Q@ 9 Transducers

7/29/2019 Q@ 9 Transducers

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Q. 1. What is a transducer?

Ans. The device which converts an energyfrom one form to another is known astransducer The input energy may beelectrical, mechanical, chemical, thermal oroptical

Q 2 Give classification of transducers

Ans The transducers may be classified as

(1) Electrical Transducers. These are thetransducers which give electrical energy asoutput (voltage, current, frequency) etc.

(2) Displacement Transducers. Thesetransducers convert displacement (mechanicalenergy) into electrical energy

(3) Optoelectronic Transducers Thesetransducers convert light energy into

electrical energy, For example photoconductive

cell, photovoltaic cell, solar

cell, etc

Out of the above, electrical transducers are themost popular These transducers tire used for


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measurement of non-electrical quantities(force, pressure, etc.) by changing

them into electrical quantities.

Q 3 Define electrical transducers

Ans The transducers, which convert nonelectrical quantities (force, pressure, sound

Ltd) into electrical quantities are calledElectrical transducers, it then becomes easierto measure a non electrical quantity

Q. 4. What are Inverse transducers?

Ans The inverse transducers convert electricalquantities into non-electrical

quantities, e g Piezo electrical transducers Therecording devices which are used in

feed-back measurement systems are also atype of Inverse transducers

Q. 5. What are variable inductance type

transducers?


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Ans. These are analog passive transducers.They work on one of the following

Principles:

(i) Change of self inductance of a coil.

(ii) Change of mutual inductance of a coil.

(iii) Production of eddy currents.

Q. 6. Differentiate between Transducers

and Inverse transducers?

Ans. Transducers are normally associated witha circuit which has non-electrical input andelectrical output e.g. pressure gauge, straingauge etc.

An inverse transducer is a device that convertselectrical quantities into non-electricalquantities e.g. piezo-electric crystal.

Q. 7. What is a strain gauge?

Ans. The strain gauge converts strain into achange of electrical resistance.

Q. 8. What are the types of strain gauges?


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Ans. There are four types of strain gaugesnamely:

1. Wire strain gauge

2. Foil strain gauge

3. Film strain gauge -

4. Semiconductor strain gauge

Q. 9. Give other classification of strain

gauges.

Ans. 1. Bonded

2.Unbonded

Q. 10. What is a Resistance Strain Gauge?

Ans. It is the most commonly used transducerfor measurement of displacement.

A resistance strain gauge consists of a grid offine wire cemented to a thin paper or

bakellite sheet and covered with a protectivecovering.

Q. 11. What is a semiconductor straingauge?


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Ans. A semiconductor strain gauge employs asemiconductor material commonly

silicon. As the resistively of semiconductor ishigher than a conductor, it is possible to

have a sensitive element as shorter, wider andsimpler.

Q. 12. What is a thermocouple?

Ans. A thermocouple consists of dissimilarmetals which when joined produce an

e.m.f when their one junction is heated.

Q. 13. What is thermocouple gauge usedfor detection of leaks in vacuum systems.

Ans. The thermocouple gauge consists of anelectrically heated wire operating at a

temperature of approximately 320K mounted

inside a glass or metal envelope.

Q. 14. Write desired properties of thermo-couple metals.


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Ans. 1. They should not melt within the rangeof temperature requirements.

2. They should not corrode at their highesttemperature.

3. The emf generated by the couple mustremain constant for a given temperaturethroughout its life.

Q. 15. What type of scale is used in

thermocouple meters?Ans. In a thermocouple meter, there is asquare law scale.

Q. 16. What is the main application ofthermocouple meter?

Ans. It is mainly used to measure radiofrequency currents having frequencies tothousands of megahertz.

Q. 17. Can a thermocouple measure AC.

Ans. Thermocouple is capable of measuring ACof high frequencies very accurately. It is based


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on the principle of generation of heat due tocurrent.

Q. 18. What is Hall Effect transducers,how it can be used for measuring

displacement?

