Thrmistors as Temp Sensors
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Transcript of Thrmistors as Temp Sensors
8/6/2019 Thrmistors as Temp Sensors
http://slidepdf.com/reader/full/thrmistors-as-temp-sensors 1/20
ON
Thermistor as
Temp sensors
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Thermistor is a combination of the words thermal andresistor. The Thermistor was invented by SamuelRuben in 1930.
A Thermistor is a type of resistor used to measuretemperature changes, relying on the change in itsresistance with changing temperature.
Thermistors can measure temperatures across the
range of -40 - 150°C.
Thermistors are made of semiconductor materials
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�Thermistors can take various shapes; rod, disc, washer, bead
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Thermistors come in two varieties; NTC (negative thermal
coefficient) and PTC (positive thermal coefficient).
If k is positive, the resistance increases with increasing
temperature, and the device is called a positive temperature
coefficient (PTC) thermistor .
If k is negative, the resistance decreases with increasing
temperature, and the device is called a negative temperature
coefficient (NTC) thermistor .
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Electrical resistance of a metal depends on the temperature.
The basic principle of thermistors is that; change in
temperature changes its resistance, this change can be convertedto electrical signal.
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To be useful, systems must interact with their environment.
To do this they use sensors.
Sensors is examples of transducers.
A transducer is a device that converts one physical quantityinto another
Examples include:
� A mercury-in-glass thermometer (converts temperature into
displacement of a column of mercury)
�a microphone (converts sound into an electrical signal).
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Almost any physical property of a material that changes inresponse to some excitation can be used to produce a sensor
widely used sensors include those that are:
resistive
inductive
capacitive
piezoelectric
photoresistive
elastic
thermal.
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Resistive thermometers
Typical devices use platinum wire (such a device is called
a platinum resistance thermometers or PRT).
Linear but has poor sensitivity.
A typical
PRT element A sheathed PRT
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Thermistors
materials with a high thermal co efficientof resistance
Sensitive but highly non-linear .
A typical disc thermistor A threaded thermistor
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PN junctions
A semiconductor device with the properties
of a diode (we will consider semiconductors anddiodes later).
Inexpensive, linear and easy to use
Limited temperature range (perhaps -50rC to
150 rC) due to nature of semi conductor material.
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Resistive devices
can be very simple
Where the resistance of the device changes with the quantity being
measured,this change can be converted into a voltage signal using apotential divider ± as shown .
The output of this arrangement is not linearly related to the change
in resistance.
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Switches
Switch interfacing is also simple
Can use a single resistor as below to produce a voltage
output.
All mechanical switches suffer from switch bounce.
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Precision
A measure of the lack of random errors (scatter).
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Linearity
Maximum deviation from a µstraight-line¶ response.
Normally expressed as a percentage of the full-scale value.
Sensitivity
A measure of the change produced at the output for a given
change in the quantity being measured.
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Major specifications to be considered while using a thermistor .
R esistance temperature curve: it varies from thermistor to
thermistor and the specifications are provided by the manufacturer .
R esistance tolerance: The specifications for this is provided by the
manufacturer
Beta tolerance: This depends on the material being used for the
thermistor
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They are used as resettable fuses.
They are used in time delay circuits.
PTC Thermistors are used in motor starting circuits.
The PTC Thermistor can provide a combination of heater and
thermostat in one device.
They are used as µliquid level¶ and µflow sensors¶.
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�Based on the application, the type of Thermistor is decided.
�Thermistors can cost from 20Rs and above.
�Some online stores for buying thermistors.
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` Sensitivity: This allows thermistors to sense very small
changes in temperature.
` Accuracy: Thermistors offer both high absolute accuracy and
interchangeability.
` Cost: For the high performance they offer, thermistors are
very cost-effective.
` R uggedness: Because of their construction, thermistors are
very rugged.
` Flexibility: Thermistors can be configured into a wide varietyof physical forms, including very small packages.
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` Typically only self-heating is a design consideration. Proper
care must be taken to limit the sensing current to a low enough
value that self-heat error is minimized to an acceptable value.
` Non-linearity can be addressed by software or by circuitry, and
moisture induced failure by glass encapsulation