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Long ground wire

Discussion in 'Electronic Design' started by [email protected], Aug 5, 2008.

  1. Guest

    Let's say you have a circuit that uses an opto-isolator to trigger a
    microcontroller with simple logic high/low. (npn output)

    The opto will have a steady DC voltage (through a resistor of course)
    on it's led but will use a switched ground to trigger the led.

    Now, let's say the pair of wires going to the switch is a couple
    hundred feet or even more.

    Everything works fine -- but of course, the led isn't going to be
    falsely triggered by noise, etc -- it needs it's 5 ma or so to "kick

    Wouldn't it still be good practice to do something to eliminate or
    reduce the effects of the long ground wire (acting as an antenna?)
    which would connect directly to the ground plane of the PCB?

    Certainly a well gorunded shield covering the pair would (probably?)
    suffice but would you also want possibly an inductor of some sort
    between the PCB and the ground (common) wire?

    To keep noise out of the PCB ground circuit?

    Actually what I have breadboarded works perectly, no issues, but I
    don't have a very noisy environment either.

    Thanks for any help.
  2. Rich Grise

    Rich Grise Guest

    Use a twisted pair. This could be shielded, but it's probably not

    To protect the LED from glitches, just put a .01 ceramic cap in parallel
    with the LED.

    Good Luck!
  3. Everything works good until a lightning storm.
  4. mkr5000

    mkr5000 Guest

    Using a twisted pair.....and like I say, all is fine.

    Of course, the opto is the way to go with something like this....I'm
    just sketchy on what happens when the entire ground plane of the PCB
    has a few hundred foot long "leg" on it.

    I'm talking about the IC pin grounds and everything else....I didn't
    know whether a choke in series would be a good idea?

    Of course, plenty of .1 caps on the supply buss.

  5. mkr5000

    mkr5000 Guest

    Phil --
    That's brilliant.

    You're saying just supply direct DC to the LED and then parallel 2
    resistors to the ground connection on the board (or even one would be
    an advantage).

    I'm just being a perfectionist and although this circuit has worked
    fine for a year or so, I was always concerned about least
    doing it that way would do no harm and you'd have the peace of mind of
    a "buffer" from your ground to the outside world.

    You're a smart guy.

    Have you seen this done in any equipment?
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