Sensors and Transducers

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1 Sensors and Transducers Transducers-Transducer is a device which converts one form of energy into another form of energy. Electrical transducers are those which convert one form of energy into electrical energy. Selection criteria of transducers. o o o o o o o o Operating Principle: The transducers are selected on the basis of operating principle it may be resistive, inductive, capacitive, optical etc. Operating range: The range of transducer should be appropriate for measurement to get a good resolution. Accuracy: The accuracy should be as high as possible or as per the measurement. Range: The transducer can give good result within its specified range, so select transducer as per the operating range. Sensitivity: The transducer should be more sensitive to produce the output or sensitivity should be as per requirement. Loading effect: The transducer s input impedance should be high and output impedance should be low to avoid loading effect. Errors: The error produced by the transducer should be low as possible. Environmental compatibility: The transducer should maintain input and output characteristic for the selected environmental condition.

2 Active transducer Passive transducer What is The transducer which generate the output in the form of voltage or current, without any external energy source is known as active transducer. The passive transducer means the transducer whose internal parameters like capacitance, resistance & inductance changes because of the input signal. Additional Energy Source Not Require Require Working Principle Draw energy from the measurand source. Take power from the external source which changes the physical properties of transducer

3 Eddy current sensor Eddy current sensor consists of a coil placed in magnetic field in such a way that electric field is produced which in turn creates Foucault current or eddy current. The sensor is used in a way that the coil kept in a magnetic field is given pressure or displacement as input which varies the field such that there is variation in eddy current. The transducer is self-generating such that it doesn t need external power supply. It is easy to install. The external forces can disrupt the transduced signals very easily. Potentiometer A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. [1]

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5 When used as a potentiometer, connections are made to both ends as well as the wiper, as shown. The position of the wiper then provides an appropriate output signal (pin 2) which will vary between the voltage level applied to one end of the resistive track (pin 1) and that at the other (pin 3). The potentiometer is a three-wire resistive device that acts as a voltage divider producing a continuously variable voltage output signal which is proportional to the physical position of the wiper along the track. Linear Variable Differential Transformer (LVDT) Definition: The Linear Variable Inductive Transformer converts the linear displacement into an electrical signal. It works on the principle of mutual induction, i.e., the flux of the primary winding is induced to the secondary winding. The output of the transformer is obtained because of the difference of the secondary voltages, and hence it is called a differential transformer. Construction of LVDT The basic construction of the LVDT is shown below in the figure. The P is the primary winding of the LVDT and S 1 and S 2 are the secondary winding of the transformer. The secondary winding is wound on the cylindrical former. The secondary winding has an equal number of turns, and it is placed identically on both the side of the primary winding.

6 The alternating current source is applied to the primary winding. The soft iron core is placed inside the iron core. The displacement which is to be measured is attached to the arm of the iron core. The high permeability metal is used for the core so that the harmonics are less and null voltage is easily obtained. The displacement measured longitudinally, which reduces the eddy current losses. The whole of the arrangement is placed inside the stainless steel housing, and their ends provide the electrostatic and electromagnetic shielding. The frequency of the alternating current applied to the primary winding lies between 50 to 20 khz. The LVDT works on mutual induction principle. The current is applied to the primary winding which produces the magnetic field, and this field induces the current in the secondary windings.

7 The output voltage of the secondary winding S 1 is E S1 and that of the S 2 is E S2. The secondary voltage signal is converted into an electrical signal by connecting the secondary winding in series opposition as shown in the figure below. The output voltage of the transducer is determined by subtracting the voltage of the secondary windings. E 0 = E S1 E S2

8 The output voltage of the secondary winding is equal when the core is in the normal position. When the soft core moved towards left the flux linked in S 1 is more as compared to S 2. The output voltage of the winding S 1 is more than the S 2 but it is in phase with the primary voltage. Similarly, when the soft iron core move towards right the magnitude of the flux linked S 2 is more than S 1. The output voltage is -180ºC out of phase with the primary winding.

9 The change in output voltage is directly proportional to the displacement of the core. Any displacement will increase the flux of one of the secondary winding and on the other hand, reduces the other. The curve between the output voltage and displacement is shown in the figure above.

