PPTC Fuse
A polymeric positive-temperature-coefficient (PPTC, PTC, PolySwitch) device is a resettable overcurrent protector. At normal current it has tens of milliohms of resistance; under sustained overcurrent it self-heats, the polymer matrix expands, and resistance jumps by a factor of 10⁴–10⁶, choking the fault to a trickle until power is removed and the polymer cools and re-densifies.
A semi-crystalline polymer (typically polyethylene) is loaded with conductive carbon-black particles. At room temperature the carbon forms a continuous network across the polymer crystals — low resistance. Under fault, joule heating melts the crystalline phase; the polymer expands, the carbon network ruptures, and resistance soars. When power drops and the device cools, the polymer re-crystallises and carbon contact reforms — the fuse 'resets'.
In plain terms
A piece of conductive plastic that gets so hot under fault current that the plastic expands and breaks the conductive path; cool it and the path closes again.
Why designers use it
- USB ports and battery packs that need automatic recovery after a short.
- Telecom line cards where a re-fuseable element saves field truck rolls.
- Consumer electronics that need affordable repeat-trip overcurrent protection.
Best for
- USB ports
- Battery packs
- Telecom
Key specifications
- Hold current (I_H): 0.05 – 10 A
- Trip current (I_T): ≈ 2 × I_H
- Time to trip: 0.1 – 30 s
- Voltage: 6 – 600 V
- R_min cold: 10 – 200 mΩ
When not to use it
- Where deterministic open-on-fault is required — a PTC trips slowly and re-arms; a regular fuse opens fast and stays open.
- On safety-critical mains rails — PTCs are not certified one-shot disconnects.
- On fast-rising fault currents — PTC response time is in the order of seconds.
Common mistakes
- Treating resettable protection as a substitute for the complete circuit's safety requirements.
- Assuming a tripped device provides galvanic isolation or can be safely touched.
- Choosing only by nominal current while ignoring voltage, fault-current and temperature limits.
Where you will find it
- Every USB port on a laptop is protected by a PPTC: short the data lines and the PPTC trips within seconds, the host firmware sees no current, and once you unplug the offending cable the PPTC cools and the port comes back on its own.
- Lithium-ion battery packs include a PPTC in series with the cells: a stuck-on charger or external short triggers the PPTC, limiting the cell current to a milliamp trickle until the fault clears.
- Telephone-line interface cards in a central office use PPTCs to ride out lightning surges — the PPTC trips, the line card survives, and the device resets after the surge passes.
A short history
The polymeric PTC overcurrent protector was developed by Raychem Corporation in the 1970s under the trade name PolySwitch. Raychem's research grew out of its work on conductive polymer self-regulating heat-trace cables; engineers realised that the same temperature-driven resistance jump could be exploited as an overcurrent fuse. Raychem was acquired by Tyco in 1999 (now TE Connectivity), and PolySwitch is still the dominant PPTC brand. Bourns (Multifuse), Littelfuse (PolyFuse), and others ship compatible parts.
Good to know
- A resettable protective device must still be selected for the actual voltage, fault current and temperature conditions.
- Do not touch an energized or faulted component to check whether it has tripped. Disconnect power and use appropriate measurement and safety procedures.