usb hub power adapter question

[ruffblaza]

i bought a 4 port usb hub today and in the back it has a usb male to usb male connection, and it also has a slot for dc 5.0v but i don't get what is the difference cause the usb hub works with out the adapter and straight with usb male cable so what is the power adapter for? does it give more power or something? better transfer rates?

 

[Phillip Corcoran]

When you plug in several USB devices at the same (especially a portable hard drive) there often isn't enough "juice" available via USB to power them all because power at each port is limited and fixed, as is the total amount of power for all the ports added together.

You'll know when you've reached the limit because a device will keep triggering the connected and disconnected sound without you even touching it..

A powered hub solves that problem, but of course it will work without mains power too provided the USB ports can satisfy the power demands on their own.

Even where sufficient juice through USB isn't an issue, a hub gives you the convenience of more USB ports, but it still pays to get a mains powered hub rather than one without a mains adapter 'cos you never know when you might come up against that power issue.