gian_nichele:
So, the ground from the board, the source pin on the mosfet and the fan are going to the ATX ground. All the others are going to a GND pin on the board.
The heater supply comes from a 12V source on the ATX.I thought it was kind of mandatory to power the Arduino with more than 5V? Should I put the 5V wire from the ATX directly on the VIN pin then (if there is no drop)?
I'll try that. You mean I should use a completely unrelated ground, right?
What I meant is if the heater is using it's own yellow/black pair of power wires.
Read the Uno's product pages. Here's the short/dirty version.
USB power is 5volt, powering on the 5volt pin is possible, but not recommended (if you don't know what you're doing), V-in needs a minimum of 6.6volt, and the DC socket needs a minimum of 7,5volt.
There is no isolated ground in an ATX supply. All black wires run to the same star ground inside the supply.
Leo..