CH-1 TRANSDUCERS Marks: 17 (25)
Introduction:
Definition: A transducer is defined as a device that converts one form of energy into another form. They are also called Sensors.
While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a transducer.
The energy transmitted by transducer may be electrical, mechanical, acoustical (sound), electromagnetic, chemical or thermal. The nature of electrical output from transducer depends on basic principle involved in the design. The output may be analog, digital or frequency modulated. Basically, there are two types of transducers electrical and mechanical.
An electrical transducer is sensing device. In this, physical, mechanical or optical quantity to be measured is transformed directly by a suitable mechanism into an electrical voltage or current proportional to input measured.
Active and passive transducers:
(a) Active transducer: It generates an electrical signal directly in response to physical parameter. It does not require an external power source for its operation. Active transducers are self generating devices. They operate under energy conversion principle and generate equivalent output signal without any external supply.
e.g. thermocouple, photovoltaic cells, microphones, loudspeakers, thermometer, electric bulb, electric motor etc.
(b) Passive transducer: It requires external power with them. They produce change in electrical quantities like resistance, capacitance. For example, thermistor , it changes its resistance with temperature . But it requires electric supply to produce current variation in circuit. LVDT needs electric supply to perform its function.
e.g. strain gauge and thermistors, antenna, LVDT.
Selection of transducer:
Transducer should be selected by considering following points-
Transducer should have wider operating range.
Transducer should
to environmental conditions like temperature, pressure etc.
Transducer should respond to smallest change to input variable.
Operating power supply of respond to weak signal
Frequency response of transducer should be constant.
Transducer should be compatible transducer should be low.
Characteristics of transducers:
1. Input characteristics:
a) Types of input and operating range.
b) Loading effect
2. Transfer characteristics:
a) Transfer function
b) Errors
c) Transducer response
3. Output characteristics:
a) Type of electrical output
b) Output impedance
c) Useful output range
Input Transducers:
Input Transducers convert a quantity to an electrical signal (voltage) or to resistance (which can be converted to voltage). Input transducers are also called sensors
Examples: LDR converts brightness (of light) to resistance.
Thermistor converts temperature to resistance.
Microphone converts sound to voltage.
Output Transducers
Output Transducers convert an electrical signal to another quantity.
Examples: Lamp converts electricity to light.
LED converts electricity to light.
Loudspeaker converts electricity to sound.
Motor converts electricity to motion.
Heater converts electricity to heat.
Resistive transducers:
In these transducers, resistance changes due to change in physical phenomenon.
(a) Resistance pressure transducers (strain gauges): In these cases, change in pressure results in resistance change of sensing element.
A strain gauge is basically a long very thin strip of resistive material. It is bonded to the surface of an object on which strain is to be measured. When a force acts on the object to cause a slight elongation, the strain gauge also lengthens proportionally and its resistance increases.
There are two types of strain gauges,
(b)Resistance thermometer: It is temperature transducer. The resistance of conductor changes when its temperature changes. This property is utilized for measurement of temperature. It is transducer used to measure electrical resistance of conductor to determine the temperature.
Resistance thermometers are also called resistance temperature detectors (RTDs). Many RTD elements consist of a length of fine wire wrapped around a ceramic or glass core. The RTD wire is a pure material, typically platinum, nickel, or copper. The material has an accurate resistance/temperature relationship. As RTD elements are fragile, they are often housed in protective probes.
(c) Thermistor: It is also temperature transducer. The electrical resistance of most materials changes with temperature. Thermistors are non-metallic resistors (semiconductors) made by sintering mixture of metallic oxides such as manganese, nickel, cobalt, copper and uranium. Thermistors have negative coefficient (NTC) i.e. resistance decreases as temperature rises. The resistance of thermistors may decrease by 5% for each of 10C rise in temperature. It converts temperature to resistance. Fig. shows graph of resistance versus temperature for thermistor.
The thermistors are made in the form of beads, disc, washer or rod forms.
Thermistors are non-linear devices over temperature range. They are chemically stable.
Types of thermistors:
Negative temperature coefficient (NTC thermistor) This type of thermistor has the property where the resistance decreases with increasing temperature, i.e. k is negative. Read more about the NTC thermistor
Positive temperature coefficient (PTC thermistor) This type has the property where the resistance increases with increasing temperature, i.e. the value of k is positive. Read more about the PTC thermistor
Application (Temperature measurement):
For measuring temperature, a Wheatstone bridge is used. Out balance voltage ΔV, can be measured and related to the resistance of the thermistor. Any change in temperature causes a change in thermistor resistance and corresponding change in circuit voltage.
