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EMI / EMC Design practices

Discussion in 'Electronic Design' started by Devendra, Apr 19, 2007.

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  1. Devendra

    Devendra Guest

    Hello All,

    I am designing a controller based on 24VDC supply voltage. It is a
    process indicator based on a Freescale 16 bit controller and
    communicates with external devices using CAN or RS485.

    I want to subject the product to CE testing.

    I just want to know that whether the X and Y suppression capacitors
    will be required for a 24VDC powered controller also!? I have put
    across the line (X) capacitors as 0.1uF and 0.01uF.

    Also whether my circuit ground should be isolated from the chassis
    ground, because my unit will be subjected to some tests in which the
    transient voltages are applied between supply lines (230Vac, 50Hz) and
    Earth (connected to Chassis of my controller).

    Kindly let me know your suggestions about standard design practices,
    for EMI EMC compliance.

    Thanks in advance.

    Best Regards,
    Devendra Jogdeo
  2. Eeyore

    Eeyore Guest

    X and Y caps are safety ratings for capacitors to be connected to the AC line.
    There's no need for any on 24v DC circuits.

    Where ?

  3. Eeyore

    Eeyore Guest

    You want someone to write a book for you for free ?

  4. Paul Mathews

    Paul Mathews Guest

    Capacitors for EMI reduction are used to provide intentional paths for
    parasitic currents which would otherwise cause stronger radoated or
    conducted EMI. They are never required, per se, except in the sense
    that, if you use capacitors for this purpose and they are conductively
    connected across the mains or from mains to protective ground or any
    accessible conductive components, then those capacitors must have the
    proper safety rating such as X, Y, X2, etc. In some cases, equipment
    powered by 'safety' low voltages may still require safety rated
    capacitors. A DC/AC inverter is a good example.
    As a good starting point for a couple of years of intensive study of
    design practices, I recommend Keith Armstrong's 'Compliance Club' EMI
    tutorial pages, easily found via web search.
    Paul Mathews
  5. Barry Lennox

    Barry Lennox Guest

    X and Y caps are not used in 24 V DC PSUs.

    You need to get a book. Try "EMC for Product Designers" by Tim
  6. Eeyore

    Eeyore Guest

    PSUs ? I'm sure they are used on the mains input filter.

  7. Devendra

    Devendra Guest

    Regarding X and Y capacitor, I mean to say....

    X : Across 24V incoming power supply and
    Y: 24VDC line to Earth and Circuit Ground to Earth....

    Also Circuit Ground and Earth should be isolated or not?
  8. Barry Lennox

    Barry Lennox Guest

    I'm not about to spend too much time analysing the OPs text, I'm
    confused if the input is 24 vdc or 240 vac. But it also appears that
    he wants to put them across the 24 v line. It's all too hard.
  9. Eeyore

    Eeyore Guest

    It is totally unnecessary to use X and Y parts here. They are not subjected to
    mains voltages. Ordinary capacitors would be fine.

    More problematic would seem to be that you placed them here in the circuit
    without any reason.

    Depends on your design strategy. For good EMC behaviour circuit ground should be
    firmly connected to equipment chassis (frame ground) *in many places* in fact.

  10. Devendra

    Devendra Guest

    Also Circuit Ground and Earth should be isolated or not?

    Thank you very much for your suggestions.

    Best Regards,
  11. MooseFET

    MooseFET Guest

    On Apr 20, 6:01 am, Eeyore <>
    I disagree with that suggestion. You never want to provide a path by
    which the circuit can directly drive currents through the chassis.
    The enclosure should be well connected to its self so that there are
    no gaps or bad electrical connections.

    I have had to resort to making special mounting hardware to mount a
    power supply that connected its internals to the chassis in more than
    one place. The chassis had about 1.5Vp-p on it from corner to
    corner. When it was bolted into the rack it screwed up everything
    else in the rack.
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