Resettable fuse

A polymeric positive temperature coefficient device (PPTC, commonly known as a resettable fuse) is a passive electronic component used to protect against overcurrent faults in electronic circuits. They are actually non-linear thermistors, however, and cycle back to a conductive state after the current is removed, acting more like circuit breakers, allowing the circuit to function again without opening the chassis or replacing anything. These devices are often used in computer power supplies, largely due to the PC 97 standard (which recommends a sealed PC that the user never has to open), and in aerospace/nuclear applications where replacement is difficult.

These devices are sold by different companies under various trademarks, including PolySwitch (Tyco Electronics), Everfuse (Polytronics), Polyfuse (Littelfuse) and Multifuse (Bourns).

A PPTC device has a current rating. When the current flowing through the device, (which has a small resistance in the on state) exceeds the current limit, 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. The trip current can be anywhere from 20 mA to 100 A.

A polymeric PTC device comprises a non-conductive crystalline organic polymer matrix that is loaded with carbon black particles to make it conductive. While cool, the polymer is in a crystalline state, with the carbon forced into the regions between crystals, forming many conductive chains. Since it is conductive (the "initial resistance"), it will pass a given amount of current, called the "hold current". If too much current is passed through the device, the "trip current", the device will begin to heat. As the device heats the polymer will expand, change from a crystalline state into an amorphous state. (Since the device physically expands, it must be given some empty space on the PCB.) The expansion separates the carbon particles and breaks the conductive pathways, 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 ("latching" functionality).

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. This cooling usually takes a few seconds, though a tripped device will retain a slightly higher resistance for hours, slowly approaching the initial resistance value.


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