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

[ModernRonin]

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.

http://www.circuitprotection.com/catalog/fundamentals/PSWFundamentals.pdf

[Zero999]

Useful guide.

Another common name is polyswitch or polyfuse.

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

[Pyr0Beast]

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

[Zero999]

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

[alm]

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).

[Zero999]

I suppose you're right, all polyswitches are PTCs but not all PTCs are polyfuses, the temperature vs resistance characteristic needs to have an sudden enough change at a certain temperature to act as a fuse.