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I just bought two 7 port usb hubs: LogiLink UA0124. Just to run into a strange situation.

I did some servicing to my PC today and disconnected the main power cord from the PSU. Only to find out that all my usb devices(connected to other usb ports on the PC) are still powered up! At first I thought that I somehow accidentally managed to make a perpetuum mobile!!!

When my euphoria run out I figured out that the hubs are giving a constant 5 volts to the PC port (backfeeding), even when I turn off the switch on the usb hub! I also noticed that the processor fan has a small movement like twitching every 5 seconds...

So there must be some weird current running through my motherboard. What should I do? This does not seem like some no name cheap product. So I wonder why is it designed in this way. Is it OK when the PC is powered up? I mean does the default voltage of the PC usb port counteract with this one and everything is fine? In that case I should only watch out when turning off the PC? Or is this something serious that could damage my other components?

UPDATE

As per request in the comments, I'm adding few pictures here. And many more on this LINK (LogiLink 7 port USB HUB dismantled)

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  • \$\begingroup\$ Can you open the enclosure of your hub and take a picture of the PCB? Properly designed hubs usually have (at least) a hub controller IC and power switch IC(s). It's also possible to cut corners in the design and leave out the power switches. Then it's more likely to back-power the PC. \$\endgroup\$ – Nick Alexeev Jul 12 '13 at 2:25
  • \$\begingroup\$ Is this a powered USB hub? It shouldn't do this and it can cause weird problems on the PC side. \$\endgroup\$ – pjc50 Jul 12 '13 at 7:27
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    \$\begingroup\$ Yeah, looks like the +5V line from your PC is directly connected to the +5V line of the hub's own PSU as long as the hub is switched on... \$\endgroup\$ – us2012 Jul 12 '13 at 11:53
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    \$\begingroup\$ the german Computer Magazin c't had not long ago (issue nr. 18/2014) an very interesting article about this theme - if you understand german language it's worth reading it: "Strom von hinten/USB-Hubs können PCs und Notebooks beschädigen" ("Current from behind/USB-Hubs can damage PCs and Notebooks"). For example they say that the old version of a Belkin F5U404cw USB-Hub was working correctly, but the new one (!) wasn't - little strange?! :-) \$\endgroup\$ – user52564 Sep 4 '14 at 12:00
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    \$\begingroup\$ Perhaps you could include a bit of a translation from that article to make it a clear answer to the question for anyone that can't read the article? \$\endgroup\$ – PeterJ Sep 4 '14 at 12:23
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I eventually ended up cutting the +5V(red) line going from the hub to the PC. The only drawback is that the hub obviously needs to be connected to its own power supply to work.

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