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The Raspberry Pi's USB ports can only supply about 100ma of 5V current; this comes directly off the 5V input power to the board. The usual solution is to use a powered hub to support devices that need more of that, but I have an application where I have a single USB device, and want to keep the setup as simple as possible, as it will be in an enclosure with other equipment.

Comments in this question seem to suggest that a setup like this would work:

USB Y cable with VCC disconnected on one side

since both devices were being powered from a common source; I'm planning to try this with a USB charger with two jacks and and a cable like this, hoping I can carefully open the cable and identify VCC and snip it.

Is there anything else I should be aware of?

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  • \$\begingroup\$ A, are you sure your usb device needs more than 100ma? And B, do you have a Rev 1 or 2 or later version of the RPI? Because the 100mA limit is only for the Rev 1, which has a poly fuse. See: raspberrypi.stackexchange.com/questions/2024/… \$\endgroup\$ – Passerby Jan 27 '14 at 2:45
  • \$\begingroup\$ @Passerby - the answer you linked states that there is still a 1.1A polyfuse on the input to the pi. I'm looking at a high gain WiFi adapter; not sure which one yet, but I haven't found one that's reported to work directly off the Pi's USB port (using this page as a reference) \$\endgroup\$ – TomG Jan 27 '14 at 2:52
  • \$\begingroup\$ Most seem to say that they work with a late RPI revision. And remember, most of these might have been added before they dropped the 100mA polyfuses. With a 2 Amp supply, you should be fine, and if it doesn't work at that point, then you can try the cable hacking. \$\endgroup\$ – Passerby Jan 27 '14 at 3:29
  • \$\begingroup\$ @Passerby - true, there's no good reason not to give it a try. I'm actually hoping to get away with a lower powered device that I know works, I'll be doing a field test later this week to see. \$\endgroup\$ – TomG Jan 27 '14 at 3:31
  • \$\begingroup\$ Possible duplicate of Raspberry Pi and USB modem - can I connect it like this? \$\endgroup\$ – Dmitry Grigoryev Jun 27 at 23:57
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The connection scheme you are using will work fine even if you use two separate supplies, one for the board and one for the device (5v each).
As long as the Vcc wire is not connected between the device and board you can power the board from the USB port of the PC and the device from a wall wart.

When you are using the same PSU to power both the device and board (like in your image) there in no need to break the Vcc connection between them, they are connected in the PSU junction anyway so they may as well connect through the usb wire too.

In other words you can use the Y cable you show to power the device and board without a need for any modification BUT while they are powered like that you can't connect the board to the USB port of the PC, if you want to do that then you should leave the Vcc connection open. The Vcc wires shouldn't be connected between devices that don't share the same power supply (unless there is a protection mechanism like a diode).

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  • \$\begingroup\$ Thanks. The only downside I see is that I would lose the protection of the polyfuse. Having had one board saved by the polyfuse when I made a bad GPIO connection, I think I'll try to find a clean way to disconnect VCC on the link. \$\endgroup\$ – TomG Jan 27 '14 at 16:33
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I have a new model B, and have successfully powered a number of USB devices directly from the Pi itself. While I've never broken out a meter to check, I have high confidence that if you have a quality USB power supply, you will be able to use whatever is not consumed by the Pi itself for peripherals on the main USB 5v rail.

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