APPLIED ELECTRONICS

Presented by :-SHAKIR ALI

SOOMRO

Lecture-01

Topic:

PHOTOTRANSISTOR

INTRODUCTION

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.

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 collector-to-emitter leakage current i.e. I(CEO), this is called the dark current and is typically in the range of nA.

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 OFPHOTOTRANSISTOR

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. Three Lead Phototransistor.2. 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.

Phototransistor Bias Circuit

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”

Introduction The first NTC thermistor was discovered in 1833 by

Micheal Farady, who reported on the semiconducting behavior of silver sulfide.

A thermistor is a type of resistor whose resistance

varies with temperature.

Thermistors are the detector of temperature, which converts temperature into Electrical energy.

We can say that thermistor detects the temperature and gives the signal in the form of electrical energy to the circuit.

Its resistance is sensitive to the temperature.

Its resistance increases or decreases with the temperature.

Thermistors differ from resistace temperature detectors (RTD) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals.

The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range [usually -900C to 1300C].

Response Time of Thermistor

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’.

Basic Operations

Thermistors can be classified into two types depending on the sign of ‘k’ (first-order temperature coefficient of resistance).

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.

Curve between Resistance and Temperature of Thermistor

This curve shows that Resistance and Temperaturehas inverse relation with each other.

Symbol

Schematic Symbol of Thermistor

Shapes of Thermistor 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

Advantages

Thermistor is chemically stable.

It is used in nuclear environment.

Thermistor is for series –parallel arrangement for using power handling capacity.

It is also used to measure the temperature.

Applications NTC thermistors are used as resistance thermometers in

low-temperature 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

Introduction

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.

Two alternative circuit symbols are shown in figure below:

SYMBOL

OR

BIAS CIRCUIT

Equivalent Circuit

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.

Construction

A solar cell consists if P-type and N-type semi-conductor 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 P-N 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), Ga-Ar (Gallium Arsenide), In-Ar (Indium Arsenide).

Working Principle

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

Applications

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.

Advantages

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”

Introduction

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.

Symbol

or

Schematic Symbol of LDR

Structure

It a simple resistor packed in glass casing.

Working Principle A photoresistor is made of

a high resistance semi-conductor.

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 time-delay.

Theory 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.

Curve between Resistance & Temperature

This curve shows that Resistance and Temperaturehas inverse relation with each other.

Applications

LDRs or Photoresistors come in many different types.

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 far-infrared detectors available, and are used for infrared astronomy and infrared spectroscopy.