10 The curve is linear for small displacement and beyond this range, it starts to deviate from the straight line. Advantages of LVDT The following are the advantages of the LVDT. 1. High Range The LVDT has the very wide range for measurement of displacement. Their displacement range is from 1.25 mm to 250 mm. 2. High Input and High Sensitivity The LVDT gives a high output and also there is no need for amplification. The sensitivity of the transducer is also very high. 3. Rugged It can tolerate the high degree of shock and variation, especially when the core is loaded with the help of spring. 4. Low Hysteresis The LVDT has low hysteresis because of which their repeatability is excellent. 5. Low Power Consumption The LVDT consumes power less than 1W power. Disadvantages of LVDT The disadvantages of the LVDT are shown below in details. 1. Large displacement is required for getting the considerable differential output. 2. The LVDT transformer is very sensitive to the stray magnetic field. 3. The performance of the transducer is affected by the vibrations. 4. The dynamic response is controlled mechanically by the mass of the core and electrically by the frequency of the current. 5. The performance of the LVDT is affected by the temperature. Uses of LVDTs The following are the major applications of LVDTs. 1. It is used for measuring the displacement having a range from few mm to cm. The LVDT directly converts the displacement into an electrical signal.

11 2. It is also used as the secondary transducer. The LVDT is used as a device for measuring the force, weight and pressure. Some of the LVDT used for measuring the load and pressure. The LVDTs are mostly used in servo mechanisms and other industrial applications. PIEZOELECTRIC TRANSDUCER. The direction, perpendicular to the largest face, is the cut axis referred to. X axis is the electrical axis Y axis is the mechanical axis Z axis is optic axis(holding the crystal together) If an electric stress is applied in the directions of an electric axis (Xaxis), a mechanical strain is produced in the direction of the Y-axis, which is perpendicular to the relevant X-axis. Similarly, if a

12 mechanical strain is given along the Y-axis, electrical charges will be produced on the faces of the crystal, perpendicular to the X-axis which is at right angles to the Y-axis. Some of the materials that inherit piezo-electric effect are quartz crystal, Rochelle salt, barium titanate, and so on. The main advantages of these crystals are that they have high mechanical and thermal state capability, capability of withstanding high order of strain, low leakage, and good frequency response, and so on. The main principle of a piezoelectric transducer is that a force, when applied on the quartz crystal, produces electric charges on the crystal surface. The charge thus produced can be called as piezoelectricity. Piezo electricity can be defined as the electrical polarization produced by mechanical strain on certain class of crystals. The rate of charge produced will be proportional to the rate of change of force applied as input. As the charge produced is very small, a charge amplifier is needed so as to produce an output voltage big enough to be measured. The device is also known to be mechanically stiff. For example, if a force of 15 kilon is given to the transducer, it may only deflect to a maximum of 0.002mm. But the output response may be as high as 100KiloHz.This proves that the device is best applicable for dynamic measurement. The figure shows a conventional piezoelectric transducer with a piezoelectric crystal inserted between a solid base and the force summing member. If a force is applied on the pressure port, the same force will fall on the force summing member. Thus a potential difference will be generated on the crystal due to its property. The voltage produced will be proportional to the magnitude of the applied force.

13 Piezoelectric Transducer can measure pressure in the same way a force or an acceleration can be measured. For low pressure measurement, possible vibration of the amount should be compensated for. The pressure measuring quartz disc stack faces the pressure through a diaphragm and on the other side of this stack, the compensating mass followed by a compensating quartz. Applications 1. Due to its excellent frequency response, it is normally used as an accelerometer, where the output is in the order of (1-30) mv per gravity of acceleration. 2. The device is usually designed for use as a pre-tensional bolt so that both tensional and compression force measurements can be made. 3. Can be used for measuring force, pressure and displacement in terms of voltage. Advantages 1. Very high frequency response. 2. Self-generating, so no need of external source. 3. Simple to use as they have small dimensions and large measuring range.

