What Is a Resettable Fuse (PTC / Polyfuse)?
A resettable fuse (PTC / polyfuse) is a protection element that, instead of melting and being consumed under overcurrent, sharply raises its resistance to bring the current to a safe level and returns to normal when cooled.
How Does It Work?
The conductive-polymer material inside it rapidly raises its resistance as it heats up (Positive Temperature Coefficient). When overcurrent heats the material the resistance shoots up and the current drops to a trickle; once the fault is cleared and the element cools, the resistance falls and the circuit conducts again.
Difference From a Classic Fuse
A classic fuse blows once and must be replaced. A polyfuse "resets" itself; once the fault is gone it works again without intervention.
| Property | PTC (Polyfuse) | Classic Fuse |
|---|---|---|
| Reuse | Yes (resets) | No |
| Response | Limits the current | Cuts the circuit |
| Speed | Slower | Can be fast |
Thermistor Relationship
A polyfuse actually behaves like a PTC thermistor; but here the goal isn't to measure temperature but to protect the circuit under overcurrent.
Where Is It Used?
USB ports, motherboards, battery packs, toys and consumer devices where overload can be frequent and fuse replacement is inconvenient.
Common Mistakes
- Expecting a full cut: a PTC doesn't zero the current, it only limits it to a safe level.
- Speed expectation: where very fast protection is needed it can fall short alone.
- Continuous fault: keeping it in the tripped state wears the element.
Frequently Asked Questions
Does it really reset itself?+
Does it fully replace a fuse?+
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