I accidentally gave 15V to a Raspberry Pi 3 B+ Micro USB slot, when I observed some smoke I instantly removed in from the power supply.
What can I do to restore my Raspberry Pi?
(1) Gave 15V to Rpi3B+ Micro USB slot, smoke coming out, instantly unplugged.
(2) How to save the poor abused Rpi cat?
Replacing the blown polyfuse might be a solution.
It took me 4 minutes to remove the old polyfuse, and another 2 minutes to replace a new one. I rebooted the Rpi and found everything OK.
Now I am going to measure the self heal time of the Rpi3B+ polyfuse.
/ to continue, ...
Long answer to be shortened or removed.
Ah, let me see. If only smoke coming out, and plug pulled fast, there is 20% that the poor cat's life can be saved.
(1) Firstly, let us take a look at the circuit and do a postmortem.
(2) You see, only two wires are connected to the micro USB connector. So troubleshooting should be dead easy.
(a) Use a multi-meter to do open-short tests, check continuity of J1 pin1, pin5 to pp2, pp3，pp7 etc.
(b) See if F1, D7, C84 (47uF) are still alive.
(c) Even above 3 poor souls all dead, you can still do something.
(d) Usually the big 47uF guy raises the smoke alarm signal first. My experience is that if I pull the plug lightning fast (Note 1), then there is 80% chance it is still working, no harm done, ... :)
(3) Checking out the real thing
Now I am checking out the real thing: a broken Rpi3B+ with two GPIO pins already fried, but power circuit still working. I think the big black guy next to the micro USB connector, labelled KE 7J677 (pp7 test points at the back side of PCB, pp2, pp3 near by) should be the 47u cap.
Rpi4B has a similar component (see the bigger photo below)
(3) Checking out the Test Points
I found the test points at the back side of PCB easy to locate and therefore the components easy to be identified. The critical components though SMD are pretty large in size, therefore newbie friendly to mess around.
Actually, you don't need to do the dirty job on spot, because you can extract two power wires out of the Rpi PCB and set up your "off site" test rig, say 10 cm from Rpi.
It would be nice if the polyfse, Zener, etc are blown off and become open circuit, effectively isolated from the to be replaced components. And even the old/broken components stay in place, new components can join in parallel, sort of good and bad guys working hand in hand.
Of course I don't guarantee that nothing won't explode. But your broken Rpi will sooner or later goes to the dust/trash bin/can anyway, you have nothing to lose, but only gain in "advanced" polyfuse and Zener circuit theory and practice (yes, OK, hacker practice/tricks).
Anyway, the Rpi power circuit seems easy to hack. So, without further due, So let us start messing around, ... :)
(4) Waiting two days or more for the polyfuse to self heal
Now I think if 15V is applied to the Rpi, two things might happen at the same time:
(a) Polyfuse F1 blows,
(b) By pass cap C84 heats up and raises blue smoke alarm.
As I said earlier, the capacitor might have little harm and recover immediately.
However, the polyfuse might take two or more days to recover. (Appendix A)
Therefore, I think the OP should wait for two or more days for the polyfuse to recover. During the waiting time, he might also like to use a multi-meter to check if the polyfuse resistance is decreasing to a very low resistance.
(5) Replacing blown out Rpi SMD polyfuse by through hole polyfuse
Actually it is dead easy to locate the poly fuse: it is the big and rich rectangular guy with 24k gold hat and shoes, as shown below. To confirm, I used a multi-meter to measure the resistance and found it very very low resistance, both direction, as low as pure gold! (definitely not Zener or 47 uF capacitor, which have different bipolar resistance values) Now I am going to remove it and do some experiments to verify that my guesses are correct! :)
/ to continue, ...
(5) Bourns Polyfuse
Appendix A - Polyfuses explained - elinux
Note - this eLinux explanation seems NOT applicable to Rpi3/4 which requires a much bigger current. - tlfong01 2019oct17hkt1017)
A "polyfuse" is a fuse made from a polymer. It is a fuse that:
after it has "blown" will self heal; that is after several days, it will (almost) behave as if nothing has happened.
There are three polyfuses in a RPI. One is used to protect the whole board ... There are also two smaller polyfuses, one to protect each USB port...
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.
End of answer