Ans A Hall Effect transducer is based on theeffect of magnetic fields on moving

charges.

A Hail Effect transducer is located within a gap.The output voltage in this hafi effecttransducer depends upon the strength of themagnetic field in the gap. Any displacement inthe ferro-magnetic Structure changes the

magnetic field and hence the output voltage.This method can be used to measuredisplacement down to 0.001 inch

Q. 19. What are the possible applicationsof Hall Effect Transducers?

Ans. 1. It can be used to measure strength ofa magnetic field in terms of an electricalvoltage..


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2. It can also be used to measure currentwithout cutting the conductor and withoutmaking any electrical connections between

conductor circuit and the meter.

3. Hall effect transducer can be used as ananalog multiplier.

Q. 20. What are the points to be

considered for selecting transducer?

Ans. 1. Range .

2. Sensitivity

3. Electrical output characteristics -

4. Physical environment

5. Errors

Q. 21. What are Pyrometers?

Ans. Pyrometers are instruments, which areused for the measurement of very hightemperatures.

Q. 22. Name the types of Pyrometers.


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Ans. 1. Resistance pyrometers.

2. Thermo-couple pyrometers

3. Radiation pyrometers

4. Optical pyrometers i

Q. 23. What is LVDT?

Ans. It is a Linear Voltage DifferentialTransformer.

Q. 24. Define Gauge Factor of a strain

gauge.

Ans. Gauge factor is defined as the ratio of

unit change in resistance to unit change inlength.

Gauge factor, K = AR/R

AL IL

where, R = Resistance of gauge wire

AR = Change in resistance

L Length of wire

AL = Change in length


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Q. 25. Give Gauge Factor for fewmaterials.

Ans. The table lists gauge factors for differentcommonly used wire materials.

Table 1.

Material Gauge factor

nickel 12.10manganin 0.47nichrome 2.0Constanant 2.1platinum 4.8Carbon 20.0

Soft iron 4.2Doped crystals 100 to 5000

Q 26 Mention applications of a resistance

strain gauge for measurement of nonelectric quantities.

Ans 1 Strain gauges are extensively used foranalyzing dynamic strains in complex


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structures such as in Bridges, Automobiles,Roads, etc.

2 Resistance wire strain gauges are very usefulfor measurement of Tension, Torque, Forceand Stresses in structures.

3. Strain gauges are used in load cells and loadrings to measure force producing strain.

Q 27 Strain in a beam is being measured

with the help of a strain gauge attachedto the beam. The resistance of the straingauge (having a gauge factor of 3)

changes from 150 ohms to 151 ohmswhen strained. Determine the strain in thebeam to which the gauge has been

attached. I

Ans. Strain (AL/L) = Change in resistande/Unstrained resistance

Gaugefactor

= 1/150

3


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= 0.0022 Ans

Q. 28. What is a SYNCHRO TRANSDUCER?

Ans. Synchro transducer is a POSITIONMEASURING DEVICE.

Q. 29. What are optical pyrometers?

Ans. The optical pyrometers are used forhigher temperature measurement up to3500C such as temperature of furnaces,molten metals and other heated materials.

Q. 30. Give advantages of RadiationPyrometers.

Ans. 1. They can measure very hightemperature.

2. They can be kept away from hot body.

3. They can measure temperature of moving

object.

Q. 31. Give disadvantages of RadiationPyrometers.


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Ans. 1. They are costly.

2. High maintenance is required.

3. Each pyrometer has individual calibration.

Q. 32. What are the different types ofstrain gauges ? Name four resistancematerials used in wire and foil gauges.

Ans. Nichrome, constantan, nickel, platinum.

Q. 33. Name two light sensitive

transducers.

Ans. These are

1. Photo conductive cell 2. Photo voltaic cell

Q. 34. What is an Eddy currenttachometer?

Ans. This meter can measure rotational speed

of a shaft. This works on the principle of eddycurrents.

