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What's a PTC device? The acronym stands for "positive temperature coefficient".

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I didn't know, and maybe other people don't either.

Basically it's a little device that heats up very quickly when too much current flows through it, and increases its resistance in a non-linear fashion due to heating, thus cutting off surges of current.


Useful guide.

Another common name is polyswitch or polyfuse.

THey can also be used for overtemperature protection in transformers and motors.

Poly-switch is more like a resettable fuse than a true PTC element by my standards, tho they are very much alike.

They're so alike because they're the same thing.

Not really. PTC (positive temperature coefficient) is just about the tempco, the opposite of NTC. PTC resettable fuses/polyswitches/polyfuses do have a positive tempco, but the resistance sharply increases above a certain temperature. At that temperature something inside melts, and they trip (though a small amount of current keeps flowing). After they're tripped, they'll be slower to trip next time. There are also PTC thermistors designed as temperature sensors, these have a linear(ish) resistance versus temperature curve, and don't change when exceeding a certain temperature. These can survive much more heating/cooling cycles. Regular resistors can also be classified into NTC and PTC (eg. carbon film versus metal film).


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