I have some troubles with connecting an Arduino Leonardo board to USB and power supply at the same time.
The attached image shows how I wired the PC, the Leonardo and the MOSFET board.
When I run it from power supply it seams to work but as soon as I plug in the USB connection, the Arduino Leonardo board blows up. Afterwards, the USB connection is physically broken but the Arduino CPU still seams to work.
Thanks for help.
Hi,
You are running the high current needed for the UO4 through the Leonardo, the PCB tracks have possibly vapourised.
You should have the Power+ of UO4 going directly to the 12V power supply, not drawing current through the Leonardo.
What is the load that is being driven by UO4 and how much current does it use?
Tom... 
Hi Tom,
Many thanks for you explanation.
Is it therefore correct when I connect the 12V power supply to the power jack of the Leonardo AND to the Power+ of the UO4?
There is no wire connected on the UO4 except the ones I show in the image.
Is it still possible that this issue is caused by that?
Why did it happen when I plugged in USB?
Cheers
Your diagram lacks a couple of lines - remember that each connection requires a common (Gnd) line, so that current can flow from and to every connected device. In many cases there exist unobvious ground connections amongst the devices, which can cause shorts when everything is connected at the same time.
Provided that everything works fine until the PC is connected, check for a possible voltage between the detached PC USB connector and the Arduino Gnd. Then insert a current limiting resistor between both Gnds, and measure the current flowing through it. That current should be very low, and should not increase when you decrease the resistor value.
Thanks for the hint, I will measure that when I get a new board.
