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I am new here and I am newbie in electronic. I studied electronic a lot of years ago.

I am attempting to connect my usb hub to 5V output of my ATX of desktop pc, as explained here:

Can an ATX PSU power a USB port directly?

My hub have 7 usb with a power supply of 5V and 2A with as USB connected to mainboard as shown here:

enter image description here

So I connected the ATX of the same desktop to the hub usb as shown here;

enter image description here

everything work perfectly, my phone has charged faster, but when I turn off my pc, I noticed that the led of hard drive what was slightly light up. I suspected for one current flow back thorough the usb cable connected to computer. While when the computer is turn on the led of hard disk is completely off if the hard drive is not used.

Please, could you tell me, if can occurs problems to my mainboard using this connection? In case a diode could solve the problem of the current flow back thorough the usb cable?

Thanks in advance for your reply.

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    \$\begingroup\$ That's not supposed to happen. It's not that hard to design a circuit that only switches to the external power input if that is present, but your hub seems to be lacking that and instead seems to be connecting the USB +5V with its external power plug. That is a very bad design for a lot of reasons. You can probably solve the issue by just removing the VUSB line from your PC-Hub USB cable, but really, some Hub designer should really feel ashamed. (also possible: the current flow is external USB Device->Hub->shared 5V->barrel plug->hard drive power rail) \$\endgroup\$
    – Marcus Müller
    Commented Dec 3, 2017 at 12:23
  • \$\begingroup\$ @MarcusMüller: You can't blame the hub designer for the "problem". This is so far outside the normal use cases for a hub that no one could be expected to anticipate it. The external power port of the hub is supposed to be connected to a continuous supply or not connected at all. Plus, it appears that the OP has bypassed the external power connector, possibly negating any protections that the hub designer did include. \$\endgroup\$
    – Dave Tweed
    Commented Dec 3, 2017 at 13:27
  • \$\begingroup\$ @DaveTweed at first I was thinking exactly the same, but then I realized that one very likely use case of these hubs is on a desk of someone who comes into his office, plugs the hub into their laptop, and only after that eventually turns on the switched power strip which powers their printer, screens, and USB hub. I honestly would expect the hub in that case to not inject current back into its supply, but then again, that's probably a cheap flyback architecture, so even that should be fine. Hm. you're right. \$\endgroup\$
    – Marcus Müller
    Commented Dec 3, 2017 at 14:12
  • \$\begingroup\$ @MarcusMüller yes, the Hub is able to work also without external power plug, though I buyed it with a power supply of 5V and 2A. \$\endgroup\$
    – Joe
    Commented Dec 3, 2017 at 14:36
  • \$\begingroup\$ @MarcusMüller What you mean for VUSB? Maybe the voltage that come from mainboard through the USB cable? \$\endgroup\$
    – Joe
    Commented Dec 3, 2017 at 14:38

1 Answer 1

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It would seem that you've created a short between an unswitched 5V output of the power supply (the Vbus of the USB port) and a switched output (the 5V wire in the disk drive cable).

Yes, this is a Bad Thing. When the PC is switched off, the unswitched output of the power supply is now overloaded trying to drive all of the loads on the switched output.

The question you linked to is about driving a single USB charging port from a standalone ATX supply — one that isn't also being used to power a PC. What you are attempting to do (powering an entire hub from a shared supply) is definitely not recommended unless you know exactly what you're doing.

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  • \$\begingroup\$ Yes, you are right. When the pc is switched off, the Vbus of USB is still on and the power go from Vbus USB to the 5V wire of the disk drive. I solved putting a diode between the 5V wire of ATX and USB Hub. Thanks for your reply. \$\endgroup\$
    – Joe
    Commented Dec 4, 2017 at 13:52

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