A resettable fuse is a polymeric positive temperature coefficient (PPTC) device that is a passive electronic component used to protect against overcurrent faults in electroniccircuits. The device is also known as a polyfuse or polyswitch.They are similar in function to PTCthermistors in certain situations but operate on mechanical changesinstead of charge carrier effects in semiconductors.
A polymeric PTC device is made up of anon-conductive crystalline organicpolymer matrix that is loaded with carbon black particlestomake it conductive. While cool, the polymer is in a crystalline state, with thecarbon forced into the regions between crystals, forming many conductivechains. Since it is conductive (the "initial resistance"), itwill pass a current. If too much current is passed through the device thedevice will begin to heat. As the device heats, the polymer will expand,changing from a crystalline into an amorphous state.Theexpansion separates the carbon particles and breaks the conductive pathways,causing the device to heat faster and expand more, further raising theresistance. This increase in resistance substantially reduces the current inthe circuit. A small (leakage) current still flows through the device and issufficient to maintain the temperature at a level which will keep it in thehigh resistance state. Leakage current can range from less than a hundred MA atrated voltage up to a few hundred MA at lower voltages. The device can be saidto have latching functionality. The hold current is the maximum current atwhich the device is guaranteed not to trip. The trip current is the current atwhich the device is guaranteed to trip.
When power is removed, the heating due tothe leakage current will stop and the PPTC device will cool. As the devicecools, it regains its original crystalline structure and returns to a lowresistance state where it can hold the current as specified for the device. This cooling usually takes a fewseconds, though a tripped device will retain a slightly higher resistance forhours, slowly approaching the initial resistance value. The resetting willoften not take place even if the fault alone has been removed with the powerstill flowing as the operating current may be above the holding current of thePPTC. The device may not return to its original resistance value; it will mostlikely stabilize at a significantly higher resistance (up to 4 times initialvalue). It could take hours, days, weeks or even years for the device to returnto a resistance value similar to its original value, if at all.
A PPTC device has a current rating and a voltage rating.