Why is Arduino ground connected to Power supply module ground? The stepper motor works the same without that ground connection.

 

Often motor power circuit common and Micro IC's common are bonded to a GND star point, object is 'noise' reduction/prevention.

 

hi Jopee.
Where is the Arduino +V supply? on that image
E

 

Also reduces system GND loops.

 

hi Jopee.
Where is the Arduino +V supply? on that image
E

Arduino USB is connected to PC.

 

Stepper motor works the same without that ground connection.

The stepper motor may be driven by a bridge connection, which doesn't require a ground connection for the motor.

 

hi J,
If that lower left PSU module is not connected to 0v/Gnd on its input, then the control signals from the Arduino would not have a 0v/Gnd reference and would be floating.
The fact that the project works without that link shows that the 9Vdc power source is connected somehow to the USB ground.
E

Note: the link is to ensure that project would work even if the 9V PSU being used was isolated from ground/floating

Better image.

 

All GNDs are not at the same potential.

Current flows in GND wires. When current flows, there is a potential difference.
Hence, ideally, you want every component to have a separate GND connection back to the power source. It matters most when there is a lot of current flowing in the connection.

 

The stepper motor may be driven by a bridge connection, which doesn't require a ground connection for the motor.

But usually isolated from grounded mains supply by transformer(s).
Every CNC system I have worked on references power common to star point GND wherever possible.,

 

hi J,
If that lower left PSU module is not connected to 0v/Gnd on its input, then the control signals from the Arduino would not have a 0v/Gnd reference and would be floating.
The fact that the project works without that link shows that the 9Vdc power source is connected somehow to the USB ground.
E

Note: the link is to ensure that project would work even if the 9V PSU being used was isolated from ground/floating

Better image.
View attachment 338822

There is no connection between PS module and USB ground.

 

hi
What PSU is driving that PCB power supply?
Can you show the power connections and power supply type?
E

For example: the circuit I posted showing no link and an isolated PSU driving power PCB will not work.
There is no common ground 0V for the Arduino logic signals.

 

hi
What PSU is driving that PCB power supply?
Can you show the power connections and power supply type?
E

For example: the circuit I posted showing no link and an isolated PSU driving power PCB will not work.
There is no common ground 0V for the Arduino logic signals.

 

The connection is not "A Ground", it is a COMMON, which serves as a return for the drive signal connections shown in the photo in post #1. Common is a much better description of how the return path actually works. It is a great way to confuse beginners, as well.

 

Arduino USB is connected to PC.

Although this means of supply is connected to Earth GND on a typical MOBO, I agree the best term to describe it in this application is Power Common.