PolyFuses, good to use?

[BrendanP]

I'm finalising the pcb design for a NiMH charger built around the MAX712. I have read a resettable polyfuse (actually, I think PolySwitch is the correct name) is a good idea in such a circuit as a safeguard.

They only cost a couple of bucks, are they a good idea? Is there a downside to them?

 

[MFB]

They act pretty much like a wire fuse but reset when the current is removed. Only problem I have found is that their response time depends on the level of overload (a bit over the limit takes longer to 'blow' than a lot over the limit) but OK for power supply protection, because we are talking in the range of hundreds of mSec.

 

[hippy]

As an extra safety precaution I think they are a good idea. I've used them when adding things to my car's electrical system and they work. Very useful for when a fuse isn't easily accessible.

Most fuses have a delay time to blowing and a tolerance to short term over-current, so a bit of mix and matching can help, for example 3A polyfuse, 5A slow-blow, 10A fast-blow etc, but it's one of those black arts to get right - My principle; extra protection doesn't usually do any harm.

Probably the one place you don't want to use a polyfuse is where a cut-off should be permanent. Fault mode, polyfuse blows, circuit cuts off, polyfuse resets, circuit fires up again, fuse blows cycles - That may apply to battery charging.

 

[Dippy]

Well, not quite:
"As long as the applied voltage remains at this level, the device will remain in the tripped state (that is, the device will remain latched in its protective state). Once the voltage is decreased and the power is removed the device will reset."
http://www.circuitprotection.com/07catalog/polyswitchTechFund.pdf

Do remember, they come in all sorts of voltage and current ratings and speeds. They also need to be temperature derated just like many other components.
Selection choice requires a bit of study and their performance is as discovered by MFB and described in Data Sheets.

Here's a good lookup table from one manufacturer:
http://www.farnell.com/datasheets/75185.pdf

They have their places where they are ideal, though I'm not so sure with battery charging.

PS. I've just re-read and understood the scenarion you describe...

 

[BrendanP]

Thanks for the link Dippy.

My understanding from the Tyco pdf is like yours Dippy in that once the device 'trips' and stops current flow it stays that way until the voltage is removed ie. plug pack turned/ unplugged. I don't think they cycle on/off.

I read in the Tyco sheet that they have to be good for 6,000 cycles to get UL approval.

 

[premelec]

they are basically a positive temperature coefficient thermistor with sharp breakpoint - so they respond to the temperature of the bulk material - watts = current x voltage and initially the voltage and resistance is low - then when it heats it switches to higher resistance and therefore higher watts at same current [i^2*r].

IF you cool the unit externally, or reduce the current to a point where it cools then it "resets".... An important feature to note is that external heating will cause it to "blow" sooner and cooling "blow" later... Usually this is OK - I like 'em.

They are deinitely in the slow blow category - and there aren't a lot for high voltage interruption either... but they sure beat having to replace blown fuses...

They are also made with a particularly sharp temperature transistion point for use as a soft thermostat - they'll regulate their own temperature - useful at times for a simple thermostatic functions...

 

[gengis]

I've used them for about 7 years now and I'm satisfied. They are slow to react to an overload and it pays to think of them as very slow blow. The load on the ones I've been getting doesn't have to be "removed" for it to reset, but it does have to be reduced below about 25% of its normal rated current. While not a perfect device, it is ideal for a lot of applications.

A battery charger seems like a good application to me - if you don't need it to reset on its own.

My motorbike has the turn signals and brake lights on one fuse - I had a turn signal short and spent a few days driving without a brake light as a result - poly fuse would have been the perfect fuse in that case.