Clearing doubts on grounding electronics circuitry

[Jimbo bob]

Hi everyone

I have been having some doubts regarding the grounding of circuitry. At some point last year I had an Arduino Uno (a microcontroller board) on a vehicle. The Engine Control Unit (ECU) of the vehicle was sending some serial data to the Uno through a voltage level converter chip to provide appropriate voltage levels for the data to be received by Uno. The Uno has a power jack and a USB port and the unit can be powered through both. When the ECU was on and connected, I tried to communicate to Uno and powered it up using the USB port only, I could not receive anything through my computer and the light indicating the follow of received data on the converter chip board, was blinking faintly.

However, once I powered the power jack on the Uno, using a ground and +12V coming from the ECU, I started receiving data through my USB and the blinking of the receiving indicator light became noticeably brighter.

Now based on my understanding the reason I was not receiving the data in the first instance was because my computer did not have a common ground with ECU. Am I correct?

This brings me to my actual question :

Must not one always have a common ground between the signal circuitry and the power circuitry? For example imagine a strain gauge circuit. I am powering the bridge connected to the gauge through the ECU and want to input its voltage output as a signal to another input of ECU. Is not that necessary for the ground of the signal of the circuit to be connected to the ground of power?

Many thanks in advanced

 

[Arouse1973]

Hello Jimbo.

Jimbo said:
"Now based on my understanding the reason I was not receiving the data in the first instance was because my computer did not have a common ground with ECU. Am I correct?"

Yes you are correct.The signal energy and power supply energy must both have a return path to complete the circuit.

The answer to the second question is no. You can isolate both power and signal lines to other pieces of equipment so they become their own isolated circuit. I do this quite a bit if I am connecting to equipment in noisy environments of just don't trust other designers to have enough protection for my circuit when I connect it.
Adam

 

[Jimbo bob]

Hello Jimbo.

Jimbo said:
"Now based on my understanding the reason I was not receiving the data in the first instance was because my computer did not have a common ground with ECU. Am I correct?"

Yes you are correct.The signal energy and power supply energy must both have a return path to complete the circuit.

The answer to the second question is no. You can isolate both power and signal lines to other pieces of equipment so they become their own isolated circuit. I do this quite a bit if I am connecting to equipment in noisy environments of just don't trust other designers to have enough protection for my circuit when I connect it.
Adam

Hi Arouse1973

The signal energy and power supply energy must both have a return path to complete the circuit.

So the power and signal must have the same return path, right? Then is not what you are saying in answer to my second question contradicting your first answer? If the power and signal lines can be isolated then how can they have the same return path?

Cheers

 

[BobK]

Circuits can be isolated by using a transformer or an optocoupler. In these cases there is no electrical connection between the two circuits and the signal is carried across by magnetic field or light.

If you want an electrical connection, there has to be a common ground. Otherwise, the voltages between the two circuits are unpredictable and current cannot flow from the output to the input.

Bob

 

[Arouse1973]

Hi Arouse1973


So the power and signal must have the same return path, right? Then is not what you are saying in answer to my second question contradicting your first answer? If the power and signal lines can be isolated then how can they have the same return path?

Cheers

In your first example yes, but the second answer no and I gave you the reason why. Sorry if I confused you.
I did say:

"The signal energy and power supply energy must both have a return path to complete the circuit"

and not

"The signal energy and power supply energy must both have the same return paths to complete the circuit"

Thanks
Adam

 

[Gryd3]

"The signal energy and power supply energy must both have a return path to complete the circuit"

and not

"The signal energy and power supply energy must both have the same return paths to complete the circuit"

On occasion... both are true.
Take a look at the signal wire and look up the signalling protocol to determine if it's a grounding issue, or a signalling issue... USB for example has a signal + and -, and both wires are used for differential signalling. The same thing happens with Ethernet cable. The 8 wires are used as 4 pairs. Differential signalling compares the voltage on one wire to the other to determine if it's a 1 or a 0. There does not need to be a common ground for this. When you start dealing with things like 1-wire communication protocols, or using any kind of signalling that does not specify a return path, then a common ground is needed.

It almost sounds like the USB port does not properly power the converter board... what are the power requirements for it?

 

[Arouse1973]

Yes differential signalling can use the other wire in the pair as a return, as all signals must have a return path. I think what Jimbo was talking about was serial data from say a UART on the ECU which was going to a UART on the UNO.

Either way unless isolated the signal will only have one way to go and that's back to the ECU. If isolated then effectively the signal is copied and now two return paths exist, the original back to the source (ECU) and now the isolated version of this signal back to the isolated source.

I doubt there was anything wrong with the actual USB side of it only the fact the UNO couldn't process what was coming in on the UART?

That's my guess
Adam

 

[Jimbo bob]

I doubt there was anything wrong with the actual USB side of it only the fact the UNO couldn't process what was coming in on the UART?

I believe that was the case. Because establishsing a wireless link using xbees and powering the Uno using ECU worked fine. It also worked fine when not only the USB was connected, but also Uno was powered by ECU.

It almost sounds like the USB port does not properly power the converter board... what are the power requirements for it?

The power requirements of the USB were same as the USB port of a computer. However, I doubt that the USB was causing any issues. I achieved the same result using other computers.

 

[Arouse1973]

Yes as I first thought! Jimbo are you now happy with how return paths work or would you like some examples to help you understand?
Adam

 

[Jimbo bob]

Yes as I first thought! Jimbo are you now happy with how return paths work or would you like some examples to help you understand?
Adam

I believe that it is clear now for me thanks to you, BobK and Gryd3.