[off-topic] Powered USB Hub

[Milan Pospisil]

Hi Jim,
          Generally speaking two voltage sources of uneven voltage
connected together will create flow of current. (or electrons, whichever
way you want to look at it) And of course, the nominal voltage between
those two will be the result of the difference between those two potentials.
 Say for example that the output of your Raspberry Pi is 5.0 Volts, and the
connected external power source (the USB hub) is 5.1 Volts. Thus the
resulting voltage(the difference of potentials between them) will be 5.1 -
5.0 = 0.1 Volts, or 100 mV and the resulting current trying to flow inwards
will be given by this voltage and the internal output resistance.
 In this case 100mV does not represent a great deal of power, but should
the difference rise to several volts or more, than of course that might be
a reason for concern. The manufacturers are aware that this can happen and
most power supplies have inbuilt protective circuitry.
However, that is only in general, as I am not familiar with the R-Pi
arrangements, perhaps someone else might want to comment here specifically.

Regards,
             Milan Pospisil

On 10 February 2013 13:55, Jim Householder <nofixed at westnet.com.au> wrote:

> Hi,
>
> I bought a USB 3 powered hub from Woolworths for $17 to use with my
> Raspberry Pi.  I've used the same model with my Toshiba notebook with no
> problems.
>
> The odd bit is that when I plug the powered hub into the R-Pi, it powers
> it up.  I would have expected the power in the hub to go only to its ports
> and not back up the line to the computer.
>
> Is there a risk of problems if the hub's nominal 5 volts is marginally
> higher than the computer's supply and electrons flow the wrong way?
>
> Thanks
> Jim
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