Lecture Slides for the course Electronic Instrumentation-II delivered by me to Diploma in Electronics/Instrumentation Engineering.

1. BIE-601
Electronic Instrumentation-II
Unit-4
Display Devices & Recorders
Lecture-3
Mohd. Umar Rehman
umar.ee.amu@gmail.com
17. 4. 2020
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2. Electroluminescent Displays
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3. Electroluminescent Displays
Electroluminescence: It is the phenomenon due to which a suitable
material (usually semiconductor) emits lght in response to passage
of electric current or to a strong electric field. It is a non-thermal
generation of light.
Electroluminescence is the result of recombination of electrons and
holes in a semiconductor. The excited electrons release energy as
photons (light).
Prior to recombination electrons & holes may be separatedeither
by the doping material as in a PN junction.
Coomonly used EL materials are Inidium Phosphide (InP), GaAs,
GaN, ZnS + Mn: Yellow-Orange color, ZnS + Cu: Green color.
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4. EL Displays...Contd
The form of the material used is either thin film (for information
display) or powder (for lighting).
For lighting purposes, powdered Phosphor was used and primar-
ily works as backlighting for LCDs. Most common color for this
material is green-cyan. However, voltages required are very high
(60 - 600 V).
For medical instruments or digital displays of automobiles, thin
film Phosphor is used which provides very high resolution.
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5. EL Displays Contd
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6. Basic Structure of EL Display
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7. Structure...Contd
All ELDs have the same basic structure.
A metallic electrode, usually Aluminium, is placed over the phos-
phor material in a pressure cell by vacuum evaporation, so as to
form an electrical connection on the other side of the phosphor.
This results in a capacitive device, because of poor conduction paths
in phosphor.
An AC field applied across the chosen segment(or dot) and Al elec-
trode excites the phosphor, resulting in emission of light.
In film type structures, the EL powder structure is replaced by a
phosphor film deposited on a glass substrate.
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8. Advantages of EL Displays
1. It can be produced in any shape and size.
2. Paper thin (0.25 mm - 0.5 mm), light weight, flexible.
3. High strength.
4. Low power consumption.
5. Variable brightness.
6. High visibility: can be viewed from long distance and in poor visi-
bility conditions like fog.
7. Long life: up to 20,000 hours.
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9. Disadvantages of EL Displays
These disadvantages are concerned only with Phosphor based EL
displays.
1. The luminous output is not very good, therefore not suited for
very large displays.
2. The output gets reduced over time.
3. Phosphor based flexible EL displays are not very durable.
4. Requirement of high voltages.
5. When using DC source, converters are required which may
produce audible noise.
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10. Liquid Vapour Displays (LVDs)
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11. Liquid Vapour Displays (LVDs)
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12. LVD I
LVDs are the latest in economical display technology.
They employ a new reflective passive display principle and
depend on the presence of ambient lights for their operation.
The above figure shows the structure of a typical LVD cell.
It consists ofa transparent volatile liquid encased between two glass
plates and side spacers.
The rear glass plate has a black background and the front glass
surface in contact with the liquid is roughened, so that the liquid
wets it.
In its simplest form, an LVD consists of a roughened glass surface
wetted with a transparent volatile liquid of the same refractive
index as that of the glass. The rear surface is blackened.
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13. LVD II
The transparent electrode is heated by using a voltage drive, which
is the basis for the display function.
In the OFF condition of display with no voltage applied across
the transparent electrode, the viewer sees the black background
through the front transparent glass electrode and the liquid.
To achieve an ON condition of the display, a voltage is applied to
the transparent electrode. This causes sufficient heat in the elec-
trode, which evaporates the liquid in contact with it, and a com-
bination of vapour film and vapour bubbles is formed around the
roughened glass surface.
As the refractive index of vapour is approximately 1, there is a dis-
continuity established at the interface between the front glass plate
and the liquid, which gives rise to light scattering. This makes it a
simple display device.
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14. LVD III
The organic liquid selected for LVD should have the following
features.
1. Refractive index close to that of the glass plate.
2. Minimum energy for vapourising the liquid in contact with the
roughened surface.
Features:
The electrical heating of a thin film of liquid adjacent to the rough-
ened surface using transparent electrodes and the applied voltage,
makes it an unusually good display with a better contrast ratio
than an LCD. The speed of operation of LVDs is low.
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