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Most USB hubs and active/amplified extension cables allow you to connect an external power supply to the downstream USB port so you can run devices that need more power than the upstream USB port can supply, and/or avoid any voltage drop from the lengthy cable.

Many of the cheaper devices however, simply connect the external power connector in parallel with the power lines in the USB cable. This means the external power supply can backfeed power up the USB cable and into the source device (such as a computer.)

Is this situation bad? I am guessing that if both the computer and external power supply are switching PSUs then they will simply adjust the amount of power each puts on the line and it won't cause any problems, but is this thinking correct? What if the external power supply is not of the switching variety?

Are there any situations where this could cause real problems?

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  • \$\begingroup\$ Think embedded systems, that you want to power off. \$\endgroup\$ – Ignacio Vazquez-Abrams Jun 15 '14 at 4:01
  • \$\begingroup\$ I've seen a PC that went into happy mumbling mode when powered down and needed USB chain removed toallow reboot. \$\endgroup\$ – Russell McMahon Jun 15 '14 at 9:23
  • \$\begingroup\$ I've noticed two issues with it: 1) I had a desktop Pentium 4 computer fail to boot (like Russell said), and 2) audio interference from the computer/data when USB-powered speakers were plugged in to the hub. Good question you had there... I think maybe I need to find a mini-USB to USB that has one of the power wires not hooked up. I could probably neatly slice into a micro-USB and cut out a bit of high side power wire (probably best to keep the ground wire intact, though the shielding might be enough for tying the grounds together). \$\endgroup\$ – user75574 Oct 19 '14 at 13:38
  • \$\begingroup\$ Here's a related thread where the O.P. is complaining that an improperly designed USB hub is back-feeding his PC. \$\endgroup\$ – Nick Alexeev Oct 19 '14 at 15:34
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That is super terrible design... but will it break anything? Good question..

If you try to backfeed a voltage into a supplied voltage, the difference in the two will determine the amount of current that flows. So if the supplied current is 5.1V and the applied voltage is 5.2V, 0.1V will cause current to flow into the 5.1V supply, unless it's specifically designed to prevent current from flowing backward. (this would depend on how the switching converter is designed)

So say you have a long cable, and 0.1V of mismatch... that might be around 5 ohms... That is 0.020A, or 20mA. That is enough current to run an LED, but usually not enough to do damage to a power circuit. A much worse condition (as Russell points out) is when the computer is OFF and there is power being backfed.

Most USB power systems are going to have a resettable fuse somewhere around 500mA to 1.5A, so that would protect things in an extreme case, but the real answer is that it depends on the computer, and it depends on the design.

My best advice is to stay away from USB hubs that are of mindblowingly poor design.

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  • \$\begingroup\$ I think I just managed to damage my computer with such usb hub, so yes those should be avoided. Thankfully (and hopefully) only one ram slot has died. Such powered hub might work ok for the most part, but might cause a critical failure during boot or under heavy load. Computer failing to boot while the hub is connected is a common symptom. \$\endgroup\$ – jahu Apr 6 '17 at 22:32

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