If a circuit is connected to a separate power supply from an Arduino that communicates with it, should I connect the grounds together?

Question

I am using an Arduino (actually an onboard co-processor AVR on an x86-based SBC) to control 120 Neopixel LEDs. Obviously, the power requirements of this many LEDs requires a separate supply.

I can't just use a single large power supply for both, because the SBC requires 11-15V if using the DC input connectors. The Neopixel strings require 5V.

I believe that I should connect one of the ground pins of the SBC to the ground terminal of the large power supply that the Neopixel is using so that communications works properly and won't damage the Neopixels somehow.

However, is it possible that doing this could somehow hurt the SBC? That would be bad, because it was about $350. I suspect there will be no issue, but I wanted to ask engineers, because again, $350.

The way I understand it is that if there's no path to the positive terminal on the PSU, that there's no potential between the SBC and the PSU's ground, but it does provide a common "floor" so that the two devices can safely interact. But can the connection to the data input on the Neopixels from the SBC potentially be dangerous in this scenario?

If there's a risk of damage, I suppose I can get a voltage regulator to bring the 5V PSU up to 12V and then power the SBC from it.

Answer 1

Your board outputs logic levels like +5V or 0V to control the LEDs, these are voltage, and voltage is relative, it is the difference in electric potential between two points. You can't have a voltage if you have only one wire, you need two. One of them is often called "ground" or "0V" and it is used as a voltage reference relative to which all other voltages are measured.

If you don't connect grounds, then the receiving chip on the other end has no way to know about that reference, so it will have no idea what voltage it is receiving. So it won't work.

can the connection to the data input on the Neopixels from the SBC potentially be dangerous in this scenario?

If you don't connect the grounds together then both devices will be at random potential relative to each other, which means that if current has to flow it will flow via what's connected, which is the IO pins of your chip, instead of just through copper. This is when it fries.

You would worry about connecting grounds if they were at different potentials, for example if you have long cables between devices, and these devices are earthed to different places in different buildings. Also if both devices are earthed in a way that creates a long enough loop of Earth wire in the building that is closed by your ground connection, because when lightning strikes in the vicinity it will induce current into conductive loops. This is why Ethernet uses isolation transformers, for example.

But that would not be the case if you use two power supplies powered from the same socket. No worries in your case.

Answer 2

Connecting the grounds between two systems that are in physical close proximity to each other is normal and done frequently.

If however your systems are far apart (on the order of 20+ ft.) , then you'll want to use some sort of isolation system to allow them to communicate while maintaining separate grounds.