NTC, or Negative Temperature Coefficient, thermistors are electronic devices whose electrical resistance varies in inverse proportion to environmental temperature. In other words, the resistance of an NTC thermistor decreases as the temperature rises, and vice versa.
An NTC thermistor is typically a semiconductor, made from oxides of cobalt, copper, nickel, iron or titanium, pressed into a small bead, disk or wafer. Varying the combinations of metal oxides and temperature to which they are heated allows a range of temperature characteristics to be produced.
An NTC thermistor initially has a high resistance, which limits the current that can flow. However, power is dissipated as heat, which raises the body heat of the thermistor. This lowers the resistance of the thermistor and increases the current flow, which, in turn, increases the power dissipated. This cycle continues until thermal equilibrium is reached.
An NTC thermistor has what is known as a zero-power resistance. This is its resistance when power dissipation as heat is negligible.