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There are other posts indicating that bad things can occur if you are providing 5V from the external connector and plug in a USB cable with its own 5V supply. Various solutions have involved cutting VBUS in the cables or just not using the usb connectors. Both of these seem like suboptimal work arounds.

I've used other systems with similar configurations and the solution was to place a Schottky diode inline with the external supply to prevent back feeding.

Is this an acceptable solution to permit both an external 5V supply AND allow the usb connectors to be used as is, and with normal USB cables? Maybe there is an official document on how to do this that I haven't found yet?

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The Pi Zero family has minimal (primitive) power circuitry and no protection.

All 5V connections (including USB) are directly connected (which is in violation of USB specs) so it doesn't make any difference how you power it.
Back feeding is the least of your worries.

The USB port is intended to power peripherals.
It CAN be used to power the Pi (because Raspberry Pi Ltd. left off the circuitry which would prevent this and is incapable of negotiating); whether this works depends on the external device. A standards compliant device would only supply 150mA to the Pi.

In fact connecting a powered Pi Zero USB to a computer without a hub risks damage to both.

If you want to do this you should isolate 5V (how you do this doesn't matter) - I have used Shottky diodes on an old PiB so I could use normal USB devices powered by the Pi.

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  • Is the issue with directly connecting to a computer because you'd be feeding the 5V external back through the device connector (which shouldn't be supplying power) to the computer? I guess I should be using a network connection or the serial connection if I'd like to connect a computer to the pi zero? Jan 21 at 3:10
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    @ChrisMorgan I am not sure I understand your comment. A normal USB peripheral should be powered by the the host to which it is connected. Most computers (including the Pi) expect to provide power to USB peripherals so you are interconnecting 2 5V sources (with unpredictable results). The PiZero can act as an OTG device (i.e. client) but lacks any circuitry to disable power feed.
    – Milliways
    Jan 21 at 3:37
  • Right, so even though the ports are OTG its power, at least through the power port, isn't acting correctly as an OTG port and thus could do bad things if you plug it into a usb host that provides power, like your PC. Jan 26 at 2:24

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