Presented by :SHAKIR ALI SOOMRO
Lecture-01
Topic:
PHOTOTRANSISTOR
INTRODUCTION
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The phototransistor is a transistor in which base
current is produced when light strikes the
photosensitive semiconductor base region.
The collector-base P-N junction is exposed to incident
light through a lens opening in the transistor package.
When there is no incident light, there is only a small
thermally generated collector-to-emitter leakage
current i.e. I(CEO), this is called the dark current and is
typically in the nA range.
When light strikes the collector-base pn junction,
a base current is produced that is directly
proportional to the light intensity.
Since the actual photo generation of base current
occurs in the collector-base region, the larger the
physical area of this region, the more base
current is generated.
A phototransistor does not activated at every
type of wave lengths of light.
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The phototransistor is similar to a regular BJT except that the base current is
produced and controlled by light instead of a voltage source.
 The phototransistor effectively converts variations in light energy to an electrical
signal
 The collector-base pn junction is exposed to incident light through a lens opening in
the transistor package.
 The phototransistor is a transistor in which base current is produced when light
strikes the photosensitive semiconductor base region.
 When there is no incident light, there is only a small thermally generated collectorto-emitter leakage current i.e. I(CEO), this is called the dark current and is typically
in the range of nA.
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When light strikes the collector-base pn junction, a base current, Iλ, is
produced that is directly proportional to the light intensity.
This action produces a collector current that increases with Iλ .
Except for the way base current is generated, the phototransistor behaves
as a conventional BJT.
In many cases there is no electrical connection to the base
The relationship between the collector current and the light-generated
base current in a phototransistor is IC = βDC * Iλ .
6
SYMBOL OF
PHOTOTRANSISTOR
A typical phototransistor is designed to offer a large area to the
incident light, as the simplified structure diagram in Figure:
Phototransistor are of two types.
1.
2.
Three Lead Phototransistor.
Two Lead Phototransistor.
1.
Three Lead Phototransistor:
In the three-lead configuration, the base
lead is brought out so that the device can
be used as a conventional BJT with or
without the additional light-sensitivity
feature.
2.
Two Lead Phototransistor:
In the two-lead configuration. the base
is not electrically available, and the
device can be used only with light as the
input. In many applications, the
phototransistor is used in the two-lead
version.
Typical collector characteristic curves. Notice that each
individual curve on the graph corresponds to a certain
value of light intensity (in this case, the units are m
W/cm2) and that the collector current increases with
light intensity.
Phototransistors are not sensitive to all
light but only to light within a certain
range of wavelengths. They are most
sensitive to particular wavelengths. as
shown by the peak of the spectral response
curve in Figure.
Lecture-2
Topic
“Thermister”
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first NTC
thermistor
was with
discovered
in 1833 by
ItsThe
resistance
increases
or decreases
the temperature.
Micheal Farady, who reported on the semiconducting
behavior of silver sulfide.
Thermistors differ from resistace temperature detectors (RTD) in
A the
thermistor
is a type
of resistor
whose resistance
that
material used
in a thermistor
is generally
a ceramic or
varies while
with RTDs
temperature.
polymer,
use pure metals.
Thermistors are the detector of temperature, which
The
temperature
response isinto
alsoElectrical
different; RTDs
are useful over
converts
temperature
energy.
larger temperature ranges, while thermistors typically achieve a
0C
higher
precision
within
a limited temperature
[usually -90
We can
say that
thermistor
detects therange
temperature
and
to gives
1300C].the signal in the form of electrical energy to the
circuit.
Its resistance is sensitive to the temperature.
The temperature at which resistance starts to
vary in order to activate the thermistor, the
time taken during that period is known as
‘Response Time’.
Thermistors can be classified into two types depending on
the sign of ‘k’ (first-order temperature coefficient of resistance).
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If ‘k’ is positive, the resistance increases with increasing
temperature, and the device is called a positive
temperature coefficient (PTC) thermistor, or Posistor.
If ‘k’ is negative, the resistance decreases with
increasing temperature, and the device is called a
negative temperature coefficient (NTC) thermistor.
Resistors that are not thermistors are designed to have
a ‘k’ as close to zero as possible, so that their resistance
remains nearly constant over a wide temperature
range.
This curve shows that Resistance and Temperature
has inverse relation with each other.
Schematic Symbol of Thermistor
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Thermistor has different shapes such as, Disc type
Thermistor, Washer type Thermistor, Bead type
Thermistor, Bulb type Thermistor.
Bead type Thermistor is shown in figure below:
NTC thermistor, bead type, insulated wires
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Thermistor is chemically stable.
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It is used in nuclear environment.
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Thermistor is for series –parallel arrangement
for using power handling capacity.
It is also used to measure the temperature.
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NTC thermistors are used as resistance thermometers in lowtemperature measurements of the order of 10 K.
NTC thermistors can be used as inrush-current limiting
devices in power supply circuits. They present a higher
resistance initially which prevents large currents from
flowing at turn-on, and then heat up and become much lower
resistance to allow higher current flow during normal
operation. These thermistors are usually much larger than
measuring type thermistors, and are purposely designed for
this application.
NTC thermistors are regularly used in automotive applications. For
example, they monitor things like coolant temperature and/or oil
temperature inside the engine and provide data to the ECU and,
indirectly, to the dashboard.
