USB hub power

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I have a USB2 4-port hub with a power supply, but one manufacturer has vaguely said that the power supply may not always be necessary and my manufacturer said nothing at all. This certainly appears to be the case if you want to use it for a few small items such a flash drives and a Wacom tablet, as I do. What is the power supply for, and when can you get away with not using it?

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here’s something about USB power.

John

PS Later… And also some old stuff from Microsoft!

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Thank you John. Your links have left me scrutinizing everything I have for rated power, and I can’t help wondering how many past posts over USB problems may have been due to overloads.

I have beside me my Benq 5250C scanner that has no wiring other than a single USB cable, and the scanner has a rating of 5V and .5A. That is right at the limit, according you your links, and my laptop, a Toshiba Satellite Pro, has two USB sockets and might be running on battery. I also have an external HP DVD burner, but that has its own power supply. I even have a lamp on a flexible (stiff) cable that is made to plug directly into a USB connector. I’ve never found a use for this (you have probably seen them), but it has to have a mechanically stable plug, such as that on a computer, to stay in place and I have no idea how much power that would draw. My Graphire tablet draws 40 mA.

I don’t know how many users are sensitive to such things, but my sensitivity has just skyrocketed. Just plug it in and the power comes through the USB connection,easy as pie. Now it seems there are strings attached.

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Unpowered USB hubs do not pass through any power at all, so can only be used to connect devices that have their own power supply. Otherwise, you could have four devices drawing power off a single USB port on the box, which would blow it (and possibly your motherboard). A powered hub is able to supply power to all its ports from its own power supply, but may not require this to connect a device that has its own power supply. If the documentation doesn’t tell you this, then you can find out through trial and error. It should either work or not without risking the device, the hub or the PC, as long as you are not attaching devices that seek to draw their power from USB.

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