So a while ago I answered a question on Arduino.SE with regards to a person wanting to shield there Arduino from EMI( Electromagnetic Interference) that was coming from some AC they were feeding into a shield on the Arduino, and thus they asked if it would be plausible to use a shield in between the Arduino and the other device to stop interference. Thus I brought out some points to help that person.
Some things I answered in it:
Using the DC ground as a shield is often done, if you look at the EMI cages on cellular phones, WiFi modules, and other noise generating devices/susceptible-to-noise devices, they are connected to ground. It gives the noise somewhere to go, instead of your signal lines. Thus using the DC ground is defiantly an option
i.e.:
Don't do a direct DC ground to AC ground unless you know what you are doing as this will create a ground loop, this will change the way stuff behaves and will make your circuit no longer a floating device, or in another way it won't be and isolated circuit, but now referenced to mains ground/earth.
Some things I answered in it:
- Using DC Ground
- Using AC Earth/Ground
- Isolating circuitry
- Extra, PCB layout
Methods
DC Ground
If you connect the DC ground from the main board you should use aferrite bead(FB)
between the two grounds, this will ensure that you don't get noise
getting back into the ground of the main board after you have tried to
shield it. You should try to match the ferrite bead to the noise
frequency you experiencing (use a scope if you have one).Using the DC ground as a shield is often done, if you look at the EMI cages on cellular phones, WiFi modules, and other noise generating devices/susceptible-to-noise devices, they are connected to ground. It gives the noise somewhere to go, instead of your signal lines. Thus using the DC ground is defiantly an option
i.e.:
Main ground]--------[__FB__]-------[shield AC Ground
You could use the AC earth line on the top part of the shielding board. The coupling part might be a bit over kill, leave space for it to test to see if it helps when you make the board. Also AC earth can be used for shielding but is used for the safety aspect of earth leakage.Don't do a direct DC ground to AC ground unless you know what you are doing as this will create a ground loop, this will change the way stuff behaves and will make your circuit no longer a floating device, or in another way it won't be and isolated circuit, but now referenced to mains ground/earth.
Other stuff
I would also make sure how you drive the relay is isolated from the control circuitry, although from what the OP had described:If you share power rails between the relay board and the main board, try filtering the power coming from the Arduino shield or the power that is being supplied and shared between the two. Also the control lines can be isolated between the controller and the relay with opto-couplers, this might stop noise coming back into the circuit from the relay part.if I move the shield further away I also don't see the issues.
PCB designs
I have made a initial design idea with Eagle, and put it into a GitHub repo, thus please change, improve or add to the design. I plan on adding more designs to make it more multi-design so that it can meet different needs.
Repo: EMI_SHIELD
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