Q. 35. What is Tacho generator?


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Ans. This. can measure rotational speed froma distance.

Q. 36. What is stroboscope?Ans. This is an equipment which producespulses of light at a controlled frequency. This,is used to measure rotational speeds of shaftwith more accuracy.

Q. 37. Mention major applications ofL.V.D.T.s?

Ans. 1. These are used for direct displacementmeasurement e.g., thickness measurement,gauging machine profile, surface roughness

etc.

2. LVDTs have also been used for force andpressure measurement.

Q. 38. Enlist advantages of LVDT.

Ans. These are

1. Frictionless instruments.

2. Hence, long mechanical life


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3. Infinite resolution.

Q. 39. What is V.R.I.T. ?

Ans. It stands for Variable ReluctanceInductive Transducer. It is used for measuringdisplacement.

Q. 40. What are the essential components

of a Thermocouple instrument that can be

used for measurement of an unknowncurrent or voltage?

Ans. 1. The HEATER ELEMENT that carries thecurrent to be measured.

2. The THERMQCOUPLE having its hot junctionin contact with the heater and its cold junctionat room temperature.

3. A sensitive PERMANENT MAGNET MOVINGCOIL METER that operates on the thermale.m.f generated in the thermocouple.

Q. 41. What are, the various advantagesof thermocouple instruments?


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Ans. The thermocouple instruments have thefollowing advantages:

1. The thermocouple instruments correctlyindicate the R.M.S. value of current or voltage.

2. These instruments are not affected by straymagnetic fields.

3. These instruments can be used over a widefrequency range.

4. They can measure a.c. as well as d.c.

Q. 42. Give disadvantages of

thermocouple instruments.

Ans. The thermocouple instruments have the

following disadvantages:

1. The overload capacity of thermocoupleinstruments is small as compared to that ofother instruments.

2. The fuses dont provide any protection asthe heater wire may burn before the fuseblows off.

Q. 43. Define : Thermoelectric Inversionand Neutral Temperature.


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Ans. Thermoelectric inversion means reversalof polarity of thermoelectric e. m.f As thetemperature of the hot junction is increased

above the temperature of the cold junction, thethermoelectric e.m.f increases proportionally.After a certain temperature for the hot

junction, the e.m.f starts decresing withincrease in temperature. This transitiontemperatures is called the NEUTRALTEMPERATURE.

Q. 44. What is the principle of operation ofa RESISTANCE THERMOMETER?

Ans. The variation in resistance of a metalwith temperature is the basis of temperaturemeasurement in a resistance thermometer.Platinum or Tungsteri wire is wound on aformer to give a resistance in the range of 10ohms to 10 kilo-ohms.

Q. 45. Give advantages and disadvantages

of resistance thermometers.

Ans. (a) Advantages : These provide themost accurate method for temperature


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measurement. These have a simple installationand need only copper cables for long rims.

(b) Disadvantages : These need energisingcurrent. These are large in size as

compared to thermo couples.

Q. 46. Give advantages and disadvantages

of thermocouples.

Ans. (a) Advantages These have a widetemperature range, are versatile and are

simple in application. These are available in avariety of package configuration. They

are available in an insulated cable housing or

ultra fine wires.

(b) Disadvantages : Thermocouples need atemperature reference for their

operation. They need great attention wherehigh accuracy is needed. They need

extension cables for long runs.

Q. 47. What are the advantages of

semiconductor strain gauge?


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Ans. 1. They are free from hysteresis andcreep error.

2. They have good frequency response.

3. They are of small size.

Q. 48. Give disadvantages ofsemiconductor strain gauge.

Ans. These semiconductor gauges areexpensive, brittle, highly sensitive to

temperature variations and have poor linearity.

Q. 49. Mention one disadvantage of LVDT.

Ans The major disadvantage of LVDT is thehazard of induced noise in an

Electro magnetically noisy environment:However, this can be overcome with proper

shielding.

