1. Transducers
Saneeju M Salu

2. Transducer
Any device that converts energy in one form to energy in another from
Device which converts one physical quantity or condition to another
Physical quantity – heat , intensity of light, flow rate, liquid level, humidity etc
Transducer
Physical
Quantity
Electrical
Quantity
(Current/ Voltage)

3. Classification of transducers
Based on principle of transduction
Active & passive
Analog & digital
Based on Electrical Principle

4. Based on principle used
Mechanical – strain guage,
Thermal – thermistors,thermocouples
Magnetic – LVDT
Optical – photoconductive, solar
Acoustical - microphone
Chemical - pH
Nuclear – ionization chamber
Biological – ECG, EEG

5. Passive & Active transducer
Device which derive power reqd. for transduction from auxiliary power source
- externally powered
Eg : resistive, inductive, capacitive
Without power they will not work
Active
They do not require external power source
Eg: solar cell

6. Analog & Digital transducer
convert I/p quantity into an analog o/p
Analog o/p- a continuous fn. Of time
Eg. Strain gauge, L VDT, thermocouple
Digital
Converts I/p into an electrical o/p in the form of pulses

7. Based on Electrical Principle
Variable – resistance type
Strain & pressure gauge 2) thermistors
3. Photoconductive cell 4) chemical conductive meter
b) Variable – inductance type
Linear Variable Differential Transformer (LVDT)
Eddy current gauge
c) Variable – capacitance type
Capacitor microphone 2) dielectric gauge 3)pressure gauge
d) Voltage – divider type
Potentiometer position sensor 2) pressure actuated voltage divider
e) Voltage generating type
1) Solar cell 2) Thermocouple

8. Tachometer
A tachometer is a sensor device for measuring the rotation speed of an object such as the engine shaft in a car. This device indicates the revolutions per minute (RPM) performed by the object.

9. Electromagnetic Flowmeter
The principle of operation for the magnetic flowmeter is based on the Faraday’s Law of Electromagnetic Induction.

10. Flowmeter
When a conductive liquid flows through the magnetic field, a small voltage (u) is induced. This voltage is proportional to the velocity of the flow and is accurately measured by two stainless steel electrodes mounted opposite each other inside the metering pipe

11. LINEAR VARIABLE DIFFERENTIAL TRANSFORMER(LVDT)
AN LVDT transducer comprises a coil former on to which three coils are wound.
The primary coil is excited with an AC current, the secondary coils are wound such that when a ferrite core is in the central linear position, an equal voltage is induced in to each coil.
The secondary are connected in opposite so that in the central position the outputs of the secondary cancels each other out.

12. LVDT contd…
The excitation is applied to the primary winding and the armature assists the induction of current in to secondary coils.
When the core is exactly at the center of the coil then the flux linked to both the secondary winding will be equal. Due to equal flux linkage the secondary induced voltages (eo1 & eo2) are equal but they have opposite polarities. Output voltage eo is therefore zero. This position is called “null position”

13. Now if the core is displaced from its null position toward sec1 then flux linked to sec1 increases and flux linked to sec2 decreases. Therefore eo1 > eo2 and the output voltage of LVDT eo will be positive
Similarly if the core is displaced toward sec2 then the eo2 > eo1 and the output voltage of LVDT eo will be negative.

14. LVDT Advantages Applications
It gives higher o/p voltage for small changes in core position
Temp range-265oC to 600oC
It is available in radiation resistant design for operation in nuclear reactors
Applications
In accelerometers
Jet engine control
Measurement of roll position
Measurement of material thickness in hot strip

15. Piezoelectric Transducer
In piezoelectric induction the measure and is converted into a change in electrostatic charge q or voltage V generated by crystals when mechanically it is stressed as shown in fig

16. Strain Gauge
A strain gauge (or strain gage) is a device used to measure strain on an object

17. Applications
It is rugged, easy to use
In ac applications, it is capable ofmeasuring variations upto 100KHz
It has a excellent linearity
It is less sensitive to shock and undesired vibrations
It is available in multielement

18. Light dependent resistor (LDR)

19. Photojunction devices
phototransistor
photodiode

20. Photovoltaic Solar Cells
Can convert about 20% of light power into electricity
Voltage is low (diode drop, ~0.6V)

21. Resistance Temperature Detectors (RTDs)

23. Thermistor
A thermistor is a type of resistor whose resistance is dependent on temperature, more so than in standard resistors
thermistor is generally a ceramic or polymer

