How to disable USB power?

Hi, I need my Arduino Due to work only from external power supply, not from USB power. Is there any way to modify the circuit to allow this? I still need to keep USB communication, but it should happen only when external power is on. Thanks

you do not need to do anything.

When powered from the power port, you will be getting a full 5V, through the 5V regulator. when powered from the USB port, the power first gose through a MOS FET diode and bypasses the 5V regulator. This will reduce the voltage slightly. This means, the higher available voltage will power the board. The lower voltage from the USB port, will not be used. Even if you power the board with an external 5V power supply, the higher voltage will power the board. it is automatic.

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Thanks for your response. The thing is I don't want the board to work when only USB is connected.

I got this behavior by removing the power MOSFET.

See here for details:

http://arduino.stackexchange.com/questions/13571/how-to-disable-due-usb-power

in that case, I would modify a USB cable. It is better to damage a cable, then damage the board.

I have tried the cable modification. It doesn't work and this is why I think. You only need 3 wires from the board to the ch340/ft232/whatever gnd, tx,and rx. To get from the ch340/ft232/whatever to the pc you still need all 4 USB wires because I don't think the external supply powers the ch340/ft232/whatever.

Removing the FET does work it seems with my uno clone anyway

Which wire did you cut. The 5V line goes directly to the fuse and then to the Mos FET. It should do the exact same thing as removing the FET. However, if you cut the Negative wire you will no longer have a complete circuit for the data lines. And it will not work.
also which port are you trying to use. The A.B. port as a secondary circuit at the power line goes to. although it should work the same, You should avoid using the the AB port.