LED and Fuse question

[adr308]

Will the red LED light up at all if the polyfuse is blown?

Also, how long can it take for a polyfuse to recover?

[Burngate]

Will the red LED light up at all if the polyfuse is blown?

No.
That LED is switched off if the voltage of the 5v rail after the polyfuse is below 4.6v (monitored by a APX803-46, at least on the B+, 2B & 3B Pis)
If the polyfuse has blown, that voltage will have dropped well below that.

Also, how long can it take for a polyfuse to recover?

I can't find a figure for that - Littelfuse only seem to give a spec. for its resistance 1 hour after tripping.
But if you only leave it for an hour, its resistance will be such that it's going to trip again fairly promptly.

In the STICKY: Is your Pi not booting? (The Boot Problems Sticky) it says

it will automatically recover if you give it some time, (meaning turning off the power and wait) though it may take a few days.

Elsewhere, "24 hours" has been mentioned, along with "or more"

[LTolledo]

it will automatically recover if you give it some time, (meaning turning off the power and wait) though it may take a few days.
Elsewhere, "24 hours" has been mentioned, along with "or more"

... never have tripped my RPi's polyfuses yet.... but just in case, can the "polyfuse recovery time" be somewhat "accelerated" if the polyfuse is "cooled" to a certain temperature?
just asking....

[Brandon92]

it will automatically recover if you give it some time, (meaning turning off the power and wait) though it may take a few days.
Elsewhere, "24 hours" has been mentioned, along with "or more"

... never have tripped my RPi's polyfuses yet.... but just in case, can the "polyfuse recovery time" be somewhat "accelerated" if the polyfuse is "cooled" to a certain temperature?
just asking....

Yes is its. You only need to remove the power and "cool" the PTC fuse:

The heat generated by this limited current through the higher resistance value will maintain the temperature of the PTC at a level that will cause the resistance to remain high. This thermal equilibrium condition will continue until power is removed from the circuit which allows the PTC to cool down and the resistance will decrease. The resettable feature of the PTC is based on the fact that the increase in resistance resulting from the increase in temperature is reversible. The PTC is reset or returned to the lower resistance state by removing the power from the circuit which allows the device to cool down. The unit is then ready to react to future overloads. The resistance will remain low if the cause of the overcurrent has been corrected, but if the overcurrent recurs the device will again switch to the high resistance state.

*https://www.littelfuse.com/about-us/edu ... -ptcs.aspx

[Burngate]

I too haven't blown one yet ... in the interests of science I might consider doing so, but just not yet ...

Wikipedia goes into some detail https://en.wikipedia.org/wiki/Resettable_fuse
According to that page,

As the device heats, the polymer will expand, changing from a crystalline into an amorphous state ...
When power is removed, the heating due to the leakage current will stop and the PPTC device will cool. As the device cools, it regains its original crystalline structure and returns to a low resistance state where it can hold the current as specified for the device.[6] This cooling usually takes a few seconds, though a tripped device will retain a slightly higher resistance for hours, unless the power in it is weaker, or has been often used, slowly approaching the initial resistance value. The resetting will often not take place even if the fault alone has been removed with the power still flowing as the operating current may be above the holding current of the PPTC. 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

Part of the problem is that the polymer* has two states, crystalline and amorphous, and while the transition in one direction is well defined, the opposite isn't.
So you're relying on it recrystallising in a short time, but it won't do that if it's too cold - the molecules have to move around, which takes energy.

*whatever that is - there are many different polymers. Cellulose is one and so is starch, both glucose polymers. PET is another, and so is DNA
I doubt any of those is used here, but you never know!

[Brandon92]

Interesting, I believe that I have some PTC fuses lying around. If I'm correct.
It could be a interesting test to see what happends to them

If I can find them and if I have some spare time. I will try it.

[Brandon92]

So, I did preform this test.

Equipment
Multimeter: Fluke 187 (zero out the wire resistance)
Fuse: MF-MSMF050-2 (MF-MSMF Series, 500 mA, 1 A, 15 V)
Soldering iron set to 300 decrees Celsius.

First I measure the resistance of the PTC out of the packaged: 0.3 ohm
Then I soldered a wire at both ends and measure the resistance again: 0.15 ohm

Then I set my powersupply to 5V and limited the current to 1A. And after 7 minutes of short. The PTC didn't care about it. This is still within his specification (30min @ 1A before tripping)

So, I change the current too 2A. And after 3min still no tripping. So I changed the current again, and this time to 4A. And after 1min still no tripping. So, I changed the current to 6A And this time the PTC fuse tripped between 0.5-1min. And after, the current was still 0.23A.

After that I measured the resistence of the PTC: 0.8 ohm. And after cooling it with Freeze spray the resistance dropped to 0.72 ohm.
After waiting another 18min the resistance dropped to 0.69 ohm. And after 12 day the resistance dropped to 0.65 ohm.

So, in this specifiek test this PTC is indeed after a short not restored to the original value.