14 4. Barium titanate and quartz can be made in any desired shape and form. It also has a large dielectric constant. The crystal axis is selectable by orienting the direction of orientation. Disadvantages 1. It is not suitable for measurement in static condition. 2. Since the device operates with the small electric charge, they need high impedance cable for electrical interface. 3. The output may vary according to the temperature variation of the crystal. 4. The relative humidity rises above 85% or falls below 35%, its output will be affected. If so, it has to be coated with wax or polymer material. Capacitive Transducer Definition: The capacitive transducer is used for measuring the displacement, pressure and other physical quantities. It is a passive transducer that means it requires external power for operation. The capacitive transducer works on the principle of variable capacitances. The capacitance of the capacitive transducer changes because of many reasons like overlapping of plates, change in distance between the plates and dielectric constant. The equations below express the capacitance between the plates of a capacitor Where A overlapping area of plates in m 2 d the distance between two plates in meter ε permittivity of the medium in F/m ε r relative permittivity ε 0 the permittivity of free space

15 The schematic diagram of a parallel plate capacitive transducer is shown in the figure below. The change in capacitance occurs because of the physicals variables like displacement, force, pressure, etc. The capacitance of the transducer also changes by the variation in their dielectric constant which is usually because of the measurement of liquid or gas level. The capacitance of the transducer is measured with the bridge circuit. The output impedance of transducer is given as Where, C capacitance f frequency of excitation in Hz. The capacitive transducer is mainly used for measurement of linear displacement. The capacitive transducer uses the following three effects. 1. Variation in capacitance of transducer is because of the overlapping of capacitor plates. 2. The change in capacitance is because of the change in distances between the plates. 3. The capacitance changes because of dielectric constant. The following methods are used for the measuring displacement.

16 1. A transducer using the change in the Area of Plates The equation below shows that the capacitance is directly proportional to the area of the plates. The capacitance changes correspondingly with the change in the position of the plates. The capacitive transducers are used for measuring the large displacement approximately from 1mm to several cms. The area of the capacitive transducer changes linearly with the capacitance and the displacement. Initially, the nonlinearity occurs in the system because of the edges. Otherwise, it gives the linear response. The capacitance of the parallel plates is given as where x the length of overlapping part of plates ω the width of overlapping part of plates. The sensitivity of the displacement is constant, and therefore it gives the linear relation between the capacitance and displacement. The capacitive transducer is used for measuring the angular displacement. It is measured by the movable plates shown below. One of the plates of the

17 transducer is fixed, and the other is movable. The phasor diagram of the transducer is shown in the figure below. The angular movement changes the capacitance of the transducers. The capacitance between them is maximum when these plates overlap each other. The maximum value of capacitance is expressed as The capacitance at angle θ is given expressed as, θ angular displacement in radian. The sensitivity for the change in capacitance is given as

18 The 180 is the maximum value of the angular displacement of the capacitor. 2. The transducer using the change in distance between the plates The capacitance of the transducer is inversely proportional to the distance between the plates. The one plate of the transducer is fixed, and the other is movable. The displacement which is to be measured links to the movable plates. The capacitance is inversely proportional to the distance because of which the capacitor shows the nonlinear response. Such type of transducer is used for measuring the small displacement. The phasor diagram of the capacitor is shown in the figure below. The sensitivity of the transducer is not constant and vary from places to places. Advantage of Capacitive Transducer The following are the major advantages of capacitive transducers.

19 1. It requires an external force for operation and hence very useful for small systems. 2. The capacitive transducer is very sensitive. 3. It gives good frequency response because of which it is used for the dynamic study. 4. The transducer has high input impedance hence they have a small loading effect. 5. It requires small output power for operation. Disadvantages of capacitive Transducer The main disadvantages of the transducer are as follows. 1. The metallic parts of the transducers require insulation. 2. The frame of the capacitor requires earthing for reducing the effect of the stray magnetic field. 3. Sometimes the transducer shows the nonlinear behaviours because of the edge effect which is controlled by using the guard ring. 4. The cable connecting across the transducer causes an error. Uses of Capacitive Transducer The following are the uses of capacitive transducer. 1. The capacitive transducer uses for measurement of both the linear and angular displacement. It is extremely sensitive and used for the measurement of very small distance. 2. It is used for the measurement of the force and pressures. The force or pressure, which is to be measured is first converted into a displacement, and then the displacement changes the capacitances of the transducer. 3. It is used as a pressure transducer in some cases, where the dielectric constant of the transducer changes with the pressure. 4. The humidity in gases is measured through the capacitive transducer. 5. The transducer uses the mechanical modifier for measuring the volume, density, weight etc. The accuracy of the transducer depends on the variation of temperature to the high level.

