A thermistor is a type of resistor used to measure temperature changes, relying on the change in its resistance with changing temperature.

**Thermistors**

THERMal 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.

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** **

**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**.

*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.*

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).

**Example Applications:**

1.
Temperature measurement.

2.
Time delay (self heating from large current

‘opens’ the thermistor so it can be used
as a slow switch). Heating = i^{2} 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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Mechanical : Maintenance Engineering : Condition Monitoring : Thermistors |

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