To restore it will cost somewhere between £5 and £55 depending on whether you buy a Zero or a RPi4B 4GB (or something in between).
Connecting 15V is 100% guaranteed to let the magic blue smoke escape and you can’t get it re-filled.
Plug it into a 5v 2-3amp (10-15w) power supply and check if it powers on. If not, throw it in the garbage.
(1) Gave 15V to Rpi3B+ Micro USB slot, smoke coming out, instantly unplugged.
(2) How to save the poor abused Rpi cat?
/ to continue, …
Ah, let me see. If only smoke coming out, and plug pulled fast, there is 20% that the poor child’s life can be saved.
(1) Let us 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.
(b) See if F1, D7, C84 (47uF) are still alive.
(c) Even above 3 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 Rpi3B+ with two GPIO pins 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.
(3) Checking out the Test Points
I found the test points at the back side of PCB easy to locate and identify the components. In other words, the power circuit is easy to repair.
(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 raise smoke.
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.
/ to continue, …
Appendix A – Polyfuses explained – elinux
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