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Need Help Understanding a Common Mode EMC Issue.


Junior Member level 3
Jul 6, 2012
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Melbourne Australia
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Hi Everyone and Happy New Year

We have an industrial product that basically consists of an embedded PC which communicates with some electronics and a camera about 10-20m away from the PC. The PC is located on a benchtop along side industrial production machinery while the camera is actually mounted on the machine.
The PC end is supplied from a 24v Power Pack which itself is connected to the AC supply. The PC enclosure is metal, the 0v supply is grounded to the chassis and and connected to Earth.
The 24v supplied to the PC is also tapped off and supplied to the remote camera equipment. Additionally an ethernet connection exists between the PC and camera.
The camera end is also in a metal enclosure. The metal enclosure is earthed directly onto the machine. Now the Earth on the machine may fluctuate with respect to the earth earth to which the PC is connected. This is typical on industrial equipment, and we have learned in the past that connecting the 0v ground to earth at both ends can end up in frying boards and cameras., so internally inside the camera enclosure all electronics are isolated from Earth.
I have attached a drawing which shows the setup.

My first Question is : Is this the best / correct way to attack this installation ?

The next issue arises after EMC testing. The emissions from the unit exceeded the limits. Now the diagram attached includes some EMC specific additions.
The Common Mode Choke, Additional Capacitor #1 and Additional Capacitor #2 were not in the original design. These parts have been added during EMC testing, and brought the emissions within acceptable limits. The "parasitic capacitance" are just shown on the diagram since its inevitable that these capacitances exist in the original design, even though the camera PCBs were isolated from earth at the camera. My problem is that I can not understand how the additional EMC components solve the problem!
I accept that the Common Mode Choke would make a significant difference, and in retrospect I feel we were naive to miss this. However the addition of the two extra 10nF caps "Additional Capacitor #1" and "Additional Capacitor #2" does not make sense. I have always understood that to reduce common mode emissions you need to reduce parasitic capacitance to earth and NOT increase it!. Also the "Additional Capacitance #1" is connected from the Camera enclosure +24v to Earth. Is this correct, or should it be from the 0v connection to Earth?

I'm not expecting a definitive answer to this question, but looking for some help to understand what is going on and how to do better in the future.



Super Moderator
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Apr 1, 2011
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Minneapolis, Minnesota, USA
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You have power supply wires running 30-60 feet. Are they shielded (whether or not it's usual to shield long runs of power supply wiring)? Do the wires pick up interference from nearby devices?

The rightmost equipment runs on 24V. It communicates to a computer running on 12V. Is ethernet voltage compatible at both ends?

Your voltage converters at each end likely require matching output levels. Typically signals from one device must have a current return path, which implies a common ground. Is your system compatible in this manner?

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