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.

Another application for such devices is protecting audio speakers, particularly tweeters, from damage when over driven: by putting a resistor or light bulb in parallel with the PPTC device 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. A fuse will also protect the speaker but when a fuse blows the speaker cannot operate until the fuse is replaced.

These devices are sold by different companies under various trademarks, including PolySwitch (TE Connectivity),PolySwitch is the earliest product of this type, having been invented at Raychem Corporation (now TE...
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