PTC thermistors
Non linear resistors, Thermistors

PTC thermistors are ferroelectric, semiconducting ceramics. They exhibit an "anomaly" in the variation of their resistance at the Curie temperature T_C of the material corresponding to the ferro-para-electric transition temperature. Above this temperature (see figure 2), resistance rises sharply (in the order of 10 to 100% / °C), then reaches a maximum before decreasing. In other words, the thermistor changes from a conductive to an insulating state within a few tens of degrees: it behaves like a fuse with automatic reset, since the phenomenon is reversible.