dt
d
KN
e 2
rep 



 35

• Thus the output signal from the reproduce head is derivative
of the input signal
• The magnitude of the output signal is not only proportional to
the flux recorded on the tape but also the frequency of the
recorded signal
• Since, the recording is done at constant current level for all
frequencies, and therefore, in order to have an overall flat
frequency response (i.e., a high fidelity), the reproduce head
characteristics and the characteristics of the amplifier
connected in the reproduce head circuit must be
complementary (i.e., the amplitude must have a response of -
6dB/octave as shown in figure)
• This process of compensation is known as “equalization”
36

Advantages:
1. Magnetic tape recorders have a wide frequency range from dc to
several MHz
2. They have a wide dynamic range which exceeds 50dB, this
permits the linear recording from full scale signal level to
approximately 0.3% of full scale
3. They have a low distortion
4. The magnitude of the electrical input signal is stored in magnetic
memory and this signal can be reproduced whenever desired
5. The recorded signal is immediately available with no time loss in
processing, the recorded signal can be reproduced (played back)
as many times as desired without loss of signal
6. When the information has been processed, the tape can be erased
& reused to record a new set of data
37

7. Magnetic tape recording permits multi-channel recording, i.e., a
tape facilitates the continuous record of a number of signals to be
made simultaneously, which is a great advantage especially when
recording transient & “once only” signals
8. The use of magnetic tape recorders provides a convenient method
of changing the time base, i.e., the data may be recorded at very
fast speeds (1.52 or 3.05 m/s) and played back at speeds (2.38 or
4.76 cm/s) slow enough to be recorded with low frequency
recorders like graphic recorders
38

Digital Tape Recorders
• Digital magnetic tapes are often used as storage devices in
digital processing applications
• Digital tape units are of two types:
(a) Incremental digital tape recorders
(b) Synchronous Digital Tape Recorders
(a) Incremental Digital Tape Recorders:
• They are commanded to step ahead (increment) for each
digital character to be recorded
• Input data may be at a relatively slow, or even discontinuous
rate
• In this way, each character is equally & precisely spaced
along the tape
39

Digital Tape Recorders (-contd.)
(b) Synchronous Digital Tape Recorders:
• In these, the tape moves at a constant speed (about 75 cm/s)
while a large number of data characters are recorded
• The data inputs are at precise rates upto 10’s of thousand
characters/second
• The tape is brought to speed, recording takes place, and tape
is brought to a fast stop
• In this way, a block of characters (a record) is written with
each character spaced equally along the tape
• Blocks of data are usually separated from each other by an
erased area on the tape called the record gap
• The synchronous tape unit starts & stops the tape for each
block of data to be recorded
40

Digital Recording Method:
• In the above two methods, the characters are represented on
magnetic tape by the coded combination of symbols (bits) 0 & 1 by
the IBM-NRZ format
• This format uses the change in flux direction on the tape to indicate
symbol 1 & no change in flux direction to indicate symbol 0
41

• In digital data recording, a recording field of sufficient amplitude
to produce magnetic saturation through the complete tape layer
thickness is reversed to record 1 & kept constant to record 0
• Reproduction of this recording is achieved by using a timing
signal obtained from a separate clock track corresponding to the
time when a ‘1’ or ‘0’ symbols are recorded
• In practice, larger fields are usually employed to ensure more
reliable recordings on a coated thicker tape & also to minimize
the effect of dropouts (due to some random surface
inhomogenities in tape coatings because of dirt or poor
manufacture, some portions of the tape may not be perfectly
recorded, which is called dropout)
• As a check on dropout errors, most tape systems include a parity
check
42

Advantages of Digital DataTape Recording:
• High accuracy
• Insensitivity to tape speed
• Use of simple conditioning equipment
• The information is fed directly to a digital computer for
processing & control
Disadvantages of Digital DataTape Recording:
• Poor tape economy
• The information from transducers is in analog form, hence an
ADC is required
• A high quality tape & tape transport mechanism are required
43

