I am working on a project which involves micro-controller and switches (triacs and EM relays). Due to size constraint, I need to stack one over the other. Here is unfinished relay and triac board which will go on top of micro-controller board of the same size:


PCB description: 220V to 5V offline switcher on the left. It has a switching frequency of 65 kHz. Two transistor driven electro-mechanical relays in between and 6 optically isolated triacs on the right. There are a few connectors for optional sensors.

I have given 4 holes in corners so that I can use spacers and nut bolts to put the two PCBs together. Electrical connection between the two will be made using either a cable or header pins (yet to be decided).

As such what should be the minimum distance between the two PCBs so that high voltage or whatever noise is there in ac lines don't disturb the delicate electronics below?

Is there something which I should use in between to provide added safety to my micro-controller board?

I was thinking of some plastic sheet the same size of PCB or two plastic sheets with a thin metal foil sandwiched between them but I am not sure whether they will have any impact or not.

Any suggestions regarding this will be much appreciated.


  • 1
    \$\begingroup\$ The limiting factor will (also) be clearence. As for disturbence, think more about layout than distance between boards. Noise level will be in the order or ~1/r^2 but the difference between a good and bad layout can be many orders of magnitude. \$\endgroup\$
    – winny
    Oct 6, 2016 at 8:52
  • \$\begingroup\$ @Winny - I totally agree. I have been haunted by noise issues in the past and now I take extreme care while routing. This is not the final layout. I will be taking care to isolate noisy elements from my logic signals when I start routing. Thanks for pointing it out. \$\endgroup\$ Oct 6, 2016 at 8:57

2 Answers 2


Apart from the comment from winny you also have to make sure to respect the safety of the mains side 220 V. and the rest of the circuit. If you remain in doubt you could enclose the mains side on both side of the print in a metal box. This way you could kill two birds with one stone. Safety against accidental touching and less influence from noise.

  • \$\begingroup\$ Clearence = saftey :-) \$\endgroup\$
    – winny
    Oct 6, 2016 at 9:13
  • \$\begingroup\$ @winny: I agree that clearance is safety but hardly against accidental touching. \$\endgroup\$
    – Decapod
    Oct 6, 2016 at 9:24
  • \$\begingroup\$ The plan was to keep both PCBs (micro and relay) inside a metal box. After reading your answer, seems like putting two plastic films with a metal foil sandwiched them, between the two PCBs is the best I could do. this will cover mains from all sides by metal. However there will be holes and gaps left for external wire connections and IO connector to the micro-controller board. \$\endgroup\$ Oct 6, 2016 at 9:29
  • \$\begingroup\$ Solid insulation or Earthed metal is generally better than clearance. \$\endgroup\$ Oct 6, 2016 at 9:30
  • \$\begingroup\$ Oh sorry I had left it just upvoted that time. Thanks Decapod. \$\endgroup\$ Oct 6, 2016 at 13:04

I am no expert on this and this would be a comment instead if such feature was not foolishly disabled by default rank per forum here on se.

that said, I have heard of people using emergency blankets(the shiny plastic blankets in roadside emergency kits) to shield security tags from sensors for shoplifting as the metal they are often coated with are lead, and was thinking such may be benefitial as a tool for you.


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