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Gound plane resistance calculator

Discussion in 'Electronic Design' started by Joerg, Aug 1, 2011.

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  1. Joerg

    Joerg Guest

    Folks,

    Is there an online tool, calculator or cheat sheet that lets me
    calculate the resistance between two arbitrarily chosen points in a
    ground plane?

    All I found was heavily math-laden scholarly articles and instructions
    how to program it in Excel. Reason behind this is that I am dealing with
    a ground plane right now that can't be thicker than a few micrometers.
     
  2. Joerg

    Joerg Guest

    No, I want to do two mouse clicks and read a nice little ohms value :)
     
  3. Joerg

    Joerg Guest

    Reality is different :)

    Last time an alligator clip slipped off fell onto a ground plane there
    was a flash, a loud bang, some yelling and hollering, and the wire from
    that alligator lead had vaporized.
     
  4. Nico Coesel

    Nico Coesel Guest

    AFAIK Sonnet is not the right tool for that. Its more for simulating
    strip-line designs. Off topic: Sonnet is a bit tedious to setup and it
    doesn't work wel for antennas. If I have the time I want to see how
    well the simulation matches the real world.
     
  5. Nemo

    Nemo Guest

    The resistance is more or less constant, irrespective of where you
    contact on the ground plane. If you look at the effectiveness of RFI
    shielding coatings, they specify it in "ohms per square". Square what?
    It doesn't matter (inch, metre) - something to do with topology.

    So it will probably depend only on the copper thickness. And as John
    Larkin said, the nature of your contact points. Sorry, I don't know what
    the actual value would be, but maybe you could measure it on an actual
    one ounce or two ounce copper clad board before it is etched?

    Nemo
     
  6. Joerg

    Joerg Guest

    Ok then, lets assume a needle stuck into the metal. The local resistance
    is usually negligible.
     
  7. Joerg

    Joerg Guest

    Thanks, Tim, that one looks very interesting. Got to try that out.
     
  8. Joerg

    Joerg Guest

    That would be the best method, like what Robert Widlar would have done.
    Can you still buy that stuff?
     
  9. Joerg

    Joerg Guest

    Thing is, the plane won't be square, possibly not even perfectly
    rectangular.

    Can be done but iffy, has to be a four-point measurement because of the
    low ohms. Or a controlled current drive and then voltage drop
    measurement. But I'd have to cut a new board every time we change the
    shape of the little circuit during the idea phase. And I am down to
    three scroll saw blades :)
     
  10. Joerg

    Joerg Guest

    Just installed it. That software seems powerful but also seems to have
    quite a learning curve.
     
  11. Joerg

    Joerg Guest

    Yes, it sure would but mine doesn't :-(

    Now where's the stores with that Teledeltos paper ... ?
     
  12. Joerg

    Joerg Guest

  13. Joerg

    Joerg Guest

    A large swath of foil would work. On a hard surface so you can press a
    flat kind of contact onto it. Once when we did stuff like that at a
    large semiconductor company one of the big shots came by in his
    expensive suit. "Dudes, we are not on kindergarden here"
     
  14. Joerg

    Joerg Guest

    I thought about that but it's too irregular.
     

  15. Um, won't the resistance always be the same?

    Ohms per square and all.

    Doesn't matter per square what.


    --
    Many thanks,

    Don Lancaster voice phone: (928)428-4073
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    rss: http://www.tinaja.com/whtnu.xml email:

    Please visit my GURU's LAIR web site at http://www.tinaja.com
     
  16. Joerg

    Joerg Guest

    That's a comparatively miniscule variation for such huge diameter
    changes. Or in RF-speak just a few dB :)

    Can you send a copy of that file (reply-to works)? Then I can use that
    as an example or starter. Well, maybe. I am not very good with software
    that uses pictograms. Probably I won't get to it anymore this week but
    it would be something worthwhile to learn.
     
  17. I like John's idea of using regular old copper clad and scaling it.
    You can even solder to it. Put a few amps through it and the voltages
    become quite reasonable. If you've ever searched for a short between
    power planes on a multilayer board..


    Best regards,
    Spehro Pefhany
     

  18. Haha "real live calculus"! I did some for a project, it felt like the
    first time in 20 years. And it was that same problem! It was for thermal,
    but same equations I think - the temperature rise of a circular package
    soldered to a ground plane. There was definitely a ln(r1/r0) in there :)
     
  19. Nico Coesel

    Nico Coesel Guest

    Can't you model it in pspice by modelling the ground plane as a grid
    with resistors:


    +--R--+--R--+--R--+
    | | | |
    R R R R
    | | | |
    +--R--+--R--+--R--+
    | | | |
    R R R R
    | | | |
    +--R--+--R--+--R--+
    | | | |
    R R R R
    | | | |
    +--R--+--R--+--R--+

    Dunno how many nodes Pspice can cope with tough.
     
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