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Can I supply a higher current than the 100mA the USB port will give simply by connecting an external regulated 5v to the power lines, with a diode in the positive side to isolate it all from the USB internal wiring?

Failing this very simple approach, what circuit could be used instead?

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Could you post a schematic with what you did/want to do? – suha May 29 '12 at 19:34
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Supply a higher current to where? – Olin Lathrop May 29 '12 at 19:48
Is this device a commercial device? Something you made? More info needed, IMHO. – dext0rb May 29 '12 at 20:23
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Isn't what you describe all included (and probably implemented much better than you could ever do) in an average powered USB hub? Select one that is lazy and does not limit the output current actively (beyond the limits of its own power supply). – Wouter van Ooijen May 29 '12 at 21:05
Thanks for these comments, I think I can respond to them all in one go. Background: I was trying out a Raspberry Pi with a cheap keyboard plugged into one of its USB ports. Linux would start up ok with no keyboard but failed with it plugged in. I put this down to the keyboard needing too much current from the USB port, so I need to supply the keyboard separately. Circuit: a blocking diode in the +ve power lead and a 5V psu connected at a point between the diode and the keyboard. I could buy a powered USB hub, but the circuit seemed so simple .... I just needed reassurance. – Harry Weston Jun 1 '12 at 14:26

1 Answer

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Yes, you can power a "USB powered" device fROM an external 5 Volt supply.
You could use a diode to allow the USB port to supply power when the external power supply was removed, but it would be usual to just power the load from the power supply and not connect to the USB SOURCE AT ALL.

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+1 for using a 5v supply external to USB, so obvious. – kenny May 29 '12 at 21:22
Many thanks for that reassurance, time to get the soldering iron out again. – Harry Weston May 30 '12 at 9:26

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