Resistive Type of Sensors -

Their Analysis and Applications

Debasmit Das10115039Batch : E3

IntroductionA resistive sensor is a transducer or

electromechanical 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.

𝑅=𝜌 𝑙𝐴

Examples of Resistive TransducersSliding contact devicesWire resistance strain gaugeThermistorsThermocouplesLight Dependent Resistors (LDRs)Device Action ApplicationLight Dependent Resistor Resistance falls with increasing

light levelLight 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.

PotentiometerPotential Divider Circuit

Strain Gauge 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 StrainAlso expression for Strain is,

= L/LFor metallic foil gauges, the gauge factor is usually a little over 2

Half Bridge Strain Gauge CircuitUnlike the Wheatstone

bridge using a null-balance 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 GaugeWith 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 GaugeStrain 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 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.

ThermistorsThe 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

where:  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.

The thermistor resistance-temperature relationship can be approximated by,

𝑅=𝑅𝑅𝑒𝑓 .𝑒𝛽( 1𝑇− 1𝑇 𝑅𝑒𝑓

)  

Thermistor AnalysisThe 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 thermistorsMeasurement of temperatureMeasurement of Difference of two temperaturesControl of temperatureTemperature compensationThermal conductivity measurement.Measurement of Gas CompositionMeasurement of FlowCurrent-limiting devices for circuit protection as

replacement for fuse (PTC)

Thermocouple 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 ThermocouplesA 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 : ThermocoupleSteel IndustryHeating Appliance SafetyPower Production : Thermoelectric GenerationThermoelectric CoolingDiesel EnginesGas Turbine Exhaust Temperature

Measurement

Temperature Variation of Resistive Sensors

Light Dependent ResistorThe 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 OperationAn 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 ApplicationsSmoke detection Automatic lightingClock RadiosAlarm systemsDynamic CompressorsSolar Street LampsCamera Light meters

ReferencesWikipediahttp://www.eecs.berkeley.edu/~culler/WEI/labs/la

b7-sensing/sensing.htmhttp://www.ce-transducer.com/Resistance.asphttp://forum.onestopgate.com/forum_posts.asp?TI

D=3536http://www.eee.metu.edu.tr/~koray/exp1.pdfhttp://www.brighthubengineering.com/hvac/5333

5-variable-resistance-transducers/http://ptuas.loremate.com/beee/node/5

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