Q. 50. Give some thermocouple materialsand temperature range.

Ans Table 2


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Combination ofThermocouple

materials

TemperatureRange

Copper-Constantan - 200C to 550CIron-Constantan - 20C to 750CChromel-Alumel +10% Rhodium

300C to 1000C

Platinum-Platinum +13% Rhodium

700C to 1600C

Q. 51. Mention desirable characteristics ofstrain gauges.

Ans. 1. Should have very good frequencyresponse.

2. Should have high value of gauge factor.

3. Should not have hysteresis or creep effect.

Q. 52. What are the following: Shaft

encoder, Incremental encoder andAbsolute encoder.


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Ans. 1. Shaft encoders are ANGULARMEASUREMENT devices that make use ofoptical or magnetic technique to measure the

shaft angle in terms of a digital

number.

2. Incremental encoders have simple ON andOFF coded pattern. The output is in the form ofnumber of counts between start and end ofdisplacement.

3. The absolute encoders have a coded patternthat gives a unique digital

output for each discrete displacementincrement. Different codes are

practiced for this purpose such as Binary,

Binary coded decimal and Gray

code.

Q. 53. What is the principle of operation of

HOT WIRE GAUGES?

Ans. HOT WIRE GAUGES depend for theiroperation on the thermal conductivity


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of the heated wire due to change in pressure.These are normally used to measure pressureof gases and vapours.

Q. 54. What is a Thermistor?

Ans. A thermistor is a device whose resistancevaries with temperature. These devices have avery high temperature co-efficient ofrsistance.

Q. 55. What are NTC and FTC thermistors.

Ans. The NTC thermistors have a negativetemperature coefficient of resistance

i.e. their resistance decreases with increase intemperature. These are basically ceramicmaterials.

PTC thermistors have a positive temperaturecoefficient of resistance i e their

resistance increases with temperature

Q 56 Define for thermistors

1 Dissipation constant 2. Time constant 3Break point


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Ans 1 DISSIPATION CONSTANT is the amountof power dissipated by the thermistors which

will raise its temperature by I C. Typical valueIOmW/C.

2 The TIME CONSTANT is the time taken b thethermistor to mdicate 63% of the newlyimpressed temperature input Agam it may beseveral seconds

3 The BREAK POINT of a FTC thermistor is apoint of the rapidly rising portion of itsresistance vs temperature characteristics afterthe Curie temperature, where its resistance isapproximately five times its base resistance

Q 57 Give basic requirements of a

transducer

Ans Following are the basic requirements of atransducer

1 Ruggedness It should be capable of

withstanding overloads and some safetyarrangement should be provided for over loadprotection


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2. Linearity. Its input-output characteristicsshould be linear and it should produce thesecharacteristic in symmetrical way.

3. No Hysterisis. It should be free fromhysterisis while input signal is varied from itslow value to high value and vice versa

Q. 58. What are primary and secondary

transducers?

Ans. When the input signal is directly sensedby the transducer and is converted intoelectrical form directly, such a transducer iscalled primary transducer.

When the input signal is first sensed by some

detector or sensor and its output being ofsome form other than input signal is given asinput to a transducer for conversion intoelectrical form, such transducer is calledsecondary transducer.

Q. 59. What are the advantages of anelectrical transducer?

Ans. Following are the advantages of anelectrical transducer


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1; Frictional error will be less in electricaltransducer.

2. Mass-inertia effects are reduced to theminimum possible.

3. Very small power is required for controllingelectrical system.

4. The electrical output can be amplified to anydesired value.

Q. 60. What is a spring?

Ans. A spring is a simplest transducer, whichconverts force into proportionate displacement.

Q. 61. What is the basic principle of

operation of INDUCTION TYPEtransducers?

Ans. Induction type transducers are based onthe Faradays law of electromagnetic inductionwhich talks about an e.m.f being induced in a

conductor placed in a magnetic field wheneverthere is a rate of change of flux linking theconductor.


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Q. 62. How capacitive transducers aresuperior to other transducers?

