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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, 2014 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, 2014 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, 2014 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, 2014 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, 2017 at 22:32
  • \$\begingroup\$ If the supplied voltage is 5.1V, applied one 5.2V and resistance 0,1 Ohm, expect 1mA to flow and raise suplied voltage to 5.1999V. What you written is NOT true, beacouse wast majority of power conerters, that supply USB can NOT absorb power. This converters only: 1) monitor output voltage and see it above desired level. 2) stop providing power, trying to compensate. \$\endgroup\$
    – Volodymyr Kalinyak
    Apr 4, 2020 at 10:12
  • \$\begingroup\$ @VolodymyrKalinyak 0.1V / 0.1Ω is 1 AMP, not 1mA. What you say about the converter design is generally true, but it's not guaranteed. As I said in the post, "this will depend on how the converter is designed". In any case, USB is not designed to allow paralleling VBUS, and you should never do it unless you're ok with weird behavior or even smoked gear. \$\endgroup\$
    – Daniel
    Apr 6, 2020 at 21:45
  • \$\begingroup\$ @Daniel (5.2 V-5.1999 V)/0.1 Ohm=1mA, not 1A. In any case most of the converters do NOT accept power from output. 1mA is a good value for feedback circuit of converter (from where it does know voltage is 5.1999 V inseted of 5.1 V and therefore stop providing power. \$\endgroup\$
    – Volodymyr Kalinyak
    Apr 7, 2020 at 9:59
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    \$\begingroup\$ SEO comment for frustrated others and/or future self. I had a massdrop alt keyboard (very picky about power and cables already) failing to turn on when booting my PC while an amazonbasics usb hub was plugged in. It didn't matter where the hub was connected, the backfeed impacted the entire system. PCI-e usb cards as well as all different controllers on the motherboard were not working correctly until the hub was disconnected. \$\endgroup\$
    – TrevJonez
    Aug 17, 2020 at 17:48

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