20 Temperature Transducers RTD THERMISTOR THERMOCOUPLE RTD A Resistance Thermometer or Resistance Temperature Detector is a device which used to determine the temperature by measuring the resistance of pure electrical wire. This wire is referred to as a temperature sensor. If we want to measure temperature with high accuracy, RTD is the only one solution in industries. It has good linear characteristics over a wide range of temperature. The variation of resistance of the metal with the variation of the temperature is given as, Where, R t and R 0 are the resistance values at t o C and t o 0 C temperatures. α and β are the constants depends on the metals. This expression is for huge range of temperature. For small range of temperature, the expression can be, In RTD devices; Copper, Nickel and Platinum are widely used metals. These three metals are having different resistance variations with respective to the temperature variations. That is called resistance-temperature characteristics. Platinum has the temperature range of 650 o C, and then the Copper and Nickel have 120 o C and 300 o C respectively. The figure-1 shows the resistancetemperature characteristics curve of the three different metals. For Platinum, its

21 resistance changes by approximately 0.4 ohms per degree Celsius of temperature. The purity of the platinum is checked by measuring R 100 / R 0. Because, whatever the materials actually we are using for making the RTD that should be pure. If it will not pure, it will deviate from the conventional resistancetemperature graph. So, α and β values will change depending upon the metals. Construction of Resistance Temperature Detector or RTD The construction is typically such that the wire is wound on a form (in a coil) on notched mica cross frame to achieve small size, improving the thermal conductivity to decrease the response time and a high rate of heat transfer is obtained. In the industrial RTD s, the coil is protected by a stainless steel sheath or a protective tube. So that, the physical strain is negligible as the wire expands and increase the length of wire with the temperature change. If the strain on the wire is increasing, then the tension increases. Due to that, the resistance of the wire will change which is undesirable.so, we don t want to change the resistance of wire by any other unwanted changes except the temperature changes. This is also useful to RTD maintenance while the plant is in operation. Mica is placed in between the steel sheath and resistance wire for better electrical insulation. Due less strain in resistance wire, it should be carefully wound over mica sheet. Thermistor The word Themistor can be termed as Thermal Resistor. So as the name indicates it is a device whose resistance changes with the change of the temperature. Due to there high sensitivity they are widely used for the measurements of the temperature. They are usually called the Ideal Temperature Transducer. Properties of Thermistors They have Negative Thermal Coefficient i.e. resistance of the thermistor decreases with increase in temperature They are made up of the semiconductor materials They are made sensitive than RTD (Resistance Thermometres Detector) and Thermocouples There resistance lies between 0.5Ω to 0.75Ω They are generally used in applications where measurement range of temperature -60 o C to 15 o C

22 Construction of Thermistor Thermistors are generally composed of mixture of metallic oxides such as manganese, nickel, cobalt, copper etc. Smaller thermistors are in the form of beads of diameter from 0.15 millimeters to 1.5 millimeters. Thermistor may be in the form of disks and washers made by pressing thermistor material under high pressure into flat cylindrical shapes with diameter from 3 millimeters to 25 millimeters. Characteristics of Thermistors The relationship governing the characteristics of thermistors is given below as-: R 1 = resistance of thermistor at absolute temperature; T o 1 K. R 2 = resistance of thermistor at temperature T o 2 K. β = constant depending upon material of transducer. From the above equation it can concluded that relationship between temperature and resistance is highly non linear. A thermistor exhibits

23 a negative thermal resistance temperature coefficient of about 0.05/ o C Applications of Thermistors Thermistors are used for the measurement of temperature as change of temperature produces large change in there resistance. They are used for the control of the temperature in any circuit. Thermal Compensation- They can be used for the temperature compensation in any circuit. For example in any circuit a simple carbon resistor is connected which has a positive thermal coefficient. So, if resistance of carbon resistor is to be made unaffected by the temperature so a thermistor having same negative thermal coefficient is connected in series with it. So, as due to rise in temperature resistance of resistor increases and resistance of thermistor decreases by the same amount as magnitude of there thermal coefficients is same. So, as the resistor and thermistor are in series, thus net resistance of resistance remains same.measurement of thermal conductivity of electrical materials.

