Isolate Arduino from the 12V line?

I recently ordered a Chinese delta printer that uses a RAMPS 1.4 and knock off Arduino Mega. I plan to add a heated bed and the included 10A PSU will not be enough to supply the full demand (6A bed + 7A for steppers/hotend) . I plan to replace it with an ATX PSU I have here that can handle 14A on it's 12v line. I am gonna be pushing that output a bit more then i would like so I would like to offload whatever I can to the 5V rail. At the same time I would like to be able to power the Arduino and a Raspberry Pi + 7in touchscreen running OctoPi off the 5Vsb and only run the main PSU when the machine is actually printing.

I should be able to do this as 5Vsb can provide 3A which should suffice for the electronics package. I would have the Pi and arduino running when the PSU is switched on, and have one or the other pull the green wire low to turn on the main 5V/12V lines when it is ready to print.

Is there any way to isolate the arduino from the RAMPS power? and if i did would there be a problem with having the arduino powered and not the RAMPS?

Edit: if the 14A proves not enough I have a PSU that is WAY overkill here (550W +12V 45A) but would preefer to use the cheaper PSU.

Edited 1 time(s). Last edit at 03/05/2016 05:45PM by TbirdMan.

Ok, I guess i should have just looked at the RAMPS schematic. If I remove the diode D1 will that cause any problems if the 12V lines on the RAMPS are turned off/on with the Arduino on?

No that will work fine, but be careful not to build a ground loop.

the 12V lines and 5Vsb should share the same ground, so that shouldn't be an issue AFAIK. I will have to look at how the code works but I imagine it shouldn't be too hard to ground PS_ON with the arduino when the printer starts it's warmup cycle.

You're right about 12V&5V sharing same ground, but I'm more concerned about "daisy chaining" the 5V to the RasPi.
Always use the same GND reference point. ( in your case the PSU )

I will only have the one GND reference, I will connect both RAMPS 12V to the PSU +12V and GND, the Pi to 5Vsb and PSU GND and the Pi will power the Arduino (which to my surprise is an authentic Arduino Mega). As for diode D1, my clumsy fingers have decided it's fate. while trying to remove the heatsink from a stepper driver to probe the pot to adjust it I must have shorted something and D1 popped. Fortunately nothing else seems to have been harmed.

I did a bit more research and silly me, of course I was not the first to think of this. The RAMPS even has a PS_ON pin to connect the green wire too. I have also decided to forgo the sketchy PSU that only had 14A 12V rail for a WAY overkill one I have on hand that can do 45A on it's 12V rail (I wasn't doing anything with it anyway) I am also looking into making my own power distribution board that I can just plug an ATX PSU into and breakout the PS_ON and 12V rails to go to the RAMPS and maybe add a couple extra fan headers to control some PWM fans, and the 5Vsb to a USB port to make connecting it to the Pi easier. Not sure if I will use the old mask and dissolve method or try to rig up some sort of router system on my printer.