# Why does ardunio and external platine need to be grounded together?

I got a Arduino Mega 2560 v3 which i connected to a testboard with a shiftregister, some mosfets and transistors and last but not least some LED's. Since i'll add up to 10 registers and 125 Leds i need an external powersupply. I got a 5V one for that. But if i earase the GND and 5V from the arduino and put it to the power supply it does not work anymore. (Arduino connected to USB) So if i add a line from GND to the testboards GND it does work as expected. So the question is why it does need to be like this? And is it "safe" todo so or have i done something wrong? (Please dont judge me)

I read about it that all GND need to be connected but id like to know why. I guess because i would have an potential difference betweeen the boards which would cause some issues or such.

Yes. It would.

These are electrical circuits - where a "circuit" means a closed path. You must have a return path for the current in each of the interconnections.

Just very basic electrical theory. If you connect a battery to a LED with only one wire, it will not light.

Paul__B:
Yes. It would.

These are electrical circuits - where a “circuit” means a closed path. You must have a return path for the current in each of the interconnections.

Just very basic electrical theory. If you connect a battery to a LED with only one wire, it will not light.

Thanks for the answer. I wasn’t sure if i need to “return” the clocklines since i do have 2 GND dont i? One of the Arduino(USB) and one of the powersupply from the Breadboard. Good to know.

So i can connect the GND without haveing fears of distroying my arduino? In my case i do have a 5V circuit but what would happen if i had a higher power at my breadboard circuit?
I feel bad… had electronics in class for 3 years but this is 5 years ago and i feel like i really know nothing.

Yes grounds can and should be connected together. See this for a diagram illustration the point:- http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html

Grumpy_Mike: Yes grounds can and should be connected together. See this for a diagram illustration the point:- http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html

Thanks alot! Bookmarked it so i wont forget it.

BennX: i do have 2 GND don't i? One of the Arduino(USB) and one of the power supply from the Breadboard.

The question is - are they connected at all? If the USB is connected to a laptop, it will only be connected to the mains ground if the laptop is connected to its power brick and that brick uses a three pin mains plug. If you are powering your breadboard from a "plug pack" (American: "wall wart") then the negative is not ground because the plug pack has only two supply pins. If it is powered by a bench supply, then the negative (black) is only ground if you have it linked to the ground post (generally, green), so there are two reasons why there is no connection between the different circuit grounds.

Not only do you need a common ground connection to make the circuit work, but it is absolutely critical for safety, both of your circuit and yourself since equipment faults could otherwise place mains on one of the unconnected "grounds". If that were to happen, you would prefer to see the smoke, rather than feel it.