• Display devices provide a visual display of numbers, letters, &
symbols in response to electrical input, and serve as
constituents of an electronic display system
• The basic element in a digital display device is the display for a
single character because a multiple character display is
nothing but a group of single character displays
• Classification of Displays:
• In general, the displays are classified in a number of ways:
1. According to methods conversion of electrical data into visible light:
(i) Active displays(Light Emitters)– CRTs, LEDs, Gas Discharge Plasma, etc.
(ii) Passive Displays (Light Controllers) – LCDs
2. According to the Applications:
(i) Analog Displays – Bar-graphic displays (CRT)
(ii) Digital Displays – Nixies tubes, Alphanumeric, LEDs
Digital Display Devices
44

3. According to display size & physical dimensions:
(i) Symbolic Displays – Alphanumeric, Nixie tubes, etc.
(ii) Console Displays – CRTs, LEDs, etc.
(ii) Large Screen Display – Enlarged Projection Systems
4. According to the display format:
(i) Direct View Type (Flat Panel Planar) – Segmental, Dotmatrix, CRTs
(ii) Stacked Electrode (Non-planar Type) – Nixie Tubes
5. In Terms of Resolution & Legibility of Characters
(i) Simple Single Indicator
(ii) Multi-element Displays
Digital Display Devices (-contd.)
45

Segmental Displays
• The segmental displays may be either 7 or 14 segmental ones
depending upon whether numeric or alphanumeric displays are
required
(i) Seven Segmental Display:
• This is used for numeric display, and consists of 7-segments a, b, c,
d, e, f, & g
• It displays the digits (0 to 9) by illuminating proper segments from the
group
• The display is incandescent & operates on low voltages ( 5 – 12 V)
and requires about 5 – 50 mA current when using LEDs
• LCDs are also used for segmental displays
46

(ii) Fourteen Segmental Display:
• For display of alphanumeric characters (both numerals as
well as alphabets) a 14-segmental display unit is used by
illuminating the proper combination of segments
Segmental Displays (-contd.)
47

Dot Matrices
• Dot matrices may be used for display of numeric & alpha-numeric
characters
(i) A 3x5 Dot Matrix:
• A 3x5 dot matrix may be used for display of numeric characters
(ii) Dot Matrix Utilizing 27 dots:
• Another system using 27 dots displays the numeric characters
• The dots may be square or round with 0.4 mm side or diameter
• LEDs or LCDs are used for display of dots
(iii) 5x7 Dot Matrix:
• For display of alphanumeric characters a 5x7 matrix is used
48

Rear Projection Display
• A typical block diagram of a projection display system is shown in figure
• In a rear projection display system, the projector is on one side of a
translucent screen & the viewer is on the other
• The image signal source & screen are not the part of the display system
• Each of the 12 incandescent lamps inside the projector light source when
energized by the input signal illuminates a different part of the filmstrip
• The lens system (optics) projects the illuminated part of the film onto a
viewing screen
Image Signal Source Controller
Image Engine Optics Screen
Light Source Optics Projector
Fig. Projector Schematic 50

Rear Projection Display (-contd.)
51

Nixie Tube
• NIXI = Numeric Indicator
eXperimental No. I
• A nixie tube is an electronic
device (non-planar) for displaying
numbers or other information, in
the form of a glass tube
containing multiples cathodes and
a wire mesh anode, filled with
neon & often a little mercury
and/or argon at small fraction of
atmospheric pressure
• The most common form of nixie
tube has 10 cathodes (thin wires)
in the shapes of the numerals 0-9
(and occasionally a decimal point,
but there are also types that show
various letters, signs, & symbols 52

• In its normal operation, the anode is returned to +ve dc supply
(150-220 V) through a suitable current limiting resistor, the
value of the supply being greater than the worst-case
breakdown voltage of the gas within the tube
• The gas in the vicinity of the appropriate cathode glows when
the cathode is switched to ground potential
• Since 10 cathodes have to be associated with single anode
inside the glass bulb, they have necessarily to be stacked in
different planes
• This requires different voltages for different cathodes to enable
the glow discharge
• Nixie tubes have the following characteristics:
(1) The numerals are usually large (typically 15-30 mm high) and
appear in the same base line for in-line read-out
Nixie Tube (-contd.)
55

