Question 2: How Does a PPTC ?

A PPTC consists of a polymer matrix and conductive carbon black particles. Since the fuse is conductive, current passes through it during normal operation. When an overcurrent occurs, the generated heat (I²R) causes the polymer to expand. This expansion separates the carbon black particles, increasing the resistance of the fuse. The higher resistance leads to faster heat generation and further expansion, which in turn raises the resistance more rapidly.

When the temperature reaches approximately 125°C, the resistance increases significantly, reducing the current to a very low level. At this point, a small trickle current is sufficient to maintain the fuse at this temperature and in a high-resistance state. Once the fault is cleared, the fuse cools and contracts to its original form, reconnecting the carbon black particles and restoring its resistance to a level that allows the specified hold current to flow. This process can be repeated multiple times.