When the temperature of a positive temperature coefficient (PTC) thermistor increases, what happens to its resistance?

In a positive temperature coefficient (PTC) thermistor, the resistance increases with an increase in temperature. This behavior is due to the properties of the semiconductor material used in the thermistor. Typically, as temperature rises, the charge carriers (electrons and holes) within the material become less mobile, which results in a higher resistance.

The mechanism behind this phenomenon involves the energy states of the material. When the temperature increases, more electrons may gain sufficient energy to move into a higher energy state, but in PTC thermistors, this movement can lead to reduced conductivity rather than increased, as it changes the balance of charge carriers in the conduction band and increases the resistance.

In practical applications, PTC thermistors are commonly used for overcurrent protection and in temperature sensing devices because of their predictable increase in resistance with temperature. This characteristic makes them useful in a variety of circuit applications, effectively allowing them to limit current in an overcurrent situation or signal temperature changes.