GND Servo and Arduino

Question

I was working in a little project with Arduino and two Servomotors

Why is it necessary to putting the GND from external source (necessary to power the servomotor) and Arduino GND together?

Why is not able to work properly with separated GND?

Answer 1

Whenever you interface your Arduino to a separate device -- or several -- you must establish a common ground. Perhaps your servos run off their own power of a small breadboard, or you have a sensor or a bunch of LEDs... in all cases you'll need to make a common ground for all components of the circuit.

If I'm hooking up more than a single LED or single sensor to my Arduino, I prototype on a breadboard and the first thing I do is connect the power GND pin of the Arduino to the blue/black ground rail of the breadboard. That ensures a common ground, which is vitally important as a reference 0V in any circuit you build.

In many well-designed multi-layer circuit boards for example, often one layer of copper is devoted entirely to GND so that every component will be able to have a solid connection to GND. Then it's referred to as a "ground plane" and is a common goal in good circuit board design.

Answer 2

Current flows from + to - in a circuit, from the highest to the lowest reference point. See details here and here.

The GND is a reference point usually 0v.

With different GND (I'm guessing different positive input as well), you might end up with:

• some short circuits
• independent circuits
• reflection phase change if you are working with alternating currents

Anyway in a circuit the GND should always the same (even though it can be in multiple places).