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I'm planning to design a board that is suitable for industry so EMC is so important. I have decided to use two separate power supplies(+5v and +24v) to Isolate the board from outside world. Optocouplers have been used to connect these two areas. I don't want to connect these two GND(power supply) together and I want to keep them separated. About the earth ground I read lots of articles that says it is better to connect the GND in PCB to the Earth Ground(for metal chassis). I wounder to Know if I'm doing right and having two separate GND for designing PCB of the board which one of these should be connect to the Chassis ? and How about the other one ! I would appreciate if you give me some good references to read. TNX

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It depends on final equipment ground policy which may differ. In all systems i designed i use GND star connection, which means the following:

  1. All isolated* grounds are connected.
  2. There is only one point of connection, near the power supplies.
  3. Chassis is connected in the same place with a thick wire.

*there are sometimes grounds that somehow depend on other systems and cannot be connected. Most common: minus of diode bridge in AC rectifier. Another possibility- minus of isolated supply for current sensing, which is connected to a motor phase. But since you are not asking about it, we will not discuss it.

Also on each board near each mounting hole there are capacitors that connect the chassis to grounds in AC domain. This is important, because being thick (in relation to PCB copper planes) the case usually has very low inductance and resistance and may be considered perfect ground for noises of all kinds. But on the other hands you don't want any power to pass through the case, so the connection is by caps.

Speaking of ground policy, nice to see that you use optocouplers. Hope you did not forget power filters.

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  • \$\begingroup\$ Thanks Gregory For your rapid reply I read some articles about Star Grounding but I'm still confused !. Does it mean connect all GNDs together and finally to Earth Ground? e.g. connect the GNDs of power supplies together and directly connect them to the earth ground exactly the place that they are. keep those GNDs separated on the PCB board and the mounting holes (for chasis) don't connect to GND on the board. if so, it is in contrary to This Post link \$\endgroup\$ – Mohammad Yousefi Jun 3 '15 at 11:13
  • \$\begingroup\$ It's funny :) The guy says "don't do that", but he forgets to mention "if you did what i recommended first". In his case it's the same connection as i suggest, but which star center on the case. This is a nice alternative, and if you draw it as a diagram and compare to my way you will see that it's the same more or less. The issue with that is that depending on final equipment it may not fit. Because if in any input or output port you send a GND line together with your signals, suddenly ground loop is created. So it kind of rapes you to use more and more isolation. So he is wrong, i am right:) \$\endgroup\$ – Gregory Kornblum Jun 3 '15 at 11:27
  • \$\begingroup\$ Thanks Gregory. and is it right that I keep my mounting hole out of Ground for the sake of preventing loop? \$\endgroup\$ – Mohammad Yousefi Jun 3 '15 at 14:56
  • \$\begingroup\$ Yes, mounting holes are the chassis net, connected with capacitors to GND \$\endgroup\$ – Gregory Kornblum Jun 3 '15 at 15:56

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