Thermistors are semiconductor resistors. Their electrical resistance varies according to temperature. This makes them suitable for temperature measurement, and for electronic control operations.
There are two main types of thermistor – NTC (Negative Temperature Coefficient) resistors, and PTC (Positive Temperature Coefficient) resistors.
NTC resistors have lower resistance at high temperatures, which means they conduct current more readily when they are hot than when they are cold. NTC resistors are commonly used in temperature sensors in engine management systems.
PTC resistors have higher resistance at high temperature, which means they conduct current less readily when they are hot than when they are cold. This makes them useful as current limiting protective devices in circuits, instead of fuses. As the current increases, the heat generated by the resistor increases, which reduces the amount of current passed.