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A polymeric positive temperature coefficient device (PPTC, commonly known as a resettable fuses) 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. They 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.

Another application for resettable fuses is protecting audio speakers, particularly tweeters, from damage when over driven: by putting a resistor or light bulb in parallel with the resettable fuses  it is possible to design a circuit that limits total current through the tweeter to a safe value instead of cutting it off, allowing the speaker to continue operating without damage when the amplifier is delivering more power than the tweeter could tolerate. Resettable fuses will also protect the speaker but when a fuse blows the speaker cannot operate until the fuse is replaced.

When the power and fault are removed, the device will cool. As the it cools, it regains its original crystalline structure and returns to a low resistance state 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.