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```Debasmit Das
10115039
Batch : E3
Introduction
 A resistive sensor is a transducer or electromechanical
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device that converts a mechanical change such as
displacement into an electrical signal that can be
monitored after conditioning.
Resistive sensors are among the most common in
instrumentation.
These Transducers do NOT generate electricity. Hence,
they are called passive devices.
The simplest resistive sensor is the potentiometer.
Other resistive sensors include strain gauges,
thermocouples, photoresistors and thermistors.
Theory of Operation
Resistance = (Resistivity * Length)/Area
The resistance of a material depends on four factors:
 · Composition

 · Length
=
 · Temperature

 · Cross Sectional Area
• To change the resistance of a material, you must change the value of one of the
above factors.
• When length is modified the change in resistance is direct. If you double the
material’s length, it’s resistance doubles. When the cross sectional area is
modified the change in resistance has an inverse effect, IE R = k/A. If you
double the cross-sectional area of wire, its resistivity is cut in half.
• But, Changes in composition and temperature do not change the resistivity of a
material in such a simple way.
Examples of Resistive Transducers
 Sliding contact devices
 Wire resistance strain gauge
 Thermistors
 Thermocouples
 Light Dependent Resistors (LDRs)
Device
Action
Light Dependent Resistor Resistance falls with increasing
light level
Application
Light operated switches
Thermistor
Resistance falls with increased
temperature
Electronic thermometers
Strain gauge
Resistance changes with force
Sensor in an electronic
balance
Moisture detector
Resistance falls when wet
Damp meter
Sliding contact devices
 There is a long conductor whose effective length is variable.
 One end of the conductor is fixed, while the position of the other end
is decided by the slider or the brush that can move along the whole
length of the conductor along with the body whose displacement is to
be measured.
 When the body moves the slider also moves along the conductor so
its effective length changes, due to which it resistance also changes.
 These devices can be used to measured linear as well as angular
displacement.
Construction of rotary and slider types
 The unit consists basically of a ‘track’ having a fixed resistance and a
variable contact which can be moved along and make continuous
contact with the track.
 If the track resistance is proportional to the length along the track (i.e.
linear track), the output voltage will be proportional to the movement
of the variable contact and the unit is suitable for use as a position
transducer.
 The track may comprise a film of carbon formed on a substrate or may
be a length of resistance wire wound on an insulator former.
Applications: Potentiometer
 The Potential divider is the most obvious application. In its simplest form
it is two resistors in series with an input voltage Vs across the ends.
 If only two terminals are used, one end and the wiper, it acts as a variable
resistor or rheostat.
 Potentiometers were formerly used to control picture brightness, contrast,
and color response in Television sets.
 Low-power potentiometers, both linear and rotary, are used to control audio
equipment, changing loudness, frequency attenuation and other
characteristics of audio signals.
Potential Divider Circuit
Potentiometer
Strain Gauge
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If a strip of conductive metal is placed under
compressive force (without buckling), it will broaden
and shorten.
If these stresses are kept within the elastic limit of the
metal strip (so that the strip does not permanently
deform), the strip can be used as a measuring element
for physical force, the amount of applied force inferred
from measuring its resistance.
This is the principle of a Strain Gauge.
Gauge Factor
The gauge factor is defined as:
where
∆R is the change in resistance caused by strain,
RG is the resistance of the undeformed gauge,
and
∈ is Strain
Also expression for Strain is,
∈ = ∆ L/L
For metallic foil gauges, the gauge factor is usually a little over 2
Half Bridge Strain Gauge Circuit
 Unlike the Wheatstone
bridge using a nullbalance detector and a
human operator to
maintain a state of
balance, a strain gauge
bridge circuit indicates
measured strain by the
degree of imbalance, and
uses a precision
voltmeter in the center of
the bridge to provide an
accurate measurement of
that imbalance:
Strain Gauge
With no force applied to the test specimen, both strain gauges have equal
resistance and the bridge circuit is balanced. However, when a downward force is
applied to the free end of the specimen, it will bend downward, stretching gauge
#1 and compressing gauge #2 at the same time:
Applications : Strain Gauge
 Strain gauges are used to measure force and small
displacements. They are used for analyzing the
dynamic strain of complex structures. They are used to
measure tension, torque etc.
 Types of strain gauges are:
(a) Wire strain gauges
(b) Foil strain gauges
(c) Thin film
(d) Semiconductor
Thermistors
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Thermistors work on the principle that resistance of
some materials changes with the change in their
temperature.
When the temperature of the material changes, its
resistance changes and it can be measured easily
and calibrated against the input quantity.
The commonly used thermistors are made up of the
ceramic like semiconducting materials such as
oxides of manganese, nickel and cobalt.
Thermistors can be used for the measurement of
temperature, as electric power sensing devices and
also as the controls for various processes.
Thermistors
 The most common type of thermistor that we use has a
resistance that falls as the temperature rises.
 It is referred to as a negative temperature coefficient
device(NTC).
 A positive temperature coefficient(PTC) device has a
resistance that increases with temperature.
Thermistor Analysis
The thermistor resistance-temperature relationship can be
approximated by,

