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The core of a PPTC resettable fuse is made of a high‑molecular polymer (plastic) mixed with conductive carbon black particles. Under normal conditions, the carbon black forms conductive paths, resulting in extremely low resistance. When overheated, the polymer expands, breaking the conductive paths, causing the resistance to spike dramatically, thereby cutting off the current.
Normal state: Resistance is very low (from a few milliohms to a few ohms), so the circuit operates normally.
Overcurrent / overheat: Temperature rises → polymer expands → resistance soars (high‑impedance state) → current is limited to a very low value (similar to an open circuit).
Cooling after power‑off: Once the fault is cleared and the device cools down, the polymer contracts and returns to its original state, the conductive paths are re‑established, and the circuit automatically reconnects.
Core feature: Resettable — this is the biggest difference from a conventional fuse. A conventional fuse is one‑time use (it is discarded after blowing). A resettable fuse is reusable; it provides protection in a circuit and can be used repeatedly.
