AC Ground, DC Ground, Signals

[munu]

I read a bunch of posts but I am still a bit confused about grounding. Lets say you have:

1. A DC device that received data. The communication could be a serial connection, a CameraLink connection, etc.
2. An AC/DC power supply that provides the above device in point #1 DC power
3. Some device anywhere from a few feet to lets say 100 feet away that tries to communicate with that device in bullet point #1.
4. The above device in bullet point #3 is powered by an AC outlet (possibly some 100' away from #1) but runs on DC (basically a computer)

It is my understanding that in order to have proper communication between the two devices (#1 and #3) they must have their DC ground on the same level (unless you use some differential voltage protocol). You also should put the AC ground/neutral on the same level as DC 0V. So basically you have AC ground from device #1, AC neutral from device #1, DC 0V from device #1, AC ground from device #2, AC neutral from device #2 and DC 0V from device #2 all connected together.

But doesn't that create hazards because if device #1 is on a different level than device #2 you can have current flow across the communication channel? Also wouldn't ground for one device be (slightly) different than ground on the other side? Maybe if it is just some 100mV it doesn't matter.

 

[barry]

Somebody once said to me: "Ground is where you plant tomatoes."

There's no reason your DC ground HAS to be connected to your AC ground. In fact, that's one of the reasons they make transformers. Let's, instead, talk about COMMON.

If you're communicating over a single-ended connection, your two DC commons should be at the same potential, which means you should connect the commons at both ends. If you were to also connect each of the commons to their local AC ground there's the very real possibility that there will be a large (or VERY LARGE) differential voltage and a large current will flow.

There is tons of information out there about grounding, and this is not a simple subject. You need to consider your specific application.

 

[munu]

Ok, so DC 0V to DC 0V. That makes a lot of sense unless you have some differential voltage protocol. The chassis ground and AC part isn't quite clear to me.

* You should be grounding the chassis. Should this go to DC 0V or to AC ground? I suspect it is AC ground because you want to protect the user of the device. In other words, you don't want to have a potential between the chassis and ground/earth.
* But shouldn't DC 0V also be connected to ground? And therefore AC neutral (which is connected to ground) is on the same level as DC 0V.

 

[KlausST]

Hi,

I guess you need to be more specific. And you need to do some drawing.

The fact that you describe a circuit with text and rather vague informations causes a lot of guessing.

Example:
"A DC device .. communicates"
I guess it means "DC powered", but why not say "a 5V powered microcontroller system"?
Or "a 48V telephone system" ... " or an 1.2V powered FPGA with gigabit serdes"
All are DC powered systems, which communicate...

Communication can be wired, non isolated, optocoupler isolated wired, differential signalled, single ended, AC modulated, transformer isolated, optical communication, RF communication, 2cm from microcontroller to FPGA, or 2km LoRa....analog, digital, from kHz to GHz..

Do yourself a favour and draw a somehow detailed sketch.

You get faster feedback focussed on your real requirements.

Klaus