A Thermistor is a type of resistor whose resistance varies with temperature. That means temperature changes will change the resistance provided by the thermistor. The word Thermistor is an amalgamation of the words “thermal” and “resistors”.

Thermistors are ceramic semiconductor components that usually consist of oxides of metals like cobalt, manganese and nickel, and barium titanate.

 Why metal oxides? 

Metal oxides provide a higher temperature coefficient of resistivity than any other type of metal i.e., they provide desirable electrical resistance with a significant temperature rise. This electrical resistivity can either be positive or negative where positive means more resistivity as a result of increasing temperature and negative means lower resistivity with an increase in temperature. 

Metals and alloys have positive temperature coefficients while semiconductors materials like metal oxides and insulators have negative temperature coefficients, which makes metal oxides widely chosen for building thermistors.

There are two main types of Thermistors - Negative Temperature Coefficient (NTC) thermistors and Positive Temperature Coefficient (PTC) thermistors. The temperature coefficient is a measure of the change in resistance with temperature change.

• Negative Temperature Coefficient (NTC) thermistors: NTCs have resistivity inversely proportional to temperature (resistivity increases with a decrease in temperature or vice versa). NTCs are used as inrush current limiters and temperature sensors.
• Positive Temperature Coefficient (PTC) thermistors:  PTCs have resistivity directly proportional to temperature (resistivity increases with an increase in temperature or vice versa). PTCs are used in the protection of circuits from high temperatures.