24 Thermocouple Definition: The thermocouple is a temperature measuring device. It uses for measuring the temperature at one particular point. In other words, it is a type of sensor used for measuring the temperature in the form of an electric current or the EMF. The thermocouple consists two wires of different metals which are welded together at the ends. The welded portion was creating the junction where the temperature is used to be measured. The variation in temperature of the wire induces the voltages. Working Principle of Thermocouple The working principle of the thermocouple depends on the three effects. See back Effect The See back effect occurs between two different metals. When the heat provides to any one of the metal, the electrons start flowing from hot metal to cold metal. Thus, direct current induces in the circuit. In short, it is a phenomenon in which the temperature difference between the two different metals induces the potential differences between them. The See beck effect produces small voltages for per Kelvin of temperature. Peltier Effect The Peltier effect is the inverse of the Seebeck effect. The Peltier effect state that the temperature difference can be created between any two different conductors by applying the potential difference between them.

25 Thompson Effect The Thompson effect state that when two dissimilar metals join together and if they create two junctions then the voltage induces the entire length of the conductor because of the temperature gradient. The temperature gradient is a physical term which shows the direction and rate of change of temperature at a particular location. Construction of Thermocouple The thermocouple consists two dissimilar metals. These metals are welded together at the junction point. This junction considers as the measuring point. The junction point categorises into three types. 1. Ungrounded Junction In ungrounded junction, the conductors are entirely isolated from the protective sheath. It is used for high-pressure application works. The major advantage of using such type of junction is that it reduces the effect of the stray magnetic field. 2. Grounded Junction In such type of junction the metals and protective sheath are welded together. The grounded junction use for measuring the temperature in the corrosive environment. This junction provides resistance to the noise. 3. Exposed Junction Such type of junction uses in the places where fast response requires. The exposed junction is used for measuring the temperature of the gas.

26 The material uses for making the thermocouple depends on the measuring range of temperature. Working of Thermocouple The circuit of the thermocouple is shown in the figure below. The circuit consists two dissimilar metals. These metals are joined together in such a manner that they are creating two junctions. The metals are bounded to the junction through welding.

27 Let the P and Q are the two junctions of the thermocouples. The T 1 and T 2 are the temperatures at the junctions. As the temperature of the junctions is different from each other, the EMF generates in the circuit. If the temperature at the junction becomes equal, the equal and opposite EMF generates in the circuit, and the zero current flows through it. If the temperatures of the junction become unequal, the potential difference induces in the circuit. The magnitude of the EMF induces in the circuit depends on the types of material used for making the thermocouple. The total current flowing through the circuit is measured through the measuring devices. The EMF induces in the thermocouple circuit is given by the equation Where Δθ temperature difference between the hot thermocouple junction and the reference thermocouple junction. a, b constants Measurement of Thermocouple Output The output EMF obtained from the thermocouples can be measured through the following methods. 1. Multimeter It is a simpler method of measuring the output EMF of the thermocouple. The multimeter is connected to the cold junctions of the

28 thermocouple. The deflection of the multimeter pointer is equal to the current flowing through the meter. 2. Potentiometer The output of the thermocouple can also be measured with the help of the DC potentiometer. 3. Amplifier with Output Devices The output obtains from the thermocouples is amplified through an amplifier and then feed to the recording or indicating instrument. Advantages of Thermocouple The following are the advantages of the thermocouples. 1. The thermocouple is cheaper than the other temperature measuring devices. 2. The thermocouple has the fast response time. 3. It has a wide temperature range. Disadvantages of the Thermocouples 1. The thermocouple has low accuracy The recalibration of the thermocouple is difficult. DIFFERENCE BETWEEN RTD,THERMISTOR AND THERMOCOUPLE Sensor type Thermistor RTD Thermocouple Temperature (typical) Range -100 to 325 C -200 to 650 C 200 to 1750 C Accuracy (typical) 0.05 to 1.5 C 0.1 to 1 C 0.5 to 5 C Long-term 100 C 0.2 C/year 0.05 C/year Variable Linearity Exponential Fairly linear Non-linear Power required Constant voltage or current Constant voltage or current Self-powered Response time Fast 0.12 to 10s Generally 1 to 50s slow Fast 0.10 to 10s