(d) Thermocouple: It is temperature transducer. It consists of pair of wire made up of different metals joined together at one end. Temperature difference between two ends of wire produces voltages between two ends of wires. This effect is called Seebeck effect. The work functions of two materials are different. A thermo electromotive force is generated.
Copper is used as an element and constantan is used an alloy of nickel and cooper. If both junctions are at same temperature, the output will be zero. The output is proportional to difference between the two junctions temperature. The reference junction is in an ice bath maintained at 00C.
Inductive transducer:
Linear variable differential transducer (LVDT):
Principle: It works on principle of mutual induction. It is inductive pressure transducer. It converts pressure to electric signal. Output is proportional to amount of pressure exerted on core.
Construction and Working:
It consists of a single primary winding and two secondary have equal number of turns. The primary winding is connected to an AC source.
A moveable soft iron ferromagnetic core slides or moves up and down inside the tubular body of the LVDT.
If core armature is exactly in the centre of the tube and the windings, the two induced emf’s in the two secondary windings cancel each other. So the resultant output voltage is zero. This is called null position.
As the core is displaced slightly from this null or zero position, the induced voltage in one of the secondaries will be become greater than that of the other secondary and an output will be produced.
Active transducer-Photovoltaic Cell (Solar Cell):
Principle:
Photovoltaic cell works on principle of photovoltaic effect. It collects solar radiation from the sun and actively convert that energy to electricity.
Photovoltaic cell functions similarly to large semiconductors and utilize a large-area p-n junction diode.
When the photovoltaic cells are exposed to sunlight, the p-n junction diodes convert the energy from sunlight into usable electrical energy.
The conversion of sunlight to usable electrical energy is called the Photovoltaic Effect.
Construction:
The surface layer of p-type material is extremely thin, so that light can penetrate to the junction.
The nickel plated ring around the p-type material is positive output terminal, and the plating at the bottom of the n-type is negative output terminal.
Working:
When light, in the form of photons, hits our photovoltaic cell, its energy frees electron-hole pairs. Each photon with enough energy will normally free exactly one electron, and result in a free hole as well.
If this happens close enough to the electric field, the field will send the electron to the N side and the hole to the P side.
This causes further disruption of electrical neutrality.
If we provide an external current path, electrons will flow through the path to their original side (the P side) to unite with holes that the electric field sent there.
The electron flow provides the current, and the cell's electric field causes a voltage. With both current and voltage, we have power, which is the product of the two.
Capacitive transducer:
Condenser Microphone:
Construction and working:
A capacitor has two plates. In the condenser mic, one of these plates is made of very light material and acts as the diaphragm. The diaphragm vibrates when struck by sound waves, changing the distance between the two plates and therefore changing the capacitance. Specifically, when the plates are closer together, capacitance increases and a charge current occurs. When the plates are further apart, capacitance decreases and a discharge current occurs.
A voltage is required across the capacitor for this to work. This voltage is supplied either by a battery in the mic or by external power.
Uses:
1. To record vocal of singer.
2. To capture and record sounds of musical instruments.
3. In live consorts, orchestras, public meetings etc.
Optical/Light transducers (Photo-electric transducer):
1. LDR (Light Dependent Resistor):
Construction of LDR:
Photosensitive material is arranged in the form of long strip, zigzagged across disc shaped base with protective sides.
For protection, a glass or plastic cover may be included.
Two ends of the strip are brought out.
Two metal grids are cemented to the these ends which serve as terminals for external connection
Symbol Construction
Working:
It works on principle of photo conductivity. It is also called photoresistor.
Light can provide sufficient energy to break away electrons from their atoms within material and liberating electrons from surface of material.
Free electrons and holes are created within material and hence its resistance decreases.
Such materials are called photosensitive or photoconductive materials. Cadmium sulfide (CdS) and (CdSe) are the common photosensitive materials.
When it is not illuminated its resistance is greater than 100KΩ. This is known as the dark resistance of LDR.
When illuminated, the LDR resistance falls to few ohms.
Applications/uses:
CdS type photoresistors are used in counting applications.
These are used for detecting ships and aircrafts by radiation given out by their exhausts.
CdS photoresistors are used in light meters.
As ON-OFF switch.
