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I want to power my cheap, powered USB 2.0 7-port HUB via standard PC PSU (build molex to barrell adaptor). I want to make sure it's not backfeeding into computer. I want to be able to power the hub form computer's USB if it's not connected to the molex, so cutting the power wire in the USB cable is not an option.

I want to use a diode for that purpose. The current which molex outputs is 11A, voltage 5V or 12V (I want to use 5V rather than 12V, the HUB is 5V device). Which diode shall I choose? Is the 11A Schottky diode any good for my purpose or maybe I should rather use a different diode? I should get no more that 500mA for each port, thus use max 3.5A power supply.

In that case is connecting it to the molex which will give me a lot more current on each port a good idea at all? If I connect the HUB to the molex and want to charge my phone's battery up will it get burned by getting 11A form the power source?

How do I prevent backfeed on a circuit (like a hub) into another circuit (like a computer)?

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  • \$\begingroup\$ If two 5V supply Voltages are mismatched and joined current flows, otherwise not. Ohms Law says ΔV/R=I for given circuit resistance. I would consider using PTC polyfuses for desired rated current on each port instead (e.g. 500mA). Make sure ATX supply has proper preload to keep regulated, it ought to be OK. You could add Overvoltage protection on the ATX supply or use Active 20A MOSFET to work as a directional power switch too, to supply to all ports with one \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Apr 7 '17 at 22:02
  • \$\begingroup\$ @Tony Stewart. EE since '75. Thanks for the comment. Unfortunately it exceeds my current knowledge by far. So, if we go back to the question: is schottky diode all right for my purpose, shall I choose an 11A diode for 11A current flowing from molex connector and, finally, will anything be damaged when connected to that hub but doesn't require only 1A of current? Thanks :) \$\endgroup\$ – Daniel Apr 8 '17 at 6:44
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Any shcottky diode will do fine.

You only have to consider the current rating. To be on the safe edge, it should be 5A or more.

But if you know that the devices connected to the Hub wouldn't draw 500 mA each (most devices don't), you can use a diode with a smaller current rating.

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  • \$\begingroup\$ you'll probably find the upstream port from which you're taking power is limited to 1.5A max \$\endgroup\$ – james Jan 15 '19 at 15:07

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