29 Susceptibility electrical noise to Rarely susceptible High resistance only Rarely susceptible Susceptible / Cold junction compensation Cost Low to moderate High Low PHOTOELECTRIC TRANSDUCERS PHOTOEMISSIVE PHOTOCONDUCTIVE PHOTOVOLTAIC PHOTOEMISSIVE TRANSDUCER 1. VACUUM PHOTOTUBE 2. GAS-FILLED PHOTOTUBE 3. PHOTOMULTIPLIER TUBE VACUUM PHOTOTUBE(photoemissive cell) Photoemissive cells are are the oldest and most elaborate way of turning light into electricity. They're sealed glass vacuum tubes (from which the air has been completely removed), inside which there's a large metal plate that serves as a negative terminal (or cathode) with a smaller, positively charged, rod-like terminal (or anode) running inside it. The negative terminal is made from a light-sensitive material. When light photons fall on it, they force electrons to leap out of it and these are promptly attracted to the positive terminal, which collects them and channels them into a circuit, producing electric power. This basic design is called a photoemissive cell or phototube. In a slightly different design called a photomultiplier, there's

30 Gas filled phototubes Gas filled phototubes are are the oldest and most elaborate way of turning light into electricity. They're sealed with inert gas like argon which there's a large metal plate that serves as a negative terminal (or cathode) with a smaller, positively charged, rod-like terminal (or anode) running inside it. The negative terminal is made from a light-sensitive material. When light photons fall on it, they force electrons to leap out of it and these are promptly attracted to the positive terminal, which also ionises the argon gas creating more electrons and hence more electrons are attracted towards anode. This basic design is called gas filled phototube. It provides more amplification as compared to vacuum phototube. Photomultiplier tube

31 Photomultipliers acquire light through a glass or quartz window that covers a photosensitive surface, called a photocathode, which then releases electrons that are multiplied by electrodes known as metal channel dynodes. At the end of the dynode chain is an anode or collection electrode. Over a very large range, the current flowing from the anode to ground is directly proportional to the photoelectron flux generated by the photocathode.the spectral response, quantum efficiency, sensitivity, and dark current of a photomultiplier tube are determined by the composition of the photocathode. The best photocathodes capable of responding to visible light are less than 30 percent quantum efficient, meaning that 70 percent of the photons impacting on the photocathode do not produce a photoelectron and are therefore not detected. Photocathode thickness is an important variable that must be monitored to ensure the proper response from absorbed photons. If the photocathode is too thick, more photons will be absorbed but fewer electrons will be emitted from the back surface, but if it is too thin, too many photons will pass through without being absorbed. The photomultiplier used in this tutorial is a side-on design, which uses an opaque and relatively thick photocathode. Photoelectrons are ejected from the front face of the photocathode and angled toward the first dynode. Electrons emitted by the photocathode are accelerated toward the dynode chain, which may contain up to 14 elements. Focusing electrodes are usually present to ensure that photoelectrons emitted near the edges of the photocathode will be likely to land on the first dynode. Upon impacting the first dynode, a photoelectron will invoke the release of additional electron that are accelerated toward the next dynode, and so on. The surface composition and geometry of the dynodes determines their ability to serve as electron multipliers. Because gain varies with the voltage across the dynodes and the total number of dynodes, electron gains of 10 million (Figure 1) are possible if dynode stages are employed. Photoconductive transducer 1. Photoconductive cell 2. Photodiode 3. Phototransistor Photoconductive cell(light dependent resistor)

