Thermistor. Thermistors are semiconductor devices that change resistance with changing temperature. There are two classes of thermistors: positive temperature coefficient (PTC) thermistors and negative temperature coefficient (NTC) thermistors. A PTC thermistor's resistance increases with increasing temperature. It may be used to limit current to protect a circuit, acting much like a fuse. A NTC thermistor's resistance decreases with increasing temperature. It may be used in a power supply circuit to limit current at start-up, but as its temperature increases it allows a higher current flow.