Resettable fuse recovery time

[ricko_uk]

Hi,
does anybody know how to figure out the recovery time or condition for a resettable fuse like this the MF-LSMF110X. Here is the datasheet: https://www.bourns.com/docs/product-datasheets/mf-lsmf.pdf

There is no mention that it is latching (i.e. you need to remove power before it reset) or about the reset condition/criteria. Nor does it provide any other data such as recovery time (which would imply it recovers soon after the current goes back below a certain level).

Asked a while back to the manufacturer but no reply.

Thank you

[wraper]

It's pretty much a useless parameter as it's not supposed to trip at all during normal operation. While they're resettable, they degrade quick and as you can find in the datasheet you linked, particular fuse is only good for 100 times. The condition is it cooling down, also it decreases resistance gradually, so even current way below it's trip current can keep it from resetting.

[T3sl4co1l]

Depends on thermal conductivity, part size, etc.

Which, the trip point itself depends critically on the same, so, expect a wide range of currents, and sharp derating at high ambient temperature, especially varying with amount of copper connecting to the pads, airflow, board temperature and nearby heat sources, etc.

I tend to avoid ground plane underneath chip polyfuses for this reason.

I don't know that the polymer crystallization takes much time, but that might be a factor as well, materials can supercool and remain amorphous, only crystallizing more slowly.  Or if cooled below the diffusion activation temperature, never.

Operation in general is slow, as the active volume is much greater than that of a metal-element fuse, and the permissible fault power isn't very high either -- typically you see maximum current ratings say 10 to 100 times nominal, and the maximum voltages are usually low (10s).  Whereas a common 5x20mm fuse might be rated 1kA or more breaking current.  The slowness means quite a lot of energy let-thru, making them even worse at protecting semiconductors for example.  (That said, thanks to the low voltage, it is possible to design circuits to withstand such conditions.)

There are higher voltage types BTW, which are based on a ceramic composition.  The on/off ratio is smaller, they're lossier I think?  But handy when that's all you need.

Tim

[selcuk]

Even 5x20mm glass fuses don't have exact timings. Don't expect much from a PTC.

When you use a SMD PTC it becomes a whole with the PCB. So they cool and warm together. Your PCB will alter its timing a lot while tripping and recovering. I usually leave a hole on PCB under a SMD PTC between its pads.

[T3sl4co1l]

Yeah, to be clear, fuses of any sort are far from precise, and polyfuses are no different.  The most important part is, you may get more than one cycle out of the polyfuse, while you're guaranteed exactly one with conventional types.

Tim

[Psi]

Another important thing to be aware of is that fake polyfuses/smt fuses are common.
They're a part where fake ones go undetected for ages and are easy to fake.
You should be testing each batch at some point in production.

[ricko_uk]

Thank you all