32 The photoconductive cell is a two terminal semiconductor device whose terminal resistance will vary (linearly) with the intensity of the incident light. For obvious reasons, it is frequently called a photoresistive device. The photoconductive materials most frequently used include cadmium sulphide (CdS) and cadmium selenide (CdSe). Both materials respond rather slowly to changes in light intensity. The peak spectral response time of CdS units is about 100 ms and 10 ms for CdSe cells. Another important difference between the two materials is their temperature sensitivity. There is large change in the resistance of a cadmium selenide cell with changes in ambient temperature, but the resistance of cadmium sulphide remains relatively stable. The spectral response of a cadmium sulphide cell closely matches that of the human eye, and the cell is therefore often used in applications where human vision is a factor, such as street light control or automatic iris control for cameras. As soon as light is incident on photosensitive material its electron bond loosens up and hence its conductivity increases and resistance decreases. Photodiode Working of Photodiode The working principle of a photodiode is, when a photon of ample

33 energy strikes the diode, it makes a couple of an electronhole. This mechanism is also called as the inner photoelectric effect. If the absorption arises in the depletion region junction, then the carriers are removed from the junction by the inbuilt electric field of the depletion region. Therefore, holes in the region move toward the anode, and electrons move toward the cathode, and a photocurrent will be generated. The entire current through the diode is the sum of the absence of light and the photocurrent. So the absent current must be reduced to maximize the sensitivity of the device. Diode Modes of Operation PN Junction The operating modes of the photodiode include three modes, namely Photovoltaic mode, Photoconductive mode and avalanche diode mode Photovoltaic Mode: This mode is also known as zero bias mode, in which a voltage is produced by the lightened photodiode. It gives a very small dynamic range & non-linear necessity of the voltage formed. Photoconductive Mode: The photodiode used in this

34 photoconductive mode is more usually reverse biased. The reverse voltage application will increase the depletion layer s width, which in turn decreases the response time & the junction capacitance. This mode is too fast and displays electronic noise Avalanche Diode Mode: Avalanche diodes operate in a high reverse bias condition, which permits multiplication of an avalanche breakdown to each photo-produced electron-hole pair. This outcome in an internal gain in the photodiode, which slowly increases the device response. Phototransistor The circuit symbol for npn phototransistors is shown by Figure 2 which is nothing but a transistor (with or without base lead) with two arrows pointing towards the base indicating its sensitivity to light. Similar symbolic representation holds well even in the case of pnp phototransistors with the only change being the arrow at emitter pointing in, instead of out. The phototransistor is made up of semiconductor material. When the light was striking on the material, the free electrons/holes of the semiconductor material causes the current which flows in the base region. The base of the phototransistor would only be used for biasing the transistor. In case of NPN transistor, the collector is made positive concerning emitter, and in PNP, the collector is kept negative.

35 The light enters into the base region of phototransistor generates the electron-hole pairs. The generation of electron-hole pairs mainly occurs into the reverse biasing. The movement of electrons under the influence of electric field causes the current in the base region. The base current injected the electrons in the emitter region. The major drawback of the phototransistor is that they have low-frequency response. Photovoltaic transducer The silicon wafer of the photovoltaic solar cell facing the sun consist of the electrical contacts and is coated with an anti-reflective coating that helps absorb the sunlight efficiently. The electrical contacts provide the connection between the semiconductor material and the external electrical load, such as a light bulb

36 or battery. When sunlight shines on a PV cell, photons of light strike the surface of semiconductor material and liberate electrons from the materials atom structure. Certain doping chemicals are added to the semiconductors composition to help to establish a path of the freed electrons. This creates a flow of electrons forming an electrical current which starts to flow over the surface of the photovoltaic solar cell. Metallic strips are placed across the surface of the photovoltaic cell to collect these electrons which forms the positive connection. The back of the PV cell, the side away from the incoming sunlight, consists of a layer of aluminium or molybdenum metal which forms the negative connection to the cell. Then a photovoltaic solar cell has two electrical connections for conventional current flow, one positive, and one negative. The type of solar power produced by a photovoltaic solar cell is called direct current or DC the same as from a battery. Most photovoltaic solar cells produce a no load open circuit voltage (nothing connected to it) of about 0.5 to 0.6 volts when there is no external circuit connected. This output voltage ( V OUT ) depends very much on the load current ( I ) demands of the PV cell. For example on very cloudy or dull day the current demand would be low and so the cell could provide the full output voltage, V OUT but at a reduced output current. But as the current demand of the load increases a brighter light (solar radiation) is needed at the junction to maintain a full output voltage, Vout.

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