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I just got a brand new Nexus 4 phone and am a bit dissappointed that it doesn't support USB OTG. One reason is that is doesn't supply 5 V out of its USB port, as opposed to other Android devices I tested.*

Since I have an extra USB OTG cable lying around, I am thinking about modding it to supply the 5V from somewhere else (e.g. from a USB power supply adapter). How would I do that?

  • I think I should connect Vcc from the power cable to Vcc from the child, and also connect all the grounds together (so that host (the Nexus 4) and child have the same 0V potential, relative to which D+ and D- are measured). (Is that sufficient to pull host and power supply to the same potentials? What potential do I end up with anyways? I find that a bit confusing.)

  • Do I connect the host Vcc with the power supply+child Vcc, or do I leave it dangling? It seems there is no power on it anyway, so I can regard it as not connected (in USB mode).

  • Is there anything special I have to do with the Sense, D+, or D- pins, or can I leave them connected as they are?

Sorry if this is a stupid question!

Here is a schematic of a regular USB (OTG) cables for reference: enter image description here From http://tech2.in.com/how-to/accessories/how-to-make-your-own-usb-otg-cable-for-an-android-smartphone/319982


*) There might be other problems related to its Slimport feature, though.

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  • \$\begingroup\$ The Nexus 4 doesn't support OTG at all. Even with self powered devices or otg injector cables or anything. \$\endgroup\$ – Passerby Feb 2 '13 at 23:41
  • \$\begingroup\$ @Passerby: I'm not giving up so early. I think it should be possible with a modified cable and a kernel fix. I took a quick look of the sources, and I think one reason it doesn't work ATM is that the slimport 'cannibalizes' the OTG mode. It looks like it uses the OTG sensing to enable the slimport, which in turn makes a USB OTG host usage impossible. (You can however charge via USB when the slimport is in use.) \$\endgroup\$ – jdm Feb 3 '13 at 13:04
  • \$\begingroup\$ In that case, check out the xda developers nexus 4 board for the otg thread. See what others have done. \$\endgroup\$ – Passerby Feb 3 '13 at 20:55
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What you are looking for is called a "USB Power Injector".

Many commercial (cheap) offerings exist. Here's an example. They are available in all USB connector types: A, B, mini-B, micro-B, etc.

enter image description here

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  • \$\begingroup\$ Thanks. I know about those cables, I'm trying to understand how they are wired, and what happens to the various potentials involved. \$\endgroup\$ – jdm Feb 4 '13 at 10:41
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    \$\begingroup\$ There's nothing to them. All of the wires pass straight through and the power input is tapped into the Vbus and Gnd lines. \$\endgroup\$ – DrFriedParts Feb 4 '13 at 12:25

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