I decided to build my own power supply for some stepper motors from an old transformer I had that puts out 28V AC. The AC goes through a bridge rectifier and comes out a bit shy of 40V DC, gets smoothed with a big capacitor and an LM317 and is producing voltage just fine in isolation. The GND is determined by the negative DC port of the bridge rectifier. (Its currently a floating GND) However, I am going to be using an Arduino and a Stepper motor controller to handle the motion of the steppers. They both use +5V and their GND is set by their voltage regulators. My motor controller allows separate power and signal grounds. 1) Is it safe to have two separate grounds? (One relative to the Arduino and one relative to the motor power supply, with no guarantee that they would actually both be equal) If it is safe, is it wise to have two separate grounds? 2) Can I ground the negative terminal of the bridge rectifier without changing the relative output? (IOW, will I still get the same voltage out if I ground the negative port?) 3) If I were to ground the negative port, should I use the ground wire from the mains, or the ground from the Arduino? (I'm guessing grounding a 40V power supply through a micro-controller would be bad, but is it?) 4) If I grounded the negative port to the ground-wire from the mains, would that keep the GND tame enough to share it with the Arduino?