I have a custom computer setup with a standard 5V SMPS module powering many attached devices: an Ethernet hub, USB hubs, Raspberry Pis. (To eliminate the need for 6+ wall-warts) I have a basic eBay volt+current-meter module on the output of the SMPS, to see how much load I'm pulling, and ensure the voltage is good.

I've noticed a weird behaviour when the SMPS is off: the volt-meter reads 3.8v. I've traced this to the USB hub: it has a 5v barrel connector input, but if it's connected to a PC which is on, it's sending power out of the barrel-jack 'input'. (And presumably slightly loading the USB bus)

This doesn't really matter, but I wondered what kind of circuit might prevent this behaviour, i.e making the power input truly input only. I'd prefer not to modify the USB-hub; I'm hoping for something I could install between the SMPS and the barrel-jack plug.


1 Answer 1


A Schottky diode would be the simplest solution but would cause a 0.3 V drop which you don't want.

enter image description here

Figure 1. Image source: SMPS 5V without load, fluctuation on load TOP258PN, uncredited.

Typically the SMPS is a transformer output with a diode and smoothing capacitor. The diode will prevent any high reverse current from your USB hub. There may be a couple of reasonably high value resistors on the voltage monitoring feedback circuit but these will leak little current and won't be damaged by the backfeed as, to them, it just looks like the PSU is running.

Recommendation: Don't worry about it. It shouldn't be a problem.

  • \$\begingroup\$ I'm surprised a diode works: likely due to my poor theoretical understanding. Since the polarity of the voltage out of the USB hub is still correct (not reversed), won't the diode do nothing? (Aside from the 0.3v drop across it, of course) \$\endgroup\$ Feb 23, 2021 at 11:05
  • \$\begingroup\$ A diode will block current. I wrote an explanation here. \$\endgroup\$
    – Transistor
    Feb 23, 2021 at 11:25

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.