But is there not already an implicit ground connection, because the ardunio should be grounded through its USB-cable and the PC-powercable, while the PC uses the same ground as the power supply? Furthermore, is the minus-output of the power supply connected the the ground or floating? The supply has just plus and minus output.
Yes, connect the ground pin on all 3.
I have attached a little picture to ilustrate the situation. The possible ground wire is in also included.
Pretty sure ground on a USB port is not tied to earth ground on a 3-pin AC power cord.
If you are measuring a difference in voltage potential, there is one.
And I am pretty sure it is on virtually all desktops as they all have a 3-pin power cord as well as virtually all laptops and USFF machines whose power cord has a 3-pin plug. Those with a 2-pin plug are obviously the exception.Possibly, which is why the grounds must be tied together to remove that difference. If something burns, then one of your power supplies is dangerously faulty and you found out the hard way.
Well, if you think that the black wires on your ATX power supplies are not connected to the chassis and ground pin of the IEC connector, I suggest you go test a few and be incredulous!
You are right. A simple continuity test between the USB ground pin and the ground pin would make this discussion moot.