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I was wondering if the data source (USB Host) for a USB device has to also deliver the power 5V circuit e.g. data and power need to have a common ground.

Or can I connect the data lines to the data source and have a separate power supply for the 5V line?

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A peripheral which does not draw its operating power from the USB VBus is called a "Self Powered Device" and is generally allowed by the USB specifications. – Chris Stratton Jul 31 '14 at 19:57
Thanks! Your comment would actually suffice as an actual answer. Unless "generally" means there are important exceptions that I should know of. – Professor Sparkles Jul 31 '14 at 20:00
up vote 4 down vote accepted

As Chris pointed out in the comments, you can have self-powered devices. However, even with self-powered devices you need to have a common ground to ensure that the signal levels on the data lines are within spec.

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With "plain old" USB, it doesn't matter a whole lot where the power for the device comes from if it doesn't draw power from the host.

However, with USB OTG (on-the-go), the presence/absence of Vusb (the power voltage) is an important part of the protocol for suspending device activity and doing the host negotiation. It needs to be connected even if both devices are self-powered.

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You certainly can supply power from elsewhere - bench power, another USB port, whatever. As long as it shares a ground but does NOT back-power the USB data host.

I have a spliced USB cable on my desk for this kind of thing. Trim insulation, cut red wire, add connectors to taste.

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How would it be able to back power the data host? – Professor Sparkles Aug 1 '14 at 13:41
If you apply a higher voltage to the VBus pin than the host is driving it with, power will flow into the USB port. Generally this isn't a good idea, though if you know the internal circuitry it may be workable (for example, a raspberry pi with the port polyfuses shunted or increased in rating could actually be powered this way, rather than via its power input connector). But something like a phone with a charge pump to create VBus (when in host mode) from a lower battery voltage could conceivably be unhappy indeed. – Chris Stratton Aug 1 '14 at 14:49

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