Connecting Grounds

Hi, I'm working on a project that will require 6 servos running at 5v (each one using around 200-300mA at no load). I have an external power supply built from an old computer AT supply. I can get up to 25A on the 5V rail :) In addition, I have a DC motor running from the external power supply's 5V rail as well.

Anyways, I know that to get the servos working properly, the grounds of the Arduino and the external power supply have to be connected together. So my question is: since the Arduino and External Power Supply grounds are connected together, is there any chance that back current from the DC motor can damage the Arduino? Will noise on the ground signal caused by all the motors be an issue as well?

I was thinking of using 4N25 optoisolators to completely isolate the Arduino circuits and the external power supply circuits, but the optoisolator's output is inverted, so I'm trying to figure out if this will be an issue with the PPM signal to servos.

Hi,

Once a motor starts spinning, its inertia keeps it spinning, this turns it into a generator and thus can generate a “kickback” voltage. The kickback diode routes that voltage harmlessly back into the motor so it can't damage the rest of the circuit. It is connected between the two terminals. Have a look at : http://todbot.com.s3.amazonaws.com/bionicarduino/bionic_arduino_class3.pdf

You could look at something like this : http://itp.nyu.edu/physcomp/Labs/DCMotorControl

If they're R/C servos, you won't need to worry about back EMF, but some decoupling won't come amiss.

Will noise on the ground signal caused by all the motors be an issue as well?

Could be.

but the optoisolator's output is inverted, so I'm trying to figure out if this will be an issue with the PPM signal to servos.

It will just invert the sense. That is writing a value of 0 will be the equivalent of writing 255. So if you have to write a value of v then simply analogWrite(pin, 255 -v); will do it.

By the way servos generally should use PPM and not PWM.

this turns it into a generator and thus can generate a “kickback” voltage

Not quite :- the inertia that keeps it spinning is generating a voltage in the same direction that caused the original movement. The kick back is caused by the collapsing magnetic field and causes a voltage opposite in polarity as the movement. That's what the diodes catch.