This document discusses different types of transducers, with a focus on temperature sensors. It defines a transducer as a device that converts one form of energy to another. Sensors convert a physical quantity to electrical energy as output, while actuators convert electrical energy to a physical quantity as output. There are two main types of temperature sensors - contact sensors that are physically in contact with the object being measured, and non-contact sensors that interpret radiant energy. Common contact temperature sensors are thermocouples, thermistors, and RTDs, with thermocouples being the most widely used due to their low cost and wide measurement range. Thermocouples generate voltage based on the Seebeck effect caused by joining

1. UNIT 4
Transducers
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2. Transducer
 A device which converts one form of energy to another.
 Typical energy domains are mechanical, electrical, chemical, magnetic,
optical and thermal.
 When input is a physical quantity and output electrical → Sensor
 When input is electrical and output a physical quantity → Actuator
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Sensors
Physical
parameter

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4. TEMPERATURE SENSORS
Temperature is a scalar quantity that determines the direction of
heat flow between two bodies.
Sensing methods: contact and non-contact
Contact Sensor: is in direct physical contact with the object to
be sensed to monitor solids, liquids, gases over wide range
Non-contact : Interprets the radiant energy of a heat source to
energy in electromagnetic spectrum
Monitor non-reflective solids and liquids
Temperature sensors generate output signals in one of two ways:
1. through a change in output voltage
2. through a change in resistance of the sensor‘s electrical circuit
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5. TEMPERATURE SENSORS CLASSIFCATION:
Contact Sensing
 Thermocouple (Thermoelectric)
 Thermistor (Thermal Resistors)
- Negative temperature coefficient device (NTC)
- Positive temperature coefficient device (PTC)
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6. THERMOCOUPLES
 Most common temperature sensing device.
 Accurate temperature measurements can be made with thermocouples
sensors at low cost with shop-built probes and ordinary low-level
voltmeters.
 Thermocouples can measure temperature at a point in a range of -250C to
+3500C.

Typical Industrial Thermocouple General Thermocouple
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7. Thermocouple Concept
• Principle of operation is based on the Seebeck effect, discovered
by Thomas Seebeck in 1822,
- Electrons flow from one wire to other, due to different energy
potentials of alloys
- As temperature changes, current flows
- Voltage is measured between the two alloys (Small voltage, less
than 10 mV)
• Seebeck effect: when any conductor subjected to thermal
gradient, generates a voltage.
• A temperature gradient along a conductor creates an EMF.

8. Thermocouple Concept……
• If two conductors of different materials are joined at one
point, an EMF is created between the open ends which is
dependent upon the temperature of the junction.
• As T1 increases, so does Voltage (V).
• EMF also depends on the temperature of the open ends T2.
• The junction is placed in the process, the other end is in
iced water at 0C. This is called the reference junction.

9. Thermistors: THERMAL RESISTORS
• Thermistor is a combination of the words thermal and resistor.
• Thermistor was invented by Samuel Ruben in 1930
• A thermistor is a type of resistor used to measure temperature
changes, relying on the change in its resistance with changing
temperature.
– Typically have a negative temperature coefficient (NTC),
– Resistance decreases with increasing temperature
• Thermistor can measures across the range of -40~150 ±0.35 °C
Leads, coated Glass encased Surface mount

10. Relationship b/w resistance & temperature
• Assume a simple linear relationship between resistance
and temperature for the following discussion:
ΔR = k ΔT
Where
• ΔR = change in resistance
• ΔT = change in temperature
• k = first-order temperature coefficient of resistance

11. Mechanical Sensing - Microswitch

12. Optical Sensing
• LED’s and Photodiodes
• Transmissive/Reflective
• Modulated/Unmodulated
• Light-on/Dark-on
• Fiber optic
• Solar cell

13. Transmissive & Reflective Sensors

14. Beam Pattern and Reflectance

15. Modulation
• “Chop” LED on and off at many kHz rate
• Bandpass filter after photodiode at the same
frequency as chopping
• Threshold circuit after BPF generates on/off
output

16. Magnetic Position Sensors
• Reed switches (sense permanent magnet)
• Inductive proximity sensors (eddy current)
• Hall Sensors (sense permanent magnet)

17. Inductive Proximity Sensor

18. Solar cells
Solar cells converts light energy into electrical energy. They’re mainly used
for generating solar energy and are made of single-crystal silicon PN
junctions, similar photodiodes but with a broader response curve.
• Unlike photodiodes connected in a reverse-bias configuration, solar cells
are connected in a forward-bias configuration much like typical diodes.
These cells are designed to be sensitive to sunlight instead of a narrow
range of the electromagnetic spectrum. When exposed to solar
radiation, a cell generates a potential difference of 0.58V.
• Typically, several solar cells are connected in series in a panel to output
a greater voltage. This DC voltage can drive a resistive load or be
converted to AC for transmission.

19. Capacitive Transducers
 capacitive transducers can be
made to be self-contained units.
 Some transducers work by
making one of the capacitor
plates movable, either in such a
way as to vary the overlapping
area or the distance between the
plates.
 Other transducers work by
moving a dielectric material in
and out between two fixed
plates to detect and transmit the
physical position of mechanical
parts via electrical signals
)
( farads
d
kA
C o


where,
k = dielectric constant.
A = Area of the plate
o = 8.854x10-12, in farad per meter.
d= the plate spacing in meters.

20. Differential Capacitive Transducers
 have 3 wire connections: one wire for each of the "end" plates
and one for the "common" plate.
 implementation in a bridge circuit

21. Differential Capacitive Transducers
 This bridge circuit is similar in function with strain gauges: it is
not intended to be in a "balanced" condition all the time, but
rather the degree of imbalance represents the magnitude of the
quantity being measured.

22. THANK YOU
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