24. the relationship between resistance and temperature is linear R=KT
R- Change in resistance
T- Change in temperature
K- temperature coefficient of resistance
Thermistors can be classified into two types,
If  is positive, the resistance increases with increasing temperature, and the device is called apositive temperature coefficient (PTC) thermistor, or posistor.
If  is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor

25. Attitude and wind velocity
Advantages
It is chemically stable
Used in nuclear environment
Arranged in series-parallel arrangement for increased power handling capacity
Used for measuring temp even at all places inaccessible to other thermometers
Applications
Measuring temp
Thermal conductivity
Attitude and wind velocity

26. Thermocouple
A thermocouple is a device consisting of two dissimilar conductors or semiconductors that contact each other at one or more points.
A thermocouple produces a voltage when the temperature of one of the contact points differs from the temperature of another, in a process known as the thermoelectric effect.
Thermocouples are a widely used type of temperature sensor for measurement and control

28. Types
Type E: (chromel – constantan) has a high output (68 µV/°C) which makes it well suited to cryogenic use. Additionally, it is non-magnetic. Wide range is −50 °C to +740 °C and Narrow range is −110 °C to +140 °C.
Type J : (iron – constantan) has a more restricted range (−40 °C to +750 °C) than type K, but higher sensitivity of about 50 µV/°C. TheCurie point of the iron (770 °C) causes a smooth change in the characteristic, which determines the upper temperature limit.
Type K : (chromel – alumel) is the most common general purpose thermocouple with a sensitivity of approximately 41 µV/°C. It is inexpensive, and a wide variety of probes are available in its −200 °C to +1350 °C / -330 °F to +2460 °F range
Type M: (Ni/Mo 82%/18% – Ni/Co 99.2%/0.8%, by weight) are used in vacuum furnaces for the same reasons as with type C (described below). Upper temperature is limited to 1400 °C. It is less commonly used than other types.
Type N: (Nicrosil – Nisil) thermocouples are suitable for use between −270 °C and +1300 °C owing to its stability and oxidation resistance. Sensitivity is about 39 µV/°C at 900 °C, slightly lower compared to type K.
Type T (copper – constantan) : thermocouples are suited for measurements in the −200 to 350 °C range
Chromel – gold/iron alloy thermocouples
Tungsten/rhenium alloy thermocouples
Platinum/rhodium alloy thermocouples

29. Microphone A microphone is a type of transducer.
Early microphones were invented for communication purposes.
Later modifications were made to design as the microphone was used more in entertainment industry.

30. Overview of a Microphone
Diaphragm—part of microphone which receives the vibration from sound waves.
Thickness and material of diaphragm are changed depending on the sound waves you wish to pick up.
How it works?
Electrical circuit is used to change these detected vibrations into an electrical signal that “images” the sound with an output voltage or current.

31. Characteristics of Microphone

32. Requisites of a good microphone
High sensitivity
High SNR
Flat frequency response over most of the audible frequency range
Very low distortion
It should have correct o/p impedance to match the with the line impedance
The directivity of the microphone should be such as to meet the requirement of application

33. Different Types of Microphones
Carbon
Ribbon
Moving Coil
Crystal
Ceramic
Capacitor
Electret

34. Carbon Microphone—Historical Design
Thomas Alva Edison and Sir Emile Berliner filled patents for Carbon Button Microphone in 1877.
Edison's design became the accepted design and patent.
This is a popular microphone for telephones as well as early radio and speeches.

35. Carbon microphone—Design Diagram

36. Ribbon Microphone
Diaphragm: Thin piece of metal foil suspended in a magnetic field.
Vibrations in ribbon produce a small voltage which is then stepped up by a transformer.
Diaphragm is very easily damaged by wind or loud incoming sounds.

37. Ribbon Microphone— Design Diagram

38. Moving Coil Microphone

39. Crystal Microphone

40. Ceramic Microphone

41. Capacitor Microphone

42. The Electret Microphone
It is similar to capacitor microphone that it does not require
A polarization voltage for its capacitor

43. Loudspeaker
It is a transducer which convert electrical signal to sound wave

44. Loudspeaker

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