4. Thermistors

This type of non-linear resistor is not strictly speaking a protection component. However, in certain applications, they play the role of such components, and their combination with them can improve protection in low-power ranges (< 1 kW).

Thermistors are electronic components whose resistance R varies exponentially with temperature T. They are classified into two types:

• those whose resistance decreases with temperature: negative temperature coefficient (NTC) resistors;

• those whose resistance increases with temperature: positive temperature coefficient (PTC) resistors.

The temperature coefficient is expressed by : $\alpha =\frac{100}{R}\times \frac{\text{d}R}{\text{d}T}$