Each USB (2.0) port (per spec, c.f. http://en.wikipedia.org/wiki/USB#Power) provides 500mA current at 5V (2.5W DC power).
USB3 and USB3.1 specify even higher current and/or voltage (up to 5A @ 20V for USB3.1) for more power delivery to compatible devices.
A hub is supposed to provide that amount of power for each port, but the port it's connected to usually only provides enough power for one port.
The AC adapter for the hub provides the added power so all the ports on the hub can provide full power for USB peripherals.
Some hubs provide more power for devices (like smartphones and tablets) that can draw more than the standard guarantees, the AC adapter provides that power too.


On 05/06/2014 05:25 PM, Michael Havens wrote:
> my usb hub has aport for an a/c converter. why?
> :-)~MIKE~(-:
> 
> 
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