# Are PTC fuses called some other way?

I recently learnt about the existence of PTC fuses, and I'd like to use one for an USB power supply I'm building. I guess there are PTC fuses on the PC side, but you never know.

I went happily to buy some at my local electronic supply store, but unfortunately they said they don't have resettable fuses (weird, they've got almost anything.) Since I'm spanish I didn't ask for PTC fuses, but asked for resettable fuses and explained how they worked, in case they're called something else here at Spain. They didn't have the fuses or knew where I could find them.

I've read PTC fuses are essentially thermistors, is it possible that they know them as thermistor or some other name, instead of resettable fuse?

I've heard them called polyswitches, polyfuses, multifuses and self-resetting fuses.

Just a thought the PTC could mean Positive Temperature Coefficent, which would make sense with thermistors as you get 2 basic types PTC and NTC(Negative Temperature Coefficient).

This is what Wikipedia says are some trademark names for PTC Fuses:

These devices are sold by different companies under various trademarks, including PolySwitch (Tyco Electronics),[3] OptiReset (OptiFuse), Everfuse (Polytronics), Polyfuse (Littelfuse) and Multifuse (Bourns, Inc.).

Here is the link to the wikipedia article.

• Already readed Wikipedia's article before asking, of course, but thanks anyways. – kaoD May 7 '11 at 22:10

Not a ton of experience with this, but the thermistors that I've used were dead once they had been triggered. As in, one time use.

Without knowing what you're trying to use it for, I can't say too much, but circuit breakers are basically resettable fuses. Maybe thats what you were looking for?

• I don't think thermistors are one time use. Fuses are, but thermistor are just variable resistences upon temperature changes. I guess these PTC fuses act like circuit breakers, but are esentially a special type of thermistor with a huge resistance upon reaching a certain intensity (and therefore, temperature.) These thermistors are not linear, so I guess they have a steep resistance near the limit intensity. I will ask for them as non-linear thermistors and see if I'm lucky :) – kaoD May 7 '11 at 20:46
• @JGord What do you mean by triggered? Normally thermistors don't have any trigger. They always change their resistance according to temperature and are among other things used to control fan speeds in computer power supplies and so on. Are you sure you used appropriate thermistors for your job? – AndrejaKo May 7 '11 at 21:14
• Once there is sufficient current to increase the temperature the resistance starts to increase, which leads to more power loss in the thermistor, so more heating, even higher resistance, etc, until there is a high resistance and only a small current left. When power is removed and it is cooled down it should conduct again, as before. – starblue May 7 '11 at 21:44
• @AndrejaKo: Some companies make devices whose resistance is quite low until they reach a certain temperature, whereupon the resistance increases very rapidly with temperature. What will end up happening is that the device's resistance will increase until the electrical power dissipated in the device falls to the match the level of heat the device can dissipate. Generally the devices don't work as effectively as a good fuse (they may allow 50% overcurrent to flow for a long time before they get hot enough to reduce power) but they can cycle hundreds of times without replacement. – supercat May 8 '11 at 0:54
• @supercat What you described is standard way of thermistor operation. I fail to see how is that different from overloading any other type of resistor. Such reasoning led me to make the last part of my comment:Are you sure you used appropriate thermistors for your job? . If the thermistors keep blowing up, then thermistors which can dissipate larger amounts of heat should be used, so that they don't overload when they start dissipating large amounts of heat. – AndrejaKo May 8 '11 at 8:02