=
where:
1 1
( −
)

.

T is temperature (in kelvin),
TRef is the reference temperature, usually at room temp.
(25 °C; 77 °F; 298.15 K),
R is the resistance of the thermistor (W),
RRef is the resistance at TRef,
b is a calibration constant depending on the thermistor
material, usually between 3,000 and 5,000 K.
Thermistor Analysis
 The graph of resistance against temperature is like
this.
The resistance on this graph is on a
logarithmic scale, as there is a large
range of values.
Applications of thermistors
 Measurement of temperature
 Measurement of Difference of two temperatures
 Control of temperature
 Temperature compensation
 Thermal conductivity measurement.
 Measurement of Gas Composition
 Measurement of Flow
 Current-limiting
devices
replacement for fuse (PTC)
for
circuit
protection
as
Thermocouple
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The thermocouples work on the principle of Seebeck
effect, Peltier effect and Thomson effect.
As per the Seebeck effect, when two dissimilar elements
are joined at their ends the electromotive force exists at
their junction.
As per Peltier effect, the amount of electromotive force
generated depends on the temperature of the junction
While, the Thomson effect says that the amount of
voltage generated depends on the temperature gradient
along the conductors in the circuit.
The voltage output from the thermocouple changes as its
temperature changes or the temperature of the body in
whose contact it is changes.
The voltage output is calibrated against the temperature
of the body that can be measured easily.
Thermocouple is a very popular device used for
measurement of temperature.
Thermocouple Internal Circuit
Types of Thermocouples
 A thermocouple is available in different combinations
of metals or calibrations.
 The four most common calibrations are J, K, T and E.
 There are high temperature calibrations R, S, C and
GB.
 Each calibration has a different temperature range and
environment, although the maximum temperature
varies with the diameter of the wire used in the
thermocouple.
Applications : Thermocouple
 Steel Industry
 Heating Appliance Safety
 Power Production : Thermoelectric Generation
 Thermoelectric Cooling
 Diesel Engines
 Gas Turbine Exhaust Temperature Measurement
Temperature Variation of Resistive
Sensors
Light Dependent Resistor
 The light dependent resistor consists of a length of
material (cadmium sulphide) whose resistance changes
according to the light level.
 Therefore the brighter the light, the lower the resistance.
Principle of Operation
 An LDR is made of a high resistance semiconductor.
 If light falling on the device is of high enough
frequency, photons absorbed by the semiconductor give
bound electrons enough energy to jump into the conduction
band.
 The resulting free electron (and its hole partner) conduct
electricity, thereby lowering resistance.
LDR Applications
 Smoke detection
 Automatic lighting
 Alarm systems
 Dynamic Compressors
 Solar Street Lamps
 Camera Light meters
References
 Wikipedia
 http://www.eecs.berkeley.edu/~culler/WEI/labs/lab7
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sensing/sensing.htm
http://www.ce-transducer.com/Resistance.asp
http://forum.onestopgate.com/forum_posts.asp?TID=3536
http://www.eee.metu.edu.tr/~koray/exp1.pdf
http://www.brighthubengineering.com/hvac/53335variable-resistance-transducers/
http://ptuas.loremate.com/beee/node/5
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