Common ground with external power supply?

[Tomato925]

One thing I am not clear about external power supply for motors or motor controllers. I would like to control 5 stepper motors (https://www.oyostepper.com/goods-666-Φ35x352mm-PM-Stepper-motor-WGear-Ratio-120-1-Spur-gearbox.html), each using a driver (ULN2003) from Raspi. The plan is to use a 5V power supply and connect the drivers in parallel to maintain a constant voltage of 5V. However, while searching for a suitable power supply for the converters, I came across information that a common mistake is not connecting the ground. converters to Raspi. I really don't understand this. To what extent is this necessary? Logically, it should work without a common ground, because these are two separate circuits...

Shouldn't I be happy if the Raspi doesn't encounter any possible voltage spikes from the power supply? :/

I look forward to clear answers that will clear up this mystery!

Best regards!

 

[Bluejets]

Show your circuit diagram.

 

[Harald Kapp]

Logically, it should work without a common ground, because these are two separate circuits...

No. Logically current flows only when the circuit is closed. Therefore you need a return path (common ground) to close the loop from the Raspi's output to the driver and back to the Raspi.

The only option where you don't need a common ground is when the driver is isolated between input and output. But even in that case you need a ground connection between Raspi and driver for the control current to return to the Raspi. The only diffenrece being that ground on the driver's output side is not connected to ground on the driver's input side. That is because of the isolation which prevents current from crossing the isolation barrier.

Try lighting a lightbulb with only 1 wire to see ...

 

[CamdenHarris]

Thank you so much for the suggestion.