Off-Topic: question about usb-hub power

Mehmet Erol Sanliturk m.e.sanliturk at
Thu Jul 21 13:19:47 UTC 2011

On Thu, Jul 21, 2011 at 7:12 AM, Uffe Jakobsen <uffe at> wrote:

> Hi,
> Apologies if this is a kind of off-topic for this list - since it does not
> have anything to do with FreeBSD.
> The external USB-hubs (v2.0) that I've come across comes in two categories:
> Some are willing to deliver power (500mA pr. connector) even if the hub
> isn't connected to a computer - and some that doesn't do this - the hubs
> that I have purchased from Dlink falls into the last category.
> I looking for an USB hub that is willing to deliver power without it beeing
> connected to a computer in order to drive some external units - no it is not
> a mobile-phone ;-)
> Could anyone give me a hint what specs to look for ? - before I actually
> buy another hub - the Dlink ones were not a success...
> Thanks in advance.
> Kind regards Uffe Jakobsen

For me . it is difficult to understand your question . Any links to actual
products in such questions may clarify the subject matter very much .

In market , there are mainly two kinds of USB hubs :

 - without power supply , mostly up to four ports or less :

Example :

 - with power supply , mostly with more than four ports :

Example :

If you need power in ports even it is NOT connected to a computer or there
is no power from the computer ( it is switched off )  , you need at least a
power supply powered USB hub .

Also , the following page contains explanations about power in USB hubs :

To understand whether a hub is delivering power to ports when it is not
connected to the computer or the computer is switched of , you may measure
voltages in its port pins ( 1 and 4 ) by powering the hub by its power
supply only :

If there is NO voltage , the power supply is bogus ( or eye painting ) . If
there is sufficient voltage it means that the hub is powering its ports
itself without getting power from the computer port .

Power level of power supply should be sufficiently powerful to supply 500 mA
to each port , means

( Total power rating ) = ( Number of Ports ) x 500 mA .

( This may require to buy another power supply for the hub if its power
supply is not sufficiently powerful ) .

OR ,

It is necessary to consciously NOT to exceed the total power of the power
supply by attaching less number of devices to ports :

( Power rating of power supply of hub ) / 500 mA = ( Number of devices
attachable to Ports )

( which is NOT a good selection because if this point is forgotten it is
possible to easily exceed the capacity of the power supply means destroy of
your hub or malfunctioning of data transmission , if I am correct ... ) .

I do not know whether the above answer is correct for you or not .

Thank you very much .

Mehmet Erol Sanliturk

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