(2) Nixie tubes are single digit devices with or without a decimal
point
(3) The selected cathode carries current in the range of 1-5 mA
(4) The Nixie tube can be pulse operated & hence can be used in
multiplexed displays
Applications:
• Nixies were used as numeric displays in early digital frequency
counters, voltmeters, & many other types of technical
equipments
• They also appeared in costly digital time displays in research
and military establishments
• Later, alphanumeric versions in 14-segment display format
found use in airport arrival/departure signs & stock-ticker
displays
Nixie Tube (-contd.)
56

Light Emitting Diode (LED)
• LED is a diode that gives off visible light when it is biased properly & the
mechanism of electromagnetic radiation (injection luminescence) takes
place
• This occurs in two steps:
(i) injection of minority carriers across the junction, and
(ii) the radiative recombination of minority carriers
• Two types of radiative recombination mechanisms are commonly
encountered in LEDs depending upon the band-gap characteristics of the
semiconductor material used, i.e.,
(a) direct recombination, & (b) indirect recombination
• The emission of photons as a result of recombination of electrons & holes is
possible only when both energy & momentum are conserved
• The simplest & most probable recombination process will be that where the
electrons & holes have the same (i.e., zero) value of momentum which is
the case of direct band-gap materials (e.g., GaP, GaN, GaAsP, Si2N3 etc)
and the released light energy (photon in visible spectrum)
57

Light Emitting Diode (LED) (-contd.)
• In indirect band-gap materials (e.g., Si, Ge, SiC, etc.) the conduction
band minima and valence band maxima occur at different values of
momentum and the recombination of electrons & holes result into
emission of energy into heat (phonons, which conserve momentum)
and light (photons, not in visible range)
• The probability of recombination in this case is obviously much lower
than that in direct band-gap semiconductors
• GaP is mainly used for emissions in the visible spectrum, it has a
gap energy of 2.26 eV and can be doped with ZnO to give red light
or with N2 to give green light
• LEDs are also housed in plastic bulb (the round end) that
concentrate the light in a particular direction
• LEDs are used extensively in segmental & dot matrix displays of
numeric & alphanumeric characters
58

