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I have 7 port USB hub with external supply.

I tried turning it off by removing external power but then it takes power from motherboard and is still working.

I have many external hardrives connected to it.

i was worrying if i turn off the power it may damage the motherboard by withdrawing so much power from that USB

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You wouldn't by chance have gotten a Startech? amazon.com/gp/product/B009AT5T1W/… –  BGM Feb 11 '13 at 23:30
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3 Answers

up vote 2 down vote accepted

USB ports can supply up to 500mA per port for powered ports, and 100mA per port for unpowered. Also, the hubs usually say they draw 100mA of power, that is why you usually see four port unpowered hubs (100mA for the hub, and 100mA x 4 ports = 500mA which the powered port can supply).

Not all devices require 100mA, for example, I had a mouse that take 20mA. I assume that as long as the power the 7 port hub is less than 500mA, you will be fine. If it is more, Windows is smart enough not to power on the devices to protect the USB port.

External hard drives are usually powered by themselves, so I assume their power requirements are low. If you are in Windows you can see the power requirements in the Device Manager, under Universal Serial Bus controllers. There will be a list of all the hubs. Looking at the properties of the hub there is a Power tab. It will list all the devices attached to the hub and their power requirements. Many devices will list 100mA because that is the maximum an unpowered hub will supply.

I hope this helps explain why the devices may still work, and put your mind at ease that the computer is probably smart enough to protect itself.

(The only thing about using external drives is some drive can be self-powered or bus powered. Some of those drives will spin slower if they are bus powered, and will spin faster when self powered because they can pull more current. I've only seen this once so I don't know how common this is, the external drive had a laptop drive inside. This may be something to consider if you have drives of that type.)

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re: self-powered external drives -- this is true for enclosures designed for 3.5" desktop drives, but the 2.5" laptop drive enclosures are often designed for bus power. don't know what the usual power req's are but i assume 100mA isn't enough. –  quack quixote Jan 20 '10 at 8:19
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In device manager switch to View | Devices By Connection to see the (logical) connections between USB devices to make this easier. –  Richard Jan 20 '10 at 10:39
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In my experience multiple hard disk drives were not working. In normal conditions a USB port will deliver 500MA power with 5V. When we connect multiple devices that power may not be sufficient to drive all the devices connected. That is why there is another product came in to market that have a external power supply.

I wont think the extra power taking will damage mother board of a computer until unless there is no surge current passing from the external source to the USB port and there after to the mother board. As per my understanding there are protectors for these on mother boards.

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You are right; the motherboard detects when the power limit has been reached and simply doesn't power any further devices. See here: superuser.com/questions/96563/… –  BGM Feb 11 '13 at 23:33
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I'm not quite sure what you're asking, as the question can be parsed many ways, but here is my stab at it...

  1. If your trying to unplug the hub from the PC and therefore all the accessories plugged into the hub, you should eject them first. Not doing so can potentially corrupt those hard drives.

  2. If you just want to unplug the power from the hub but still leave the hub connected to the PC, the hub will draw it's power (which it feeds to the accessories plugged into itself) from the PC. As this is usually not enough many accessories will cease to work. It is quite possible that in effect this is functionally similar to #1, where one physically unplugs the accessories before ejecting them. This, as mentioned, can corrupt them.

  3. If your concern is that the motherboard will be damaged due to the need to supply all that power, you can rest assured on that score. The motherboard will supply what the spec is (or however the manufacturer implemented it), asking for more won't provide anymore, and correspondingly there is no risk of burning out that port.

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