Thrmistors as Temp Sensors

8/6/2019 Thrmistors as Temp Sensors

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Thermistor as

Temp sensors



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



�Thermistors can take various shapes; rod, disc, washer, bead



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 .



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.



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).



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.



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



Thermistors

materials with a high thermal co efficientof resistance

Sensitive but highly non-linear .

A typical disc thermistor A threaded thermistor



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.



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.



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.



Precision

A measure of the lack of random errors (scatter).



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.



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



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¶.



�Based on the application, the type of Thermistor is decided.

�Thermistors can cost from 20Rs and above.

�Some online stores for buying thermistors.



` 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.



` 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



` www.temperatures.com

` www.wikipedia.org

` www.omega.com` www.howstuffworks.com

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Thrmistors as Temp Sensors