Thermistor

Thermistor

Negative temperature coefficient (NTC) thermistor, bead type, insulated wires
Type	Passive
Working principle	Electric resistance
Electronic symbol
Thermistor symbol

A thermistor is a type of resistor whose resistance is dependent on temperature, more so than in standard resistors. The word is a portmanteau of thermal and resistor. Thermistors are widely used as inrush current limiter, temperature sensors (Negative Temperature Coefficient or NTC type typically), self-resetting overcurrent protectors, and self-regulating heating elements (Positive Temperature Coefficient or PTC type typically).

Thermistors are of two opposite fundamental types:

Thermistors differ from resistance temperature detectors (RTDs) 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 greater precision within a limited temperature range, typically −90 °C to 130 °C.

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

where

Thermistors can be classified into two types, depending on the classification of ${\displaystyle k}$. If ${\displaystyle k}$ is positive, the resistance increases with increasing temperature, and the device is called a positive temperature coefficient (PTC) thermistor, or posistor. If ${\displaystyle k}$ 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 ${\displaystyle k}$ as close to 0 as possible, so that their resistance remains nearly constant over a wide temperature range.