Temperature Sensors and Photo Sensors

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TEMPERATU

RE SENSORS

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WHAT IS A SENSOR ?

• A sensor is a device that detects events or changes in quantities andprovides a corresponding output, generally as an electrical or optical s

•For example, a thermocouple converts temperature to an output voltaa mercury-in-glass thermometer converts the measured temperature expansion and contraction of a liquid which can be read on a calibrate

tube.

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WHAT IS A TEMPERATURSENSOR ?

• A temperature sensor is a device that gathers data concerning thetemperature from a source and converts it to an understandable/readform.

•These conversions are generally of the typeexpansion or contraction of solids, liquids or gases,

the changes in electrical resistance of conductors and semi-conductors,

thermoelectric emf’s. 

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BI-METALLIC STRIPS

• A bimetallic strip is used to convert a temperature change into mecha

displacement.

•This device consists of two different metal strips bonded together.

•The metals have different coefficients of expansion and when the temchanges, the composite strip bends into a curved strip, with the highemetal on the outside of the curve.

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 APPLICATION

•Clocks (to compensate tempechanges)

•Thermometers (Breguet’s thermometer has tri-metallic h

•Electrical devices (as circuit

breakers)

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RESISTANCE TEMPERATURDETECTORS (RTD)

•The resistance of metals increases with the increase in the temperatulinear way.

•R is the resistance at temperature T, Ro

resistance at temperature To, α is thecoefficient of temperature resistance.

•Such detectors are highly stable and have high response time of 0.5 t

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THERMISTORS

•Thermistors are small pieces of material made frommixtures of metal oxides, such as chromium, iron, etc.

•The resistance of the thermistors changes in a verynon-linear manner with the change in temperature.

•There are two types of thermistors, namely,

NTC (negative temperature coefficients)thermistor and

PTC (positive temperature coefficients) thermistor.

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•The resistance-temperature relationship for athermistor can be described as

Where Rt is the resistance at temperature t,K and β are constants.

•They can be very small thus enabling temperatures tobe monitored at virtually a point.

•They give rapid and very large changes in resistanceper degree change in temperature.

•Their main disadvantage is their non-linearity.

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TRANSISTORS(THERMODIODES)

• A junction semiconductor diode is used as a temperature sensor.

•When the temperature of the doped semiconductor changes, the mobtheir charge carriers changes and this affects the rate at which electroholes can diffuse across a p-n junction.

•When a p-n junction has a potential difference V across it, the current through the junction is a function of the temperature

Where, T is the temperature in Kelvin scale,e is the charge on an electron, andk and I0 are constants.

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•Rearranging the above equation we get

•For a constant current I, V is proportional to the temperature in the Kescale.

•Thus, change in temperature can be measured as the change in the vfor a constant current I.

•The main advantage is that this device gives voltage as a linear functitemperature.

•Similarly, for thermodiodes, the voltage across the junction between thand the emitter depends on the temperature and can be used as a meof temperature.

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THERMOCOUPLES

•If two metals are joined together, a potential difference occurs across  junction. The potential difference depends on the metals used and thetemperature of the junction.

• A thermocouple is a complete circuit having two such junctions.

•If both junctions are at same temperature, there is no net emf. Howev

there is a temperature difference, there is a net emf.

•If temperature of one of the junction is 0°C, then emf is given as

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LIGHT SENSORS

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WHAT ARE LIGHT SENSORS

•Photodiodes are semiconductor junction diodes which are connected circuit in reverse bias, so giving a very high resistance.

•Thus, when a light falls on the junction the diode resistance drops andcurrent in the circuit rises appreciably.

• A photodiode is used as a variable resistance device controlled by the

incident on it.

•Photodiodes have a very fast response to light.

•These photodiodes are called light sensors.

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PHOTOTRANSISTORS

•The phototransistors have a light-sensitive collector-base p-n junction

•When there is no incident light there is a very small collector-to-emittecurrent.

•When the light is incident, a base current is produced that is directlyproportional to the light intensity.

•This leads to the production of the collector current which is then a meof the light intensity.

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PHOTORESISTOR

• A photoresistor has a resistance which depends on theintensity of the light falling on it.

•This resistance decreases linearly as the intensity of thelight falling on the photoresistor increases.

•The cadmium sulphide photoresistor is most responsiveto light having wavelengths shorter than about 515nm.

•The cadmium selinide photoresistor is most responsiveto light having wavelengths shorter than about 707nm.

• An array of light sensors is often required in a smallspace in order to determine the variations of lightintensity across that space.

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