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ELECTRONIC INSTRUMENTATION
EKT 314/4
WEEK 4 : CHAPTER 2
TRANSDUCERS
Contents
 Definition of Transducer
 Type of Transducers
 Electrical Transducers
 Parameters
 Advantage/Disadvantages
 Classification
 Application
 Selection of Transducers
 Examples of Transducers
Photoelectric Transducers
 Primary types
 Photoemissive Cell
 Photoconductive Cell
 Photovoltaic Cell
 Photojunctions
Photoemissive Cell
 Electron emission due to the incident radiation upon photo
emissive surface.
 Also known as phototube
 Three basic types of phototube:
 Vacuum
 Gas-filled
 Photomultiplier
Photoemissive Cell (cont.)
 Vacuum Phototube
 Consists of rod anode and curvature
cathode in the vacuum glass.
 Cathode is coated with emissive materials
that emit electron when light radiation
occur on them.
 Stable, consistent characteristics over time
when operate at low voltage and protected
against excessive light.
 Moderate sensitivity due to small current
flow in the vacuum tube.
Photoemissive Cell (cont.)
 Gas-filled Phototube
 Same construction as vacuum phototube except the presence of
inert gas, usually argon into tube.
 Emitted electrons are accelerated by the electric field and cause
ionization. Anode current increases due to high collision.
 Provide gain over response up to 10 compared to vacuum
phototube.
 Not stable as vacuum type, the characteristic are not linear.
Photoemissive Cell (cont.)
 Photomultiplier Phototube
 Consist of evacuated glass envelope containing photo cathode,
anode, and dynodes (additional electrodes).
 Each dynode is at higher voltage than the previous dynode.
 Electron emitted by cathode are attracted to first dynode and
emitted again (secondary emission) to the following dynodes.
 High sensitivity and high frequency response but large in size
and expensive.
Photoconductive Cell
 Fabricated from semiconductor materials such as Cadmium




Sulphate (CdS) or Cadmium Selenide (CdSe).
The increase of current with light intensity while the voltage
remain constant makes the resistance of semiconductor decrease.
Also known as Photo Resistor, Light Dependent Resistor (LDR).
Simple, high sensitivity and low cost.
Variations of temperature may affect resistance for a particular
light intensity.
Photoconductive Cell (cont.)
Construction
b) Typical Curves of
Resistance vs.
Illumination.
a)
Photoconductive Cell (Example)
The relay of (a) of figure above is to be controlled by a photoconductive cell with
the characteristics shown in (b) in figure above. The potentiometer delivers
10mA at a 30V setting when the cell is illuminated with 400l/m2 and is
required to de-energized when cell is dark. Calculate (i) the required series
resistance, and the (ii) dark current level.
Photoconductive Cell (Example)
i) From the characteristic in (b), cell resistance at 400l/m2 is
1kΩ. Therefore,
I
30V
R1  Rcell
So
30V
30
R1 
 Rcell 
 1000  2000
I
0.01
Photoconductive Cell (Example)
ii) The cell dark resistance is 100kΩ
30
 0.3mA
Dark current =
2000  100000
Photovoltaic Cell
 Semiconductor junction devices for converting radiation




energy into electrical energy (voltage).
Also known as solar cell.
Conversion efficiency depends on the spectral content and
illumination intensity.
Main advantages is its ability to generaee voltage at fast
response.
Can be used as energy converter (power provider) directly.
Photovoltaic Cell (cont.)
Photojunctions
 Photodiodes
 Silicon diode with the lens on its case
to focus the incident of the light to
the junction.
 Without bias, operates like
photovoltaic devices or voltage
source.
 When reverse bias operates like
photoconductive device.
 Important advantage is fast response
compared to photoconductive
device. It is also small and
inexpensive.
Photojunctions (cont.)
 Phototransistor
 NPN device by addition of
junction to photodiode.
 Provide larger output
current compared to
photodiode for a given
amount of illumination on a
very small area.
 More sensitive than a
photodiode up to the factor
of 100.
Contents
 Electrical Transducers
 Classification
 Selection of Transducers
 Photoelectric Transducers
 Temperature Transducers
Temperature Transducers
 Transducers that can be used to measure temperature.
 Resistance Temperature Detectors (RTD)
 Thermocouples
 Thermistors
 Other temperature transducers
Resistance Temperature Detector
(RTD)
 Usually make use of platinum, nickel or resistance wire
elements.
 Resistance varies with the change of temperature.
 Almost all metals give high resistance when temperature
increase.
 High value of temperature coefficient is required to sense a
small changes in the temperature.
Resistance Temperature Detectors
(cont.)
 The temperature ranges and coefficient of resistane of
various resistane wire can be tabulated as in table below.
Material
Range (°C)
Coefficient of Resistance (Ω/Ω/°C)
Platinum
-200 ~ 850
0.0039
Copper
-200 ~ 260
0.0067
Nickel
-80 ~ 300
0.0043
Resistance Temperature Detectors
(cont.)
 Expression below relate the resistance of the conductors and the
temperature:




