I got arduino with RAMPS 1.4
RPI is powered by a his own power supply
RAMPS is powered by another source (pc source) 12 v and also arduino (5v)
I build like this because i want RAMPS off when printer is not working but to have RPI up all the time (I got a small web server on it for tests)
when I put pc source off, display on RAMPS is always on (because is powered from usb RPI)
so because Ifound this not so ok I used violet wire (always on 5v on pc source) to power arduino
I'm not sure if what I'm done is normal
I suppose if I unpower arduino from that 5 V will be more ok, this means will be up all the time from RPI USB
I am a little bit confuse in this situation
It sounds like we've solved the original problem and we're troubleshooting the best way to do your power setup using a thread with an old title.
Perhaps @foosel can split this thread into two so that we can continue to troubleshoot this. (Thanks)
Raspi <- 5V power adapter
RAMPS board <- 12V from a PC-based ATX power supply
Arduino <- from that same ATX power supply (purple and black)
You probably have the ON switch of the ATX power supply wired together or maybe you have an external switch. I suppose there are people who use a relay and have the Raspi control that relay. You could then turn the ATX power supply ON/OFF like this. If the RAMPS board supplies a 5V output, you might consider putting the Arduino on that.
For a while I was running:
- ramps board from its stock power supply
- raspi from an ATX power supply on the 'always on' pins, also connected to 'PSU control'
- MOSFETs/high power/fans on the ATX power supply
I liked that. I would have loved to control the RAMPS board from the ATX, but it breaks the serial connection, so it's easier to just have it always-on like the raspi too.
I went away from this setup when I converted to 24v. It's a little more complicated now 
I actually just did this and it's working perfect so far
I used the purple wire from the ATX (always on 5v) and black to power the Pi
I put the Green and Black to a relay to turn on and off the power to the ATX (when you short the Green/Black it turns the power on)
I used the PSU Control to control the relay to switch the power on and off to the printer
I only have the Pi running off the Purple wire, but, from the specs on the PSU, I wouldn't be afraid to power both an Arduino and a Pi off the same Purple 5v rail
I suppose I would have to try it to see if the Pi starts giving me a low voltage error to be sure tho
To update power response. I added the Arduino to the same purple 5v rail as the Pi (which is a B+, so it's a power hog to begin with) and I also have two HATs on the Pi, and the Arduino is powering a robot arm with 4 servos on it (which is programmed to keep moving, so it's drawing a load too)
And I'm not getting any under-voltage complaints from the Pi at all. In fact, here's a listing of my power...
pi@octopi-BPLUS:~ $ ./power
core: volt=1.2000V
sdram_c: volt=1.2500V
sdram_i: volt=1.2500V
sdram_p: volt=1.2250V
And this is the reading on the standard Pi3B with a regular Pi power supply...
pi@octopi-beta:~ $ ./power
core: volt=1.2000V
sdram_c: volt=1.2000V
sdram_i: volt=1.2000V
sdram_p: volt=1.2250V
The ATX PSU is a 450W with a 16A max on the 5V rail
I think I could probably power 2 or 3 more B+'s with no problem, so, I would check the specs on your PSU, but, I think you should be fine powering the Arduino on the same PSU
Also, keep in mind that your ramps board is going to be running on the 12V rail, so, it's not on the same circuit
Keep in mind the Pi isn't THAT much of a power hog- it does need a 2.5A power supply, but it's still hard to use more than 0.5A even with full GPU running- which, presumably, you aren't.
Now I just want an ATX PSU that has high-amp 24V rails. Ah well.