Thermistor

A thermistor is a type of resistor whose resistance varies significantly with temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush current limiters, temperature sensors, self-resetting overcurrent protectors, and self-regulating heating elements.

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, typically −90 °C to 130 °C.

Basic operation

Thermistor symbol

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

$\Delta R=k\Delta T \,$

where

$\Delta R$ = change in resistance

$\Delta T$ = 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, or 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 a $k$ as close to zero as possible, so that their resistance remains nearly constant over a wide temperature range.

Instead of the temperature coefficient k, sometimes the temperature coefficient of resistance $\alpha_T$ (alpha sub T) is used. It is defined as^[2]

$\alpha_T = \frac{1}{R(T)} \frac{dR}{dT}.$