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New to the Pi, so new in fact that my very first (a B+) hasn't even arrived in the mail yet...

I know that "backfeeding" or "backpowering" refers to powering the Raspberry Pi via a powered USB hub connected to the Pi through the latter's USB port (rather than the micro-USB power socket). I've seen lots of discussion about how to avoid it, ranging from "avoid these hubs" to even solutions involving custom USB cables that exclude the +5V wire.

But why is this such a big deal?

A USB hub runs at the same +5V that the Pi itself runs on. And the output side of the Pi's power supply is connected directly to the USB ports' +5V rail (at least that's what I've gathered from the simplified schematics I've looked at). So what does it matter that the +5V comes from the USB hub instead of the Pi's own power supply?

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There is only a problem if you have two 5V supplies fighting with each other.

I quite often back power early model Pis, but if I'm doing so I don't also power via the expansion header or the microUSB socket.

By the way you can't back power the B+ or A+ or Pi2 via the USB sockets. Circuitry has been added to stop current inrush problems which used to crash the Pi. As a side-effect this has stopped the ability to back power.

EDITED TO ADD

As JamesJones mentions in his answer you can actually backpower the A+/B+/Pi2 from USB once the Pi has been booted with power via the microUSB or expansion header.

  • 1
    Oh interesting, I thought I'd read the opposite, that the B+ et al had had circuitry removed which actually made it easier to back power. – Kromey Mar 4 '15 at 20:29
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    When I say back power I specifically mean via the USB sockets. Some (stupidly in my opinion) have started to use the term for powering via the expansion header (unfortunately that includes the Foundation). You can not back power via USB sockets on the A+/B+/Pi2. You can power via the microUSB or the expansion header. – joan Mar 4 '15 at 20:38
  • Not necessarily the 'only' way you might have a problem. You're breaking the USB specification, which means you might get undefined behaviour on some devices. Such as killing the computer you're plugged into. youtube.com/watch?v=Uh6iKilgtG0 – steveayre Apr 27 '16 at 13:46
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YES YOU CAN! BUT.... here is what you have to do for it to work out.

First off, you can't power-on the Pi2B just over a USB-Hub for example. You always need to start it up using the classical method with the micro-usb-connector.

But after it's booted up, you can happily remove the micro-usb-connector leaving it just powered up over the USB-Port/Hub.

Only Downfall: You can't reboot and will have to plugin the Micro-USB for the few secounds it take for a RPi2B to boot up, but after that, happily remove the micro-usb-connector again.

The Risks: Same as always with backpowering. No protection therefore make sure to have a stable Power-Supply/powered USB-HUB that put's out the right Voltage/Ampage without current fluctuation.

Just to be complettly straight-forward:

Plug-In Powered-USB-Hub --> Nothing Happens --> Plug-In Micro-USB-Connector without removing Hub--> Starts booting... --> Remove Micro-USB after Boot up complete. DONE.

If you need to reboot:

Kernel-Update, neeeed reboooot --> sudo reboot --> shuts-down --> nothing happens --> Plugin Micro-USB-Connector --> starts booting... --> Booted sucessfully, remove Micro-USB. DONE.

Other than that? Have fun.

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I have a pair of A+ units: Type: Model A+, Revision: 1.2, Memory: 256MB, Maker: Sony

These seem to backfeed just fine from my USB hub. I've had them running for sometime without issue.

My model Pi2 in fact does not allow backfeed at all using the same USB hub.

  • What is an A+ unit in this context? – Bex Mar 6 '15 at 9:26
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    The A+ is the stripped down model of the Pi B+ with the 40 pin GPIO header. In this context it does behave differently than the B+ when it comes to backfeeding power. – Subrosa Mar 6 '15 at 18:36

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