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I have a 7-port USB 2.0 hub (this) which comes with external power supply in its packaging. However, when I connected a lot of USB sound cards (Creative SB X-Fi Go Pro! or ASUS Xonar U3) to the hub I noticed it works (every sound card plays or records sound) even without the power supply.

What is the point of the power supply then? Could its absence affect longtime performance (e.g. for server use) of the communication along the USB bus? Is there any difference when using other kind of equipment (such as HDD)?

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migrated from unix.stackexchange.com Jul 16 '13 at 20:32

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3 Answers 3

up vote 6 down vote accepted

The power supply makes certain that each downstream port has at least 500mA (or so) available, instead of all ports sharing the 500mA available from the host.

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Practically speaking, what Ignacio said means that you can't plug several devices that require a "powered" USB contact into a hub that lacks external power.

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When you add an external power source to USB hub, it can provide 500mA for each port, there's no limit on which devices you can add. If you attach an external USB hub without power, it only has that original 500mA current to distribute to 4 or more ports.

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It likely would have less the 500 ma based on the fact the port would use some of that. –  Ramhound Jul 16 '13 at 23:14

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