Resettable fuse

They are similar in function to PTC thermistors in certain situations but operate on mechanical changes instead of charge carrier effects in semiconductors.

[1] A polymeric PTC device is made up of a non-conductive crystalline organic polymer matrix that is loaded with carbon black particles[2] to make it conductive.

While cold, the polymer is in a crystalline state, with the carbon forced into the regions between crystals, forming many conductive chains.

[4] The expansion separates the carbon particles and breaks the conductive pathways, causing the device to heat faster and expand more, further raising the resistance.

A small (leakage) current still flows through the device and is sufficient to maintain the temperature at a level which will keep it in the high resistance state.