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I have read on Wikipedia how a Polyfuse works, and other Questions have shown that shorting out some of the GPIO Pins or other "accidents" can trip this item on the Pi. Given that the failure mechanism is in some way thermal, does anyone have any experience of whether the quoted "day or so" to allow it to "recover" after tripping could be improved by "extra" cooling - say by sealing the Pi in an anti-static bag inside a tied-off polythene bag and putting the whole thing in a domestic freezer?

The Wikipedia entry notes that a typical device regains it's crystalline (conductive) state in a few seconds through natural cooling but will have a higher resistance for hours before reaching a final "reset" state, it states:

"The device may not return to its original resistance value; it will most likely stabilize at a significantly higher resistance (up to 4 times initial value). It could take hours, days, weeks or even years for the device to return to a resistance value similar to its original value, if at all."

So, does enforced cooling (refrigeration/freezing) shorten the time it takes or reduce the "extra" resistance that this polyfuse is said to gain? Whilst the former will be the most obvious for someone who has experienced this the latter may transpire to be more harmful - from my understanding of the processes this would reduce the future "safe" current level - which, if it was near the limit before, will become more marginal after a single trip cycle!

For example a Where is my RPi losing power question reports a (presumably, untripped) polyfuse presenting a 1.2Ω resistance[1] - which will be dropping in the order of 0.8 Volts if it is passing the nominal 0.7A of, given the age of the question, likely a Pi Model B. That poster was having problems because his nominal 5V Charger-type PSUs were dropping the level under load. If, following an incident that caused a trip, the resistance reset to a 10% higher level then that same drop would increase to over a Volt.

Note: I am asking this from a theoretical standpoint, I do not have a Pi that would benefit from it (at the moment anyhow) but any practical tests someone might be willing to do or any empirical evidence already gathered with a Pi would be useful reference material.

As a supplementary query, it is not clear that repeated tripping would result in a "recovery" level of resistance that is largely constant after the first instance or whether there would be an on-going degradation arising from each subsequent "trip". This is also something which I think would useful to know - even as general advice to offer to those who show signs of repeated abuse of power(supplies)!

Whilst it has been suggested that this question might be better answered on EESE in that community they could be drawing from a whole range of deployment cases (anywhere where a polyfuse device is used) and whilst the larger field may give more responses I would be sceptical about how applicable to the specific RPi case those responses would be. The device characteristics for the Pi case is narrow and well defined (no more that few different devices for all the different Pi models) so answers from practical experiences here will be very pertinent!


1 - For comparison, a quick test on a RPi 2 B that I have that has no evidence of ever tripping suggests a normal series resistance of <0.1 Ω for the main polyfuse.

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    Seems like something that would be more appropriate on EE. – goldilocks Dec 25 '15 at 15:18
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Heating it up or cooling it down won't make much of a difference. You can't really do anything about it. Let it reset by itself. Store it at room temperature while it's doing its thing.

If you're really in a hurry, replace it.

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    I do not think that "Heating it up or cooling it down won't make much of a difference." is correct - I believe that running a Pi at an "elevated" ambient temperature will increase the likelihood of the polyfuse tripping. A PDF document referenced on the Wikipedia page includes the passage: "Since PolySwitch devices are thermally activated, any change in the temperature around the device will impact the performance of the device." – SlySven Dec 25 '15 at 20:38
  • @SlySven I thought we were talking about an already tripped fuse. "Resetting the Pi's main Power Supply polyfuse faster" implied that the fuse is already blown, and you're just looking for a way to speed up the resetting process. Bruh. – Gene Dela Rosa Dec 26 '15 at 6:09
  • Perhaps an understandable thought @PandaLion98, but no I wanted to find out before such a thing occurred for me! 8-) I think I might be in a position to do some research as my present usage case means I will be powering via (a UPiS unit) the 5V GPIO pins so the main Polyfuse will not be involved and can be placed upon the sacrificial alter of scientific research...! 8-P – SlySven Dec 26 '15 at 14:40
  • @SlySven I found this: "Letting them have a rest in a cool place might help a "bad" polyfuse recover" elinux.org/Polyfuses_explained – Gene Dela Rosa Feb 11 '16 at 13:02

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