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therm istors therm al res istors

ThermistorsTHERMal resISTORS

A thermistor is a type of resistor used to measure temperature changes, relying on the change in its resistance with changing temperature. Thermistor is a combination of the words thermal and resistor. The Thermistor was invented by Samuel Ruben in 1930, and has U.S. Patent #2,021,491.

Leads, coated

Glass encased

Surface mount

therm istors therm al res istors1

ThermistorsTHERMal resISTORS

Thermistors are made of semiconductor materials (metallic compounds including oxides such as manganese, copper, cobalt, and nickel, as well as single-crystal semiconductors silicon and germanium).

Contrast <<--->> Common carbon resistors, made from carbon powder mixed with a phenolic binder glue.

Leads, coated

Glass encased

Surface mount

therm istors therm al res istors2

ThermistorsTHERMal resISTORS

Assume a simple linear relationship between resistance and temperature for the following discussion:

ΔR = k ΔT

where

ΔR = change in resistance

ΔT = change in temperature

k = first-order temperature coefficient of resistance

Source: http://en.wikipedia.org/wiki/Thermistor

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ThermistorsTHERMal resISTORS

Thermistors can be classified into two types depending on the sign of k.

If k is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor, Posistor.

If k is negative, the resistance decreases with increasing temperature, and the device is called a negative temperature coefficient (NTC) thermistor.

Source: http://en.wikipedia.org/wiki/Thermistor

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ThermistorsTHERMal resISTORS

Resistors that are not thermistors are designed to have the smallest possible k, so that their resistance remains almost constant over a wide temperature range.

Source: http://en.wikipedia.org/wiki/Thermistor

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ThermistorsTHERMal resISTORS

Thermistor-choice is based on the nominal resistance you want at the operating temperature range, on the size, and on the time constant.

Time constants are about 5 - 10 seconds. (Check this out with your thermistor).

Source: http://www.science.uwaterloo.ca/~jddmarti/p352_w2007/Thermistor_50K.pdf

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ThermistorsTHERMal resISTORS

Source: http://www.science.uwaterloo.ca/~jddmarti/p352_w2007/Thermistor_50K.pdf

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ThermistorsTHERMal resISTORS

Example Applications:

Temperature measurement.

Time delay (self heating from large current ‘opens’ the thermistor so it can be used as a slow switch). Heating = i2 R where R is the resistance and i is the current.

Surge suppression when a circuit is first energized. Current needs to flow through the thermistor for awhile to heat it so that it ‘opens’, and acts again as a switch.

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