Polyswitch

A PolySwitch (also Polyfuse and Multiswitch) is a polymer form of positive temperature coefficient resistor (PPTC) which acts as a self repairing fuse. "PolySwitch" is a registered trademark of Raychem Corporation. While the generic term for these devices is sometimes called resettable fuses, technically these are not fuses but actually non-linear thermistors.

A PPTC has a current rating. When the current flowing through the PolySwitch device, (which acts as a resistor) exceeds the current limit of, (for example) 100 mA, the PPTC device warms up above a threshold temperature and the electrical resistance of the PPTC device suddenly increases from a few ohms to a few kiloohms.

A polymer 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 due to I²R heating. As the device heats it will expand. As it expands, the carbon particles will separate and the resistance of the device will 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 PolySwitch 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.