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I'm currently trying to build a hardware USB-MIDI-Recorder based on an Arduino Leonardo. The idea is that you attach it to a self-powered device like an E-Piano that features a USB-B out port for sending MIDI packages and record those to an SD card.

It would be nice if I could actually power the recorder from the USB port of such a device directly so that I don't have to use an external power supply, however, I think those USB ports are only slave ports most of the time.

Is it still possible to draw some power from them or would that damage the device? This is probably out of specification, but is there still a chance? The recorder would probably draw less than 100 mA.

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  • \$\begingroup\$ It is possible but unlikely. Some devices out of bad design do allow backfreeding but thats rare and again bad practices. If you measure the V+ pin on that connector while powered it should not have any voltage. But if it does you may be able to (ab)use it. \$\endgroup\$
    – Passerby
    Nov 14, 2021 at 17:40

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No, it is simply not possible to draw power from USB-B device. It would not be according to specs, and it must be in fact prevented, otherwise the device would back-feed power in the wrong direction, back to for example an unpowered computer.

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