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Resettable Thermal Fuses

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ClydeCrashKop

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I just discovered these
Resettable Thermal Fuses
http://www.mpja.com/11-28-16.asp?r=226642&s=4

Assortment of 50 Tyco RUEF Series PPTC auto-reset thermal fuses. Thermistor-type devices that limit the flow of dangerously high currents during fault conditions. But unlike traditional fuses that work one time PPTC devices reset after the fault is cleared and power to the circuit is removed. UL listed, RoHS compliant
Rated: 30V
5ea. of 10
Current:
HOLD: 0.9A, 1.1A, 1.35A, 1.6A, 1.85A, 2.5A, 3A, 4A, 5A, 8A
TRIP: 1.8A, 2.2A, 2.7A, 3.2A, 3.7A, 5A 6A, 8A, 10A 16A

http://www.mpja.com/download/33414FUData.pdf
 

ClydeCrashKop

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You are right.
A polyfuse "blows", just like any regular fuse, because there is running so much current through it that it heats up. In a regular fuse that means so much current causes a thin wire to simply melt; in a polyfuse it means that a small piece of conductive (polymer) plastic gets so hot that suddenly its resistance increases dramatically, so that it gets hotter still, and things escalate to a point that the resistance goes so high that most of the current stops flowing. After the current is turned off, a long time after the fuse has cooled down the fuse slowly regains it original state and in the end becomes conductive again.
Always learning. One more tool in my bag of tricks.
 

JonSea

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Tayda Electronics has polyfuses for sale individually for between 10 - 18 cents each. They have values as low as 200mA, which is more in line with what I need.

The rated current is what they will carry. I believe it takes about twice that current to cause them to "blow" in a few seconds.
 

AnalogKid

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A polyfuse "blows", just like any regular fuse,
No, it doesn't, and the differences are important.

1. With a polyfuse there is no galvanic isolation after a trip. For a part rated for hundreds of mA or several A, the "idle" current through a tripped polyfuse can exceed UL safety limits.

2. A polyfuse has a much longer reaction time than a traditional fuse. The current break is not as quick or as clean.

3. The ON resistance and reaction time both degrade with each trip cycle. 5-10 trips and they're shot. BTW, the same is true for MOV transient suppression components.

I think of a polyfuse as last resort compromise rather than as a first-line design component. They are better than nothing, but only in a situation where nothing else will work.

ak
 

schmitt trigger

Well-Known Member
To add to AK's list:

4. The triping current varies, sometimes significantly, with the ambient temperature.

They are adequate for many applications as long as one understands its limitations.
 

ronsimpson

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A polyfuse "blows", just like any regular fuse
Poly Fuses have no vibration problems. "Regular" fuses, when running close to the trip point, react to vibration. Note "automotive" fuses assumes vibration while house hold fuses assumes no vibration.
 

AnalogKid

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To add to AK's list:
4. The triping current varies, sometimes significantly, with the ambient temperature.
And ambient temperature varies, sometimes significantly, with location on the board and within the system. Both cooling fans and protection devices tend to be near the input end of a power supply, with results that for most designers qualify as a case of "single trial learning".

ak
 

KeepItSimpleStupid

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On one relativrly "hot day", my driver's power window would not work. It took about 3 hours to cool and a 45 min trip at highway speeds didn't work.
 
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