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Tuesday, December 20, 2011

Thermistors : Introduction





Thermistors are thermally sensitive resistors and have, according to type, a negative (NTC), or positive (PTC) resistance/temperature coefficient. 

Thermistors differ from resistance temperature detectors  (RTD) in that the material used in a thermistor is generally a ceramic or polymer, while RTDs use pure metals. The temperature response is also different; RTDs are useful over larger temperature ranges, while thermistors typically achieve a higher precision within a limited temperature range [usually −90 °C to 130 °C].

Assuming, as a first-order approximation, that the relationship between resistance and temperature is linear, then:


ΔR = k ΔT
where
ΔR = change in resistance
ΔT = change in temperature
k = first-order temperature coefficient of resistance

Manufactured from the oxides of the transition metals - manganese, cobalt, copper and nickel, NTC thermistors are temperature dependant semiconductor resistors. Operating over a range of -200°C to + 1000°C, they are supplied in glass bead, disc, chips and probe formats. NTCs should be chosen when a continuous change of resistance is required over a wide temperature range. They offer mechanical, thermal and electrical stability, together with a high degree of sensitivity.

The excellent combination of price and performance has led to the extensive use of NTCs in applications such as temperature measurement and control, temperature compensation, surge suppression and fluid flow measurement.

PTC thermistors are temperature dependent resistors manufactured from barium titanate and should be chosen when a drastic change in resistance is required at a specific temperature or current level. PTCs can operate in the following modes:
•    Temperature sensing, switching at temperatures ranging from 60°C to 180°C, e.g. protection of windings in electric motors and transformers.
•    Solid state fuse to protect against excess current levels, ranging from several mA to several A (25°C ambient) and continuous voltages up to 600V and higher, e.g. power supplies for a wide range of electrical equipment.
•    Liquid level sensor.



Detailed explanation about NTC and PTC thermistors can be found in the upcoming post.


Article Source: www.designinfo.com


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