The term thermistor is a neologism of thermal resistor, i.e. heat-sensitive resistor. It is a resistor with positive or negative temperature characteristic, whose temperature characteristic has a strong non-linearity. Corresponding components are used as temperature sensors, but with linearized nonlinearities.
Thermistors are based on material-dependent temperature characteristics expressed in temperature coefficients. There are resistive materials made of semiconducting metal oxides such as aluminum oxide, iron oxide or platinum oxide, whose resistivity increases with temperature, while for others it decreases. These temperature dependencies are exploited in thermistors.
Corresponding components in which the resistance value increases with temperature, i.e. which have a positive temperature coefficient, are called PTC thermistors because they conduct current better at low temperatures than at higher temperatures. PTC thermistors are referred to as Positive Temperature Coefficients(PTC). Thermistors with a negative temperature coe fficient are hot conductors, Negative Temperature Coefficient( NTC). Their resistance value decreases with increasing temperature. RTD elements, Resistance Temperature Detector (RTD), whose best-known representative is the Pt100, also belong to the group of PTC thermistors.
Thermistors have relatively high resistance at low temperatures and very low resistance at high temperatures. They are therefore also installed as thermal protection to limit the inrush current in motors, power supplies and other electrical and electronic circuits.