Thermistors are a type of resistor that acts as a temperature-sensing element, generally made of ceramic or polymer. Thermistors exhibit a large change in resistance proportional to a small change in temperature. Thermistors come in two common types: NTC (Negative Temperature Coefficient) thermistors – In these thermistors resistance decreases as temperature rises. An NTC is commonly used as a temperature sensor, or in series with a circuit as an inrush current limiter. PTC (Positive Temperature Coefficient) thermistors – In these thermistors resistance increases as temperature rises. PTC thermistors are commonly installed in series with a circuit, and used to protect against overcurrent conditions, as resettable fuses.