Electronic – Are USB hubs that backfeed power upstream bad

power supplyusb

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?

Best Answer

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.