# How does common ground work in this Arduino project?

I just saw a video on YouTube explaining common ground with an Arduino, leading to some headaches. I'm a beginner, so I apologize for a primitive question and a primitive description.

In the video, he connected two power sources to each side of the ground line on a breadboard to share a common ground: one 12V and one 5V. He then connected the ground cable of a 5V load closer to the 12V ground side (left side) of the line and the 12V load ground cable closer to the 5V side of the line (right side). I could not stop thinking about what happens where the line intersects. If the 12V circuit is supposed to return to the 12V power source and the 5V circuit to the 5V source, will the circuits go in two different directions at the intersecting segment, and what happens to the voltage? What would happen if you put a multimeter on this part to measure the amps?

I made an image to try to explain my question with conventional path. I'm sure I'm just trying to use the wrong logic...

• I don't understand this picture... where is the supply to the Arduino coming from? A separate battery or some such? If so then that's quite relevant info. Commented Nov 28, 2023 at 14:52

Assuming the currents in the 5V and 12V circuits are equal, say 1A, the sum of current going left and right will be sum up to 0 so in the area you marked no current flows.

Now, if the currents are not equal, then there will be some amount of current to some direction, depending on which current is smaller and larger.

This is due to Kirchoff's law for currents (KCL), the sum of currents entering and sum of currents exiting a node must be equal.

• Thank you! That cleared up my headache! Commented Nov 29, 2023 at 1:55

What would happen if you put a multimeter on this part to measure the amps?

In the area marked "What happens here" you get both currents flowing but, because they are flowing in opposite directions, the net current is not has high as either of the two individual currents.

what happens to the voltage?

The voltage will still be 0 volts but, breadboard isn't a perfect conductor and, under the influence of any load currents, that voltage may be a few millivolt different on one side compared to the other.

• Thank you for a great explaination. Commented Nov 29, 2023 at 1:55

Illustrating as already pointed out, here is a simulation of what can happens.