Thermistors are also commonly used in modern digital thermostats and
to monitor the temperature of battery packs while charging.
Lecture-3
Topic
SOLAR CELL
Or
PHOTO-VOLTAIC
CELL
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Solar cells are operate on the principle of
Photovoltaic Action i.e. conversion of Light
energy into Electrical energy.
This action occurs in all semi-conductors
which are constructed to absorb energy.
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Two alternative circuit symbols are shown in
figure below:
OR
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To understand the electronic behavior of a solar cell, it
is useful to create a model which is electrically
equivalent, and is based on discrete electrical
components whose behavior is well known. An ideal
solar cell may be modelled by a current source in
parallel with a diode; in practice no solar cell is ideal, so
a shunt resistance and a series resistance component
are added to the model.The resulting equivalent circuit
of a solar cell is shown above.
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A solar cell consists if Ptype and N-type semiconductor material (
silicon, germanium and
selenium) forming a P-N
junction.
The bottom surface of the
cell (which is always away
from light) covered with a
continuous conductive
contact to which a wire
lead attached.
The upper surface has max:
area exposed to light with a
small contact either a long
the edge or around the
perimeter.
Silicon is commonly used for
fabricating solar cells, another
construction consists of P-type
selenium covered with a layer of
N-type cadmium-oxide to form PN junction.
The surface layer pf P-type
material is extremely thin (0.5
mm) so that light can penetrate to
the junction.
Power solar cells are also
fabricated in flat strips to form
efficient coverage of available
surface area.
Indecently the maximum
efficiency of a solar cell in
converting sunlight into electrical
energy is nearly 15% at present.
Another material used to
make solar cells are Th-Ar
(Thallium Arsenide), GaAr (Gallium Arsenide),
In-Ar (Indium Arsenide).
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It converts solar energy into electrical
voltage or current.
When photons of solar light are strike
with glass of solar cell, they eject into
P-N junction, so replacement occurs
in electron and holes, hence
conduction takes place.
In short circuit the voltage becomes
zero and current becomes maximum,
so power gets zero.
In open circuit voltage is maximum
and current becomes zero so also we
get power zero.
For the mentioned reasons it is not useable,
so we use it at knee point of the of circuit,
where voltage and current both in circuit, as
shown in the V-I curve.
Solar cell gives maximum voltage of 0.6v at
knee point.
For maximum voltage we use series-parallel
algorithm of solar cells, so it acts like a battery
source.
V-I Characteristic Curve
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A solar cell operates with fair efficiency, has
unlimited life, can be easily mass-produced
and has a high power capacity per weight.
It is because of these quantities that is has
become an important source of power for
earth satellites.
It can be used in commercial point of view. It is
used in solar cars, house usage etc.
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The main advantage of solar cell is the input of
the solar energy free of cost.
In future it is more beneficial for the purpose
of power generation.
Lecture-4
Topic
“LDR”
(Light Dependent Resistor)
Or
“Photoresistor”
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A photoresistor or light dependent resistor or
cadmium sulfide (CdS) cell is a resistor whose
resistance decreases with increasing incident
light intensity.
It can also be referenced as a photoconductor.
or
Schematic Symbol of LDR
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It a simple resistor packed in glass casing.
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A photoresistor is made of
a high resistance semiconductor.
If light falling on the
device is of high enough
frequency, photons
absorbed by the
semiconductor give bound
electrons enough energy
to jump into the
conduction band.
The resulting free electron
(and its hole partner)
conduct electricity,
thereby lowering
resistance.
The internal components of a
photoelectric control for a typical
American streelight.
The photoresistor is facing rightwards,
and controls whether current flows
through the heater which opens the
main power contacts.
At night, the heater cools, closing the
power contacts, energizing the street
light.It is basically light dependent
resistor. The heater/bimetal
mechanism provides a built-in timedelay.
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A photoelectric device can be either intrinsic or
extrinsic.
An intrinsic semiconductor has its own charge carriers
and is not an efficient semiconductor, e.g. silicon. In
intrinsic devices the only available electrons are in the
valence band, and hence the photon must have enough
energy to excite the electron across the entire bandgap.
An Extrinsic devices have impurities, also called
dopants, added whose ground state energy is closer to
the conduction band; since the electrons do not have as
far to jump, lower energy photons (i.e., longer
wavelengths and lower frequencies) are sufficient to
trigger the device.
If a sample of silicon has some of its atoms replaced by
phosphorus atoms (impurities), there will be extra
electrons available for conduction. This is an example of
an extrinsic semiconductor.
This curve shows that Resistance and Temperature
has inverse relation with each other.
LDRs or Photoresistors come in many different
types.
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Inexpensive cadmium sulfide cells can be
found in many consumer items such as camera
light meters, street lights, clock radios,
alarms, and outdoor clocks.
They are also used in some dynamic
compressors together with a small
incandescent lamp or light emitting diode to
control gain reduction.
Lead sulfide and indium antimonide LDRs are used for the mid infrared
spectral region.
Germinium (Ge):Copper (Cu) photoconductors are among the best farinfrared detectors available, and are used for infrared astronomy and
infrared spectroscopy.