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I have one usb device that will not work because of the limited usb current. I don't want to add a powered usb hub (saving cost and space) and I don't want to solder on my pi (polyfuse mode). Can I just "bridge" the 5V to the USB device? Original diagram by Paul Beech

Update:

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Can your power supply provide the extra current? –  Jivings Oct 31 '12 at 20:15
    
Hopefully. I want to use batteries, but first I will use a normal power supply (4A) and measure the current drawn. –  tauran Oct 31 '12 at 22:07
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Can the current flow back in from the USB interface and towards, say, shorted GPIO pins? You could end up with unexpected current paths this way unless you cut the power from the USB connector and supply USB power only straight from the power connector. Bypassing on the board would have the same issue, of course. –  XTL Nov 1 '12 at 8:56
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up vote 6 down vote accepted

I guess that should not be a problem, for the USB device certainly not a problem. Depending on the USB implementation on the RPi it might not detect your device because it doesn't draw any current. If that is the case you can use a resistor to draw a little current on the USB output. (Put this resistor between the unused 5V and GND outputs on the USB connector)

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Interesting. Is that how USB devices are detected? –  Jivings Oct 31 '12 at 20:14
    
No, not according to the standard. A variety of pull-up or pull-down resistors on the D+ and D- line determine if is it a USB host or a USB slave device, also the speed (USB 1.1 or USB 2) are detected in that way. From my head I don't remember the exact combination, if you like I can look it up for you. But on the TPS2044B (power-distribution switche) chip over-current is detected (on the 5V line), so I would not be surprised if other chip manufacturers do something with the 5V line to detect other things. –  ikku Oct 31 '12 at 21:11
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It will be a good idea NOT to use the GND of the Pi- But pull GND(black) direct from your power source too. Loopback Interference being a big problem but also some situations of back power will have to go through Pi and could cause damage to it. It might work for you but using a floating Live is NEVER a good idea. –  ppumkin Nov 2 '12 at 10:22
    
Thanks. Updated the diagram. –  tauran Nov 2 '12 at 17:43
    
In the updated diagram, shouldn't you connect the ground to the USB ground also? Otherwise D+/D- might be floating –  Alex L Nov 4 '12 at 6:14
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