Thermistors

Thermistor

Negative temperature coefficient (NTC) thermistor, bead type, insulated wires
Type	Passive
Working principle	Electric resistance
Electronic symbol
'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 thermistors are in the form of beads ,rods and discs but RTDs are in different shapes and sizes. 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.