Cannot connect USB and PSU together

Hello,

I have an Arduino mega board with a reprap 1.4. The reprap is powered by a Lascar Electronics PSU 130, 12V. The Arduino is connected with USB. When I upload some software to the Arduino, disconnect USB and power on the PSU, all works as expected (the steppers move, and the PSU reports 12V). When I leave the USB connected, the steppers just make some noise, and the PSU reports 6V. Our setup requires that the USB stays connected for data transmission.

Currently computer and PSU are connected to the same power socket. Is there anything I do wrong here, or something I can test or check?

Edit: I also got the advice to cut the 5V cable of a USB cable. Can that work?

Thanks in advance

Edited 1 time(s). Last edit at 08/08/2018 10:52AM by patrik_g.

... maybe there is a voltage difference between the GND of your computer and the PSU, what's causing high currents flowing over the shielding of the USB cable ... this can wreck the USB-port of your computer!

You can search another PSU without this differences or connect a thick wire between computer and PSU GND's to short this out ...

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Viktor
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Atmega has an onboard power selector circuit, which either uses USB power or the onboard 5V regulator. If it doesn't work, all sort of things can happen.
You can try to inspect the Atmega for cold solder or missing parts around the power circuit or replace it with another board.
Cutting off 5V in the USB cable also works, but keep the GND connection intact. Make sure, no one can replace the cable later.

Thank you for the answers. In our setup, cutting the usb was the least intrusive, and seems to work well