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I want to solder a male usb port to my Pi, so that it can be a little dongle computer. However, if I power it with a powerbank, and while it is still being powered, plug it into another power source over the soldered usb connection, will I fry it, since it is being powered twice on the same port? Thanks!

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    The +5V & GND pins on both microUSB sockets on a RPiZ are connected. Only the socket furthest from the edge of the board has D+ & D- connected. – Dougie Mar 8 '20 at 0:29
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As Dougie has commented the ports are joined together.

It is extremely inadvisable to connect the outputs of 2 switch mode power supplies together.

The results are unpredictable. It is unlikely to damage the Pi and in the best case power will only be drawn from a single supply - the higher voltage causing the other to ramp down.

Normal USB gadgets are designed to be powered by the computer to which they are connected.

I suggest 3 options:

  1. Power the Pi from the computer - whether this is feasible depends on the Pi current requirements and/or the computer.
  2. Isolate the 5V line so the Pi will not attempt to power the computer.
    This should allow normal USB gadget operation if plugged into a power source.
    I would use a Schottky diode (as I did with the very early Pi to prevent them being back-powered by USB - except the other way round).
    An "ideal" diode (using a MOSFET) would be better - as used in most Pi models, but not Pi0 or Pi4.
  3. Ignore the 5V from the computer - by not connecting the 5V line.

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