For relay control.
In street light control.
As volume control that is controlled by light levels.
2. Photodiode:
A photodiode is a semiconductor device that converts light into an electrical current.
The current is generated when photons are absorbed in the photodiode.
Photo means light and diode means a device consisting of two electrodes.
A photo diode is a light sensitive electronic device capable of converting light into a voltage or current signal.
It works on the principle of photo generation.
Photodiode Construction:
Photodiode is formed by diffusion of lightly doped P-type impurity into heavily doped N-type impurity.
Area between P-type impurity and N-type impurity is depletion region.
The contact terminal at N-type area is cathode and that of P-type area is anode.
Active area on front side is coated with anti reflection material to reduce the reflection of light and other area is coated with thick layer of silicon dioxide(SiO2).
Photodiode Working:
The circuit has reverse biased photodiode, resistor R and DC power supply.
When no light is incident on photodiode, the reverse current is extremely small. This is called dark current.
When light is incident on pn-junction of photodiode, there is transfer of energy from incident light to atom in junction. This will cause more free electrons.
This will increase the reverse current. As intensity of light increases, reverse current goes on increasing till it becomes maximum.
Applications/uses of photodiode:
Consumer electronics devices like smoke detectors, compact disc players, and televisions and remote controls in VCRs.
Automotive devices
Detection of visible and invisible light.
In optical communication instrument.
Character recognition.
Cameras
Medical devices
Bar code scanners
3. Phototransistor:
Phototransistors are either tri-terminal (emitter, base and collector) or bi-terminal (emitter and collector) semiconductor devices which have a light-sensitive base region.
Symbol
Construction and Working:
It consists of two silicon or germanium p-n junctions as ordinary transistor.
Window lens is provided on the base.
The light falls on the base region through window lens the and direct voltage is applied across emitter and collector.
The collector current and emitter current is a function of light intensity which falls on the base region.
Due to amplifier action very small changes in light intensity can be detected by photo transistor.
It has very high sensitivity for the small change in the intensity of the light, compared to LDR or photo diode.
Application of phototransistor: Phototransistors are often used in smoke detectors, infrared receivers, and CD players. Phototransistors can also be used in astronomy, night vision, optocouplers and laser range-finding.
QUESTIONS
Marks wise Questions: 1M(4), 3M(3), 4M(3)
1. Fill in the blanks.
----------- is an active transducer.
(a) Thermistor (b) Strain gauge (c) Microphone (d) Capacitor
---------- is a passive transducer.
(a) Thermistor (b) Photo-voltaic cell (c) Microphone (d) Thermocouple
Thermistor is ----------- device.
(a) Linear (b) Metallic (c) Semiconductor (d) Active
----------- converts light energy into electrical energy.
(a) Thermistor (b) Thermocouple (c) Microphone (d) Photocell
Thermocouple works on the principle of ------------.
(a) Joule’s effect (b) Magnetostriction effect
(c) Piezo-electric (d) Seebeck effect.
--------- transducers are called as self generating transducers.
(a) Passive (b) Active (c) Thermistor
7. In capacitive transducer, an electric voltage is generated by varying its --------.
(a) Area (b) Dielectric (c) Distance
8. LVDT is ------------ transducer.
(a) Resistive (b) Capacitive (c) Inductive
9. Active transducers are normally operated by --------.
(a) Input signal (b) External supply (c) None
2. Match the following.
A B
(a) Thermistor (1) Pressure
(b) Photocell (2) Seebeck effect
(c) Microphone (3) Light
(d) Thermocouple (d) temperature
(e) LVDT (5) Sound
3. State true or false
Transducer converts non-electrical quantity into analogue electrical quantity.
Strain gauge is active transducer.
Thermistor is non-linear device.
Capacitance transducer is temperature transducer.
Thermocouple is pressure transducer.
Secodaries in LVDT have equal number of turns.
4. Answer the following.
Define and explain transducer.
Explain input transducers with examples.
Explain output transducers with examples.
Explain active and passive transducers with examples.
With diagram explain bonded and unbonded strain gauges.
State principle and construction of thermistor.
Explain principle and construction of resistive thermometer..
With diagram explain working of LVDT.
Explain working of capacitive transducer with neat diagram.
State and explain principle and construction of thermocouple.
Compare active and passive transducers.
Write working principles transducers-(i) Thermistor (ii) Thermocouple (iii) LVDT
Define optical transducer. Give its examples.