Ans. 1. Their power requirements is verysmall.

2. These have a very good dynamic response,as the capacitor plates themselves have verylittle mass.

Q. 63. What are Active and Passivetransducers?

Ans. The transducers, which develop theiroutput in the form of electrical voltage orcurrent without any auxiliary source are calledthe active transducers.

Transducers in which electrical parameters i.e.resistance, inductance or capacitance changewith change in input signal are called thepassive transducers.

Q. 64. What are Analog and Digitaltransducers?


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Ans. Analog transducers convert input signalinto the output signal which is a continuousfunction of time, e.g. thermistor, strain gauge

thermocouple etc.

Digital transducer convert input signal into theoutput signal in the form of pulses.

Q. 65. What is pulse counter?

Ans. This is an electronic instrument used tocount pulses.

Q. 66. What is Float?

Ans. This is a simple device of measuring

displacement of liquid surface. In other wordsit is a water level Indicator.

Q. 67. What is the use of spring as a

transducer?

Ans. The spring can measure force byconverting it into displacement.

Q. 68. What is Bourden Pressure Tube?


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Ans. It is a transducer, which can measurepressure.

Q. 69. What is a Piezeo electrictransducer?

Ans. It is used to measure pressure byconverting into electrical energy.

Q. 70. What are the factors to be kept in

mind to select a transducer?

Ans. (a) The quantity to be measured.

(b) Accuracy required.

Q. 71. Write advantages of Flow meters.

Ans. (i) There is no obstruction in the velocity.

(ii) The output is proportional to the velocity.

(iii) They are not affected by the viscosity,

density and change in temperature.(iv) There is no moving part.

(v) They lend themselves to bidirectional flow.


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But due to complexity and high costs, their useis limited.

Q. 72. Enlist few mechanical transducersand modes of operatin.

Ans. Few mechanical transducers andtheir mode of operation is given:

Table 3.

Sr.no Mechanical

transducersMode of operation(conversion)

1. Burden tube Pressure todisplacement

2. bellow Pressure to

displacement3. diaphram Pressure to

displacement4. spring Force to displacement5. Pendulum Force to displacement6. manometer Pressure to

displacement7. Thermocouple Temperature to

current8. bimetal Temperature to

displacement


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9. Float Fluid level Pressure todisplacement

10. hydrometer Sp gravity to

displacement11. venturi Velocity to pressure12. vane Velocity to force

Q. 73. Write a note on selection of

transducer.Ans The transducer are selected on the basicsof following points

(i) Range The transducer should havesufficient range of operation

(ii) Sensitivity It should give sufficient outputper unit of measured input

(iii) Good electrical characteristics Theelectrical characteristics such as outputimpedence, frequency response, response time

of transducers output signal should becompatible with the recording device

(iv) Environment friendly. The transducerselected should be able to withstand the


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environmental conditions such temperature,moisture, etc.

(v) Errors. The errors produced by transducerand caused by environment should small.

Q 74 What are the basic requirement of atransducer?

Ans The basic requirements of a transducer

are

(i) Ruggedness It should be capable towithstand overloads

(ii) Linanty Its input-output characteristicshould be linear

(iii) Repeatability It should give same outputfor the same input, if applied again and again

(iv) High output quality The output shouldhave high signal noise ratio

(v) No hysterisis It should not give anyhysterisis effect, when varied from low

to high value or vice-versa

Q 75 Enlist advantages of Electricaltransducers


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Ans Advantages of Electrical Transducers

Usually, physical quantities beforemeasurement are converted into electricalquantity

It has the following advantages

I Amplification Attenuation of an electricalquantity can be done easily

2 The effects of friction are minimized.

3 The electrical/ electronic system can beeasily controlled and that too with little power

4 Telemetry used now-a-days in modeminstrumentation This is possible only in

electrical/electronic systems

5 The use of I C s has brought a revolution inthe field of instrumentation

6 The electrical output can be easily used,transmitted and processed

Note More Short Answer Questions are givenat the end of the Chapter


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SHORT & LONGANSWER TYPE QUESTIONS

Q 76 Describe elements and requirementsof transducers

Ans. (a) Elements. A transducer may be

thought of consisting of two closely relatedelements (Fig. 1).


