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I was trying to make a custom power source to supply power to the micro usb port on the Pi B+ and I reversed the + and - by accident and then smelled something like burned plastic. The SD card doesn't seem to boot anymore but I changed the SD card with a new one and now it seems to work still.

Is this PI still useable? Does the Raspberry Pi 3b+ have reverse polarity protection built in?

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There is no reverse polarity protection on the Pi3B+ although earlier models had an "ideal diode" (whether this would provide reverse polarity protection is doubtful - this is there to prevent back-powering). The schematics are available for download.

The Pi3B+ (and similar) do have a polyfuse and transient protection diode which probably conducted and may have protected the Pi (but I wouldn't try it myself).

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    I am a newbie in schematics so I'm asking here. – bakalolo Sep 6 '18 at 4:00
  • One way to find out is to power it (correctly) and see if it boots :) – JayEye Jul 23 at 18:42
  • The zener diode (D7 on the 3B+ schematics) would conduct if you wire the μUSB connector backwards. The MF-MSMF250/16X (MF-MSMF250/X must be a typo, it does not show up in bourns.com/docs/Product-Datasheets/mfmsmf.pdf) will take >2s to trip at 3A, by which time either the power supply or the zener are probably toast. The only components that are fed the +5V supply are the two regulators, which may or may not have reacted well to the reverse polarity. OP: please let us know whether your Pi is still alive, neither @Milliways nor I have a spare one to kill :) – JayEye Jul 23 at 19:08

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