The problem is the button was connected directly to the old power supply and your new external one does not connect to the button. The cable that was connected to it was what would be defined as the switch to turn it on. Since the cable is too short to connect to an external power supply, as of now the Saturn will only turn on and remain on infinitely, unless a different type of switch is put in place. I do not know how to fix that issue, but because of this new supply, and the button not controlling directly from the Saturn's mainboard the only thing that can be done is plugging and un-plugging it when not in use.