• Advantages of LEDs in Electronic Displays:
(i) LEDs are miniature in size & they can be stacked
together to form numeric & alphanumeric displays in
high density matrix
(ii) The light output from an LED is a function of current
flowing through it, therefore, intensity of light emitted
can be smoothly controlled
(iii) LEDs require moderate power (1.2 V of emf & 20 mA
of current) for their operation (full brightness)
(iv) LEDs are very fast as they have switching time less
than 1 ns
(v) LEDs are rugged & can therefore withstand shocks
and vibrations, they can also operate over a wide
range of temperature (0-700 C)
(vi) LEDs don’t have a filament that will burn out, so they
last much longer (more than 100000 hrs)
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Liquid Crystal Display (LCD)
• A liquid crystal is a material (normally organic for LCDs) that flows like a
liquid but whose molecular structure has some properties associated with
solids, i.e., liquid crystals are substances that exist in an odd state: sort of
like a liquid & sort of like a solid
• Thus, their molecules tend to maintain their orientation (like the molecules
in a solid) but also move around to different positions (like the molecules in
a liquid)
• Depending on the temperature & particular nature of a substance, liquid
crystals can be in one of the several distinct phases, whose nematic phase
makes LCDs possible
• Liquid crystals are affected by electric current
• A particular type of nematic liquid crystal, called twisted nematic (TN), is
naturally twisted and applying a varying electric current to these liquid
crystals will untwist them to varying degrees
• LCDs use these liquid crystals because they react predictably to electric
current in such a way as to control light passage
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Liquid Crystal Display (LCD) (-contd.)
Types of LCDs:
(1) Dynamic Scattering Type (Nematic Crystal)
(2) Field Effect Type (Twisted Nematic Crystal)
(1) Dynamic Scattering Type (Nematic Crystal):
• As shown in fig. 1 (a) & (b), the individual molecules have a rod like
appearance
• The liquid crystals are layered between glass sheets with transparent
electrodes (Indium Oxide) deposited on the inside faces
• Under no bias condition, the incident light will pass through & the liquid
crystal structure appears clear
• If a voltage (6-20 V) is applied across the conducting surfaces, the
molecular arrangement is disturbed and regions of different refractive
indices are established which reflect the incident light in different directions
at the interface of different refractive indices
• This phenomenon is referred to as dynamic scattering and causes a
frosted-glass (bright) appearance
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• The LCD does not generate its own light but depends on a external or
internal light source
• Therefore, during the day or in lighted areas, a reflector can be put behind
LCD to reflect light back (reflective mode) & under dark condition, the LCD
must have its own internal light source either behind or side of LCD
(transmissive mode)
(2) Field Effect Type (Twisted Nematic Crystal):
• The construction of field effect LCD is similar to that of dynamic scattering
type with the exception that two thin polarizing optical filters are placed at
the outside of each glass sheet
• Similar to dynamic scattering LCD, the field effect can be operated in the
reflective or transmissive mode
• In case of transmissive field effect LCD, the internal light source is on the
right & the viewer on the left as shown in fig. 2(a)
• Only the vertical component of the incident light can pass through the
vertical polarizer
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• In between the two walls of the liquid crystal, there is a general drift from
one polarization to the other
• The left-hand vertical light polarizer only permits the passage of vertical
polarized light
• If there is no applied voltage then the viewer sees a uniformly dark pattern
across the entire display but when a threshold voltage is applied (2-8 V), the
rod like molecules align themselves with the field (perpendicular to the wall)
and the light passes directly through without the 900 shift and can pass
through the second vertically polarizer and the light area is seen by the
viewer
• Through proper excitation of the segments of each digit, the pattern will
appear
• In case of reflective type field effect LCD, the horizontally polarized light at
the far left encounters a horizontally polarized filter and passes through to
the other vertical polarization, & returned to the observer
• If there is no applied voltage, there is a uniformly lit display
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• The application of a voltage results in a vertically polarized incident light
encountering a horizontally polarized filter at the left which will not be able
to pass through and be reflected which results a dark area on the crystal
and the pattern
• Field effect LCDs are normally used when a source of energy is prime
factor (e.g. in watches, cell phones, portable instruments, etc.), since they
consume very small power (in µW range) as compared to the dynamic
scattering types (in mW range)
• The cost is higher for field effect LCDs and their height is limited to about 2”,
while dynamic scattering LCDs are available upto 8” in height
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Advantages of LCDs:
• They have low power (in µW) consumption as compared to LEDs (in
mW)
• They have low cost
• Since the color generated by LCD units is dependent on the source
of illumination, so there is greater range of color choice (16.8 million
colors)
• Disadvantages of LCDs:
• They are very slow devices, having response time in the range of
100-300 ms
• They occupy a large area
• When operated on dc, their life-span is quite small because LCDs
degrade chemically, therefore, they are used with ac supplies having
a frequency less than 500 Hz
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Segmental Gas Discharge Displays
• Segmented gas discharge displays work on the principle of gas discharge
glow similar to Nixie tubes
• They are mostly available in 7 or 14-segment form to display numeric or
alphanumeric characters
• As shown in figure, each segment (including decimal point) of the 7-
segment display formed on a base has a separate cathode
• The anode is common to each member of 7-segment group, which is
deposited on the covering face plate
• The space between the anode & cathodes contains the gas
• For each group of segments, a “keep alive” cathode is also provided, which
improves the switching speed of the display by passing a small constant
current (few µA) through it and acts as source of ions
• Pins are connected to the electrodes at the rear of the base plate with the
help of which external connections can be made
• Figure shows the structure of a 7-segment display making the use of gas
discharge plasma
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Segmental Gas Discharge Displays (-contd.)
• Back electrodes of the thick film type serve as cathode segments & front
electrodes of the thin film type serve as transparent anodes
• A gas (typically Neon) is filled in the discharge space between the cathode
& anode segment
• The gas is struck between the cathode & anode of a chosen segment so
that the cathode glow provides the illumination & characters can be
displayed by activating the appropriate segments
Advantages & Disadvantages:
• The power requirements of such devices are more or less in the same
range as those for Nixie tubes (i.e., the power consumed is extremely small)
• The major disadvantage of this gas discharge tube is that high voltage (150-
220 V) is required for operating
• Therefore, high voltage transistors (special ICs) are required as switches for
the cathodes
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