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1. Sensing or Detecting Element. Thiselement senses detects the physical quantityunder measurement.

2. Transduction Element. This elementtransfers the output of the sensing elementinto, an electrical output.

In addition, a transducer may have amplifiers,power supply, calibrated scale, etc.

(b) Requirements. The transducers shouldhave the following requirements:

1. Mechanical Ruggedness. The transducershould be mechanically rugged. It should beable to withstand the working conditions andmechanical strains.

2. Linear Output I Input Characteristic. Thetransducers should have linear output/inputcharacteristic.

3. Reliability. The results of the transducersshould be reliable. The result should also bestable and should not change with temperature

and other changes.4. Good Response. If the input to thetransducer varies with time (in few cases, itdoes) the transducers should have goodresponse to the variations.


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5. Low Noise. The noise / signal ratio of thetransducers should be low so that its outputcan be directly measured with sufficient

accuracy.

Q. 77. Give comparison between Activeand Passive transducers.

Ans. Table 4.

Sr.no Active transducers Passive transducers1. They are self

generating i.e. theydevelop output(voltage or current)without any.auxiliary device,

They are not selfgenerating i.e. theyneed an externalpower source to giveelectrical output(voltage, current).

2. They draw energyfrom the quantityundermeasurement.

They need an externalenergy source.

3. They give very low

output andamplification isnecessary.

They give sufficient

output, which maynot need anyamplification.

4. Examples :Thermocouples,

Examples Inductiveand capacitive


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Piezo- electrictransducers etc.

transducers etc.

Q. 78. What are variable resistance typetransducers? list few examples.

Ans. In these transducers, resistancebetween the output terminals gets variedaccording to the quantity under outputterminals.

where R = resistance of the conductor

p = resistivity of the material of the conductor

= length of conductor

a = cross-section of the conductor.

Resistance transducers are preferred as ac anddc both supplies are suitable fa resistance

measurement.Examples of there transducers are

(1) Potentiometer


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(2) Strain gauge

(3) Thermistor

(4) Resistance hygrometer.

Q. 79. Explain the following astransducers:

(1) microphone and loudspeaker

(2) Video cameras

(3) Human ear

(4) Human eye.

Ans. (1) A microphone is a transducer, whichconverts sound into electrical energy. Aloudspeaker is a transducer which convertselectrical energy back to sound energy. Thetwo constitute a public address (PA) systemFig. (2). This principle is used in radiotransmission and reception.


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(2) The video camera is a transducer whichconverts picture (light) into electrical energy.

The picture tube is a transducer which convertsthe electrical energy back into light (picture).

The principle is used in TV transmission andreception (Fig. 3).

(3) The human ear acts as a transducer whichconverts sound into electrical pulses which goto our brain; the brain also acts as atransducer and converts back the electricalpulses into the original sound and we feel thesound.

(4) The human eye acts as a transducer whichconverts a picture into electrical pulses whichgo to our brain which, in turn also acts as atransducer and reconverts back the electricalpulses into the original picture and thus we can

see the object.

Our eye is like a camera which can take infinitephotographs black and white as well as coloursimultaneously and can send them to the


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brain. Our eye can change focal length to seenear and distant objects, while camera cannot.

Q. 80. Explain the term Hysterisis effectfound in transducers.

Ans. The output of a transducer depends notonly on the input quantity applied at present,but also depends on the input quantitiesapplied previously. A different output

is obtained, when the same input quantity isapplied repeatedly For decreasing values, agreater output is obtained than increasingvalue of the same quantity.

This phenomenon is called as Hysterisis, alltransducers suffer from this effect.

Q@ 9 Transducers

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