Resettable fuse

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A PolySwitch device is a polymeric PTC device sold by Tyco Electronics. "PolySwitch" is a registered trademark of Tyco Electronics Corporation. Similar devices are also sold by other companies; trademarks include Everfuse, Polyfuse and Multifuse. These devices are a polymeric form of positive temperature coefficient resistor (PPTC) which acts as a self-resettable fuse. While the generic term "resettable fuse" is sometimes used for these devices, technically these are not fuses but are actually non-linear thermistors.

A PPTC device has a current rating. When the current flowing through the device, (which acts as a resistor) exceeds the current limit (e.g. 100 mA), the PPTC device warms up above a threshold temperature and the electrical resistance of the PPTC device suddenly increases several orders of magnitude to a "tripped" state where the resistance will typically be hundreds or thousands of ohms, greatly reducing the current.

A polymeric PTC device comprises a polymer matrix that is loaded with carbon black particles to make it conductive. Since it is conductive it will pass a given amount of current. If too much current is passed through the device, the device will begin to heat. As the device heats it will expand. As it expands, the carbon particles will separate, breaking conductive pathways thus causing the resistance of the device to increase. This will cause the device to heat faster and expand more, further raising the resistance. This increase in resistance is sufficient to substantially reduce the current in the circuit. A small amount of current will still flow through the device and is sufficient to maintain the temperature of the device and keep it at the high resistance level.

When the power and fault are removed, the PPTC device will cool. As the device cools, it contracts to its original shape and returns to a low resistance level where it can hold the current as specified for the device.