Polyfuses

Aug 27, 2016 6 Replies
T

Reminds me, there are two different kinds of PTC.



Today's most common PTC is a polymer type, which is cheap and small, and very unreliable (100s to 1000s of cycles). They're only made to protect against single, infrequent fault conditions.



What if you need "unlimited" protection, or you have to protect something that's frequently switched (like a cycling motor or heater)?



Trick is to search for ceramic PTCs. 10k's of cycles.



Unfortunately, no one sorts them by type... so good luck finding them. Naturally, they tend to be more expensive, too, so you aren't going to find them in your usual cost-motivated search.



The Vishay parts are pretty reasonable:

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TDK/EPCOS have a line as well, but they cost five times more, and aren't searchable (their website is terrible). Not sure who else has 'em, or if there are better ways to search.



Tim


Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
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K

Or a current limiting high-side switch.

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T

Yup, but that's only when you have low voltages (at least, I haven't seen any of those for high voltages, and high power), and logic/control voltage available.

The time constant is rather short, which can be advantageous, but also might not be. Say for motor starting.

Also doesn't work on AC, at least not without a lot of hackery.

So for that kind of thing, PTCs are great!

Tim

Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
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K

True, I generally use them for The time constant is rather short, which can be advantageous, but also might

Relay. ;-)

Except that they have really limited voltage/current, too. They're also *really* sloppy (fuse sloppy).

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J

The IXYS CPC1540 is an interesting part. It's an SSR, 350 volts and

120 mA, and it's self-protecting: current and thermally limited. We sometimes use them as always-on solid-state fuses.
John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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J

The Tyco polyfuse data sheets don't mention lifetime. I have one Bourns data sheet that specifies min 100 cycles at Vmax and Imax.

I guess I should test some.

John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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G

I've got data from littelfuse on polyfuses. Basically ever event ages them, and if you leave them in the shorted state it ages them. They age in the "right" direction in that they fuse at a lower current as they age.

I could see if I could dig up the data. I'm not supposed to share it so I can't post it online. I can send you a copy, but you have to promise to burn it after reading. :^)

George H.

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