Power the pi via the data usb?


7 posts
by nadnerb » Thu Jun 28, 2012 7:07 pm
Hi all,

I've just tried out my external usb HDD with the pi running Raspbmc.
Obviously the pi can't power the HDD alone, so I have a splitter cable with one (data) plug going to the pi and the other (power) going to my laptop. All was well until I wanted to shutdown so I pulled out the power supply from the pi and it stayed on! I was looking at the thing wondering how it was still running without power then realised the usb was still providing some power.

Should the pi function like this? Is it dangerous supplying power INTO the pi via a usb out connection?
Thanks
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by bob_binz » Thu Jun 28, 2012 7:40 pm
There's a fair bit of discussion on the subject here viewtopic.php?f=29&t=8261, but the general consensus is it's not good to power into the data connectors.
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by nadnerb » Thu Jun 28, 2012 8:19 pm
OK thanks, I'll try not to do that again then!
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by squeaky3 » Thu Jun 28, 2012 9:45 pm
Not a good idea as the voltage leaking down the data cable is quite variable, and so could damage your HDD and pi as it will dip below the required voltage suddenly. I noticed this when using a powered hub. You may wish to tape off the far RH pin (5v pin) (looking at the USB with the gap at the top) when using a USB device which is powered externally. Just use a thin piece of insulation tape over this pin will stop voltage leaking back into the Pi's USB ports, and should still allow power to the HDD, although a powered hub would probably be better.
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by itimpi » Fri Jun 29, 2012 3:55 am
If you are powering the Pi via the hub then the easiest thing to do is remove the power from the hub rather than the Pi.
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by alexeames » Fri Jun 29, 2012 7:54 am
squeaky3 wrote:You may wish to tape off the far RH pin (5v pin) (looking at the USB with the gap at the top) when using a USB device which is powered externally.


I find it hard to believe that should be necessary. Surely both the external device and the Pi have been designed to be usable in such circumstances. I use mine with a Seagate external powered USB drive and it seems to be fine.

Surely a powered hub is a different beast to an externally powered HDD designed to work over USB?
Or are we saying there is a design fault in the Pi here?

I don't understand the issue. :|
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by Burngate » Fri Jun 29, 2012 3:09 pm
alexeames wrote: ... Surely a powered hub is a different beast to an externally powered HDD designed to work over USB?
Or are we saying there is a design fault in the Pi here? ... :|

Yes they're different, but he's taking the power for the HDD from his laptop via a Y cable that's connected to the Pi on the other leg.
So it ain't the HDD that's supplying the power when it shouldn't, it's the Y cable that's supplying power from the laptop back up the Pi's input - cos it can't help it, being just a bit of wire!
Solution 1: mangle the Y cable so it doesn't send power down the data leg
Solution 2: remove F1 or F2 (whichever is relevent) on the Pi

Of note: If the proper power route for the Pi is another PSU - not the same laptop - the two PSUs will be fighting. Probably, F1/2 will shortly die, removing the problem
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