EMI shield grounding

Discussion in 'General Electronics Chat' started by kender, Jun 27, 2007.

  1. kender

    Thread Starter Senior Member

    Jan 17, 2007

    I’m designing a signal conditioning board for industrial environment. The board will reside inside a plastic enclosure on a DIN rail. I need EMI shielding. The enclosure will not provide shielding, because it’ plastic. I’m going to make a small sheet metal “can” that will be soldered onto the board. The question is: should I ground it to signal ground or earth ground (i.e. the DIN rail)?

    Please let me know if I forgot to mention anything important.

  2. beenthere

    Retired Moderator

    Apr 20, 2004
    If your signal comes from the outside of the can, signal ground and earth ground had better be common. Hope your DIN rail is solidly grounded, too.

    Keeping noise off signals in industrial environments is one of the black arts.
  3. kender

    Thread Starter Senior Member

    Jan 17, 2007
    Please explain me why. The signal is coming from thermistors outside the can. Currently, the signal ground is connected to the return line of the 4-20mA pair. Can't I have the signal ground pretty much float with respect to earth?
  4. mrmeval

    Distinguished Member

    Jun 30, 2006
    The plastic sure can become an EMI shield. There is a product that sprays on.

    Be sure to download the application guide. You'll want to make that a full Faraday cage.

    * Shielding effectivieness: One to two mil coating provides 40dB - 50dB shielding across a frequency range of 5 to 1800MHz.
    * Surface Resistance approx. 0.7 Ohms
    * Dry time: 10 minutes at room temperature
    * Recoat time: 5 minutes
    * Excellent adhesion to most plastics
    * Available in both liquid and aerosol
    * Tested as per IEEE Std. 299-1997
    * Underwriters Laboratories Recognized File No.: E202609
    * Liquid version available for dipping, brushing or spray gun applications.
    * Thinner is available for adjusting the viscosity
    * Thin or clean with part #435-1L Thinner/Cleaner
    * NO CFC 'S, NO HCFC'S, Ozone Friendly
  5. mrmeval

    Distinguished Member

    Jun 30, 2006
    It would be an antenna. You could probably get TV stations on it.

  6. kender

    Thread Starter Senior Member

    Jan 17, 2007
    I thought about using such spray-on shield, although it will still need to be grounded. What's the best way to connect to such shield for grounding? I've never seen how it's done, but I can envision leaf spring or helical spring.
  7. nanovate

    Distinguished Member

    May 7, 2007
    At some point your signal ground and earth ground should connect.

    Is your current signal carried on a shielded twisted pair? You'd also want to connect that shield to earth ground at one point.

    Do you anticipate needing to shield magnetic noise for which grounded shields do not offer much protection?
  8. cumesoftware

    Senior Member

    Apr 27, 2007
    I think its better to use signal ground rather than earth, so the shielding of eventual cables/lines can be connected to this shielding as well. You can connect these grounds to the earth via capacitance.
  9. mrmeval

    Distinguished Member

    Jun 30, 2006
    Run a bolt thorough the case prior to spraying on the shield material and screw on a metal lug with a wire soldered to it first. Spray over it.
    Then hook the ground where you need it.

    If the case has bosses that the board sits on you can spray over them and make sure the board grounds are at those points. If those points are not ground then be sure to insulate the board at those points.

    Where the case meets when closing it should be sprayed over as well. Since I don't know the case or the board I can't do much more.

    Also, remember that the case will be highly conductive once you've sprayed that stuff on.

    Oh, I thought the application guide would be worth while DOH!