Hi all,
I have been given the task to create a control system for an industrial electronics device. I am not new to this but I do have a fundamental question regarding circuit layout/grounding.

The device will have various sensors and motor drives, all of these are to be incorporated into 1 control board. The motors will operate from 12VDC and the logic side from 5VDC. This will be a line powered device 120VAC 60hz.

In regards to the power supplies and grounding; I have seen on other equipment where the common side of the DC supply is bonded to the frame of the machine (earth ground from the line supply). Other machines I have seen where the common side of the DC supply was isolated from the machine frame. Which is the right choice or is it design dependent? I have built machines that would not operate correctly until I bonded the common of the DC to the machine frame to cancel out noise.

More specifically related to my project. The motor drive circuitry will have noise and I do not want this noise introduced to the logic side of the system, so my intention is to keep the power supplies (12VDC and 5VDC) isolated from each other and drive the motor control components via optoisolators. But, if I bond the common of the 12 volt supply to earth and then I bond the common of the 5 volt supply to earth will noise cross over to the 5 volt side thru the machine frame? What is the most correct way to approach a design like this? This machine will operate in a very electrically noisy environment with lots of 480 3 phase Freq drives and solenoids in surrounding equipment.

Thanks

Scott

 

Hi,
Is the frame of the machine already earthed for the AC supply?

Regards NM

 

heady subject. I think the one thing that is important is a single grounding point. If remote devices have any leakage to ground, there is a potential for grounding loop problems, which floating may overcome.

Personally, I'd start with grounding for safety reasons, apply appropriate methods for prevention, and float if required to overcome leakage that may be inherited via device characteristics.

 

"Is the frame of the machine already earthed for the AC supply?"

Yes, the machine frame will be grounded (earth bonded) by the green wire on the line supply.

Thanks,
Scott