Rt  Rref (1  aDt )
Rt is resistance of the conductor at temperature t°C.
Rref is resistance of the reference temperature (typically 0°C)
a is temperature coefficient of resistance
Dt is the difference between operating and reference temperature.
Resistance Temperature Detectors
(cont.)
 Platinum RTD is the most widely used.
 Advantages:
 Wide operating temperature range.
 Stability at high temperature.
 Linearity.
 Disadvantages:
 Low sensitivity.
 Expensive.
 Easily affected by contact resistance.
Resistance Temperature Detectors
(cont.)
 Two Lead Wire RTD
 Uses Wheatstone’s Bridge*
to measure the resistance.
 Low cost but in order to
achieve high accuracy, the
circuit must be stable and
insensitive to the variations
of ambient temperature.
Es is supply voltage, Eo is
output voltage, R1, R2 and
R3 are fixed value resistors,
RL1 and RL2 are the
Resistance of the two leads
And RT is resistance of RTD.
Resistance Temperature Detectors
(cont.)
 Three Lead Wire RTD
 Practical and accurate method
for most industrial
applications.
 The bridge circuit
automatically balance
resistance change due to
ambient temperature change.
 Third lead has no effect on the
bridge ratios and balance.
Thermocouples
 Consist of two different materials that are in thermal and
electrical contact.
 Thermoelectric effect – Seebeck effect.
 Direct conversion of temperature differences to electrical voltage.
Thermocouples (cont.)
 The junctions at different temperature causes a circulation of




current.
Open circuit will generate voltage that is relative to the Seebeck
current.
Electromagnetic force (Thomson and Peltier) come from the
conductors where the density of free charge carriers increases
with temperature.
Materials used for wire and sensing junction temperature effect
the magnitude of the voltage generated.
Reference temperature junction of thermocouple also known as
cold junction.
Thermocouples (Cont.)
 Common thermocouple materials combination:
 Platinum/Platinum-Rhodium
 Chromel/Alumel
 Chromel/Constantan
 Copper/Constantan
 Iron/Constantan
Thermocouples (Cont.)
Thermocouple output voltage with respect to temperature for various
thermocouple material.
Thermocouples (Cont.)
 If in thermocouple, the voltage
generated by the reference junction
is the same with the one generated
by the sensing junction, this give
null output provided that both
junctions is at the same
temperature.
 This can be solved by a process
known as cold junctions
compensation.
 The reference junction is now at
0°C.
Thermocouples (Cont.)
 The isothermal block is
made of material that is
good conductor of heat
but poor in electricity.
 Industries often use an
isothermal block that
contains a thermistor and
two reference junctions.
 This setup known as
electronic ice point
reference.
Thermocouples (Cont.)
 Advantages
 High speed
 Good accuracy
 Cheap
 Rugged
 Disadvantages
 Low accuracy
 Placed remote from measuring devices.
 Reference junctions compensation.
Thermistor
 Also called thermal resistor as the resistance varies as a
function of temperature.
 Manufactured in the form of beads, discs and rods.
 Most thermistors have a negative coefficient (NTC) of
temperature resistance.
 Three important characteristics:
 Resistance – Temperature
 Voltage – Current
 Current – Time
Thermistor (cont.)
 The resistance of thermistor, RT at a temperature T can be
formulated as follows:
RT  a. exp
b
T
 From the equation above, a and b are constants that can be
determined by the structure and material of thermistor.
Thermistor (cont.)
Resistance versus temperature of a NTC Thermistor
Thermistor (cont.)
 Major applications of thermistors are measurement and
control of temperature.
 Other applications of thermistors:
 Measurement of power at high frequencies.
 Measurement of thermal conductivity.
 Measurement of level, flow and pressure of liquids.
 Measurement of composition of gases.
 Vacuum measurement.
Thermistor (cont.)
 advantages
 Limitations
 High sensitivity especially
 Characteristics of
in NTC region.
 Fast response over narrow
temperature range.
 Cold junction
compensation is not
required.
 No problems on contact
and lead resistance.
 Low cost and small size.
resistance are non-linear.
 Not recommended for
wide temperature range
application.
 Need a shielded power
lines or filters and low
excitation current.
Thermistor (example)
The circuit of figure below is used for temperature measurement. The
thermistor is 4kΩ type. The meter is 50mA meter with a resistance of 3Ω, Rc is
set to 17Ω, and supply voltage Vt is 15V. What will be the meter reading at 77oF
(25oC) and at 150oF. (Kalsi2005, pg. 398)
Thermistor (example)
From the plot of temperature versus resistance, the
resistance at 25oC is 4kΩ. So, the current at 25oC is.
Vt
15
I

 3.73mA
Rt 4000  17  3
Thermistor (example)
At 150oF, the resistance is approximately 950Ω. The
meter reading will be:
Vt
15
I

 15.5mA
Rt 950  17  3
Other Temperature Transducers
 Bimetallic strip
 Two strips of different metals welded together, in the form of
straight cantilever beam with one end fixed.
 Due different thermal coefficient, one of the metals expands
more than the other when heated.
 Commonly used as thermostat.
 Simple, cheap and robust.
Other Temperature Transducers (cont.)
 Integrated circuit
 Temperature sensing element and signal conditioning
electronics in single monolithic integrated circuit package.
 Advantages; linearity; cheap and high output.
 Disadvantages; require power supply, small range, self-heating,
slow and limited configuration.
Other Temperature Transducers (cont.)
IC Type Transducer
Other Temperature Transducers (cont.)
 Radiation Pyrometers
 Operation based on Stefan-Boltzmann law – sense the
temperature from the energy radiated from heated blackbody
optically.
 Construct such that a heat is focused onto the hot junction of
thermocouple.
 Used for measure very high temperature.
 Two types of pyrometer that are fixed focus and variable focus.
ELECTRONIC INSTRUMENTATION
EKT 314/4
WEEK 4 : CHAPTER 2
END