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I have a USB device that requires more power than the RPi can provide. I don't want to use a powered hub. Couldn't I build a simple adapter that passes only the data lines and breaks out Vcc and ground to the device (no connection to RPi) such that the device could be powered by a separate source? Could the source be the same as the RPi's? Thanks for your thoughtful answers.

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Why don't you want to use a powered hub? – Alex Chamberlain Sep 3 '12 at 7:18
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Fair question. I have an application that must be battery powered so I have to keep the power requirements low. I have the proof-of-concept working; now I'm working on refinements. – CPRitter Sep 3 '12 at 10:55
I see! I'd like to see someone develop a powered USB expansion board, which would be perfect for you as it could power the RPi and the devices efficiently. – Alex Chamberlain Sep 3 '12 at 10:56
What I considered doing was removing the PWB from a powered hub, mounting it in the same box as the RPi and powering both from the same battery. But the application doesn't require 4 more USB ports, all that extra HW and battery drain or cable to connect the hub and RPi! It's headless and the two built-in ports are sufficient. All i need is a tad more juice to the USB device. – CPRitter Sep 3 '12 at 12:50
did you manage to get this setup working CPRitter. I could appreaciate some details;) thanks – jarshvor Mar 8 at 20:47

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It should be fine. You should connect the grounds together though.

If the wires are really long, you may have problems with earth loops

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Thanks for your response! I certainly am not an expert but I was thinking that since the data lines are differential, no reference would be necessary and therefor it would not be necessary to connect the grounds. Am I incorrect? – CPRitter Sep 3 '12 at 1:45
@CPRitter, You may get away with it if the usb device is "floating", but if there is a significant difference in the grounds you will run into problems. – gnibbler Sep 3 '12 at 1:54
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I would like to take this discussion one step further.

Apparently on the revison 2.0 boards, if you have a hub that puts power on a cable going into (coming from?) the USB A connectors you can power the Pi from the hub. In other words, the signal would be coming out of the A connectors and the power would be going into the A connectors. So in this case, the battery pack could be spliced into one of the USB cables. Or it could, for example, be incorporated into a keyboard assuming this application has a keyboard.

The resetable fuses protecting the USB outputs have been removed. This feature was implemented on some later revision 1.0 PCBs by replacing the fuses with links; revision 2.0 permanently implements this modification. It is now possible to reliably power the RPI from a USB hub that back feeds power, but it is important that the chosen hub cannot supply more than 2.5A under fault conditions.

http://www.raspberrypi.org/archives/1929

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