Lamyrus wrote:That sounds like I have to make sure that the PSU sense line needs more power take off then one raspberry pi. Where do I find this PSU sense lead? It should be on the 24pin header going to the motherboard? (I converted blindly an old PSU from a dell and hooked it up to a lacie nas server as that power converter broke down and it works like a charm. the lacie nas has two 3.5" 500gb drives and an arm board to be powered...)
[Public safety message: long ramble ahead]
In the dark ages of computing, ATX power supplies would provide 3.3/5/12V to the motherboard, along with some vestigial -12V and -5v (obsoleted in the current spec, was for hardware UARTs). The problem that many of the designers faced was that each motherboard would have different loading profiles for 3.3/5/12V which necessitated that they put a bit of robustness into the design.
Given that there were a specified number of connections via the ATX plug to the board, the voltage line that was typically most susceptible to skew was 3.3V. If a heavy current was drawn from 3.3V by a motherboard (typical of early 2000s models), the line could droop due to the I²R losses in the wire between the power supply and the motherboard. To compensate for this, the feedback circuit would have a separate "sense" lead running from the power supply to the ATX connector. Some would even have sense leads for both 3.3V and 0V.
Internally, the sense lead provides a more dominant feedback signal compared to local voltage sensing - the external voltage is given priority. You could "probably" remove the sense lead and the PSU will work fine, but I'd test this with a dummy load before plugging it into electronics.
Anecdotally, I had an Asus A7N8X with a very very heavily overclocked AMD Athlon that managed to melt the +5V pins in the ATX plug. The CPU core voltage was derived from 5V. Hilarity ensued when smoke literally started pouring out of my first PC while gaming
Rockets are loud.