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.