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Thread: OT: Low voltage digital circuit and shielded cable

  1. #11
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    Grounding the shield at more than one place can lead to AC currents in the shield, which can couple into the signal conductors. If a design requires grounding at both ends, maybe they are using the shield as a signal return.

    Ed
    For just a little more, you can do it yourself!

  2. #12
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    Quote Originally Posted by QSIMDO View Post
    STEADY ON, lads!
    Motorcycle, 12 volts DC...I'm using a "Motogadget m-unit" if anyone's interested and a V.2 not the new blue tooth doo-dad.

    If I don't ground the shield would it be just the same as running separate wires or is the problem the twisted pairs in there are in close proximity?
    Your thread wandered a bit, didn't it?

    You'll have no problem with ground loops (an AC phenomenon), no problem with RF interference (it's not avionics), in fact you'll have no problem at all.

    DC in those wires will not be bothered, and any switched/pulsed signals are in the audio range so capacitance is negligible.

    Ground the sheath or not - whatever is convenient.

    -js
    There are no stupid questions. But there are lots of stupid answers. This is the internet.

  3. #13
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    Quote Originally Posted by ed_h View Post
    Grounding the shield at more than one place can lead to AC currents in the shield, which can couple into the signal conductors. If a design requires grounding at both ends, maybe they are using the shield as a signal return.

    Ed
    Ummm, the AC you speak of would come from what source, Ed?

    -js
    There are no stupid questions. But there are lots of stupid answers. This is the internet.

  4. #14
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    For your motorcycle use... forget the shield and ground. Yes,m it will behave just like unshielded cable. Don't worry about the effects of twisted pairs either - you don't have balanced input/outputs.
    Last edited by lakeside53; 08-18-2019 at 07:59 PM.

  5. #15
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    Quote Originally Posted by lakeside53 View Post
    For your motorcycle use... forget the shield and ground. Yes,m it will behave just like unshielded cable. Don't worry about the effects of twisted pairs either - you don't have balanced input/outputs.
    +1
    absolutely.
    shields are over rated, and over engineered
    just wire it up and go for your bike ride

  6. #16
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    Thanks one & all!
    Better answers here than I've ever got on bike sites.
    I guess they just ride.
    Len

  7. #17
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    They are smart!

  8. #18
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    Quote Originally Posted by lakeside53 View Post
    They are smart!
    That's why we own more than one!
    Build, ride, repeat.
    Len

  9. #19
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    It's a bad idea to leave a shield floating. This will cause the capacitance of the cable to change depending on the signal passed through it.

    Will it cause problems with low-frequency digital signals on a motorcycle? Probably not, but it's almost as easy to tie the shield to ground as it is to ignore it....

  10. #20
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    Quote Originally Posted by Jim Stewart View Post
    Ummm, the AC you speak of would come from what source, Ed?

    -js
    In General, the most obvious source is the power grid. Pick two random ground points and there is likely to be at least some AC potential between them. Connect them, and you have an AC current. Varying potentials from atmospheric phenomena, like storms, would be on top of that. These examples assume true earth grounds, but the same principles apply in systems isolated from earth. On a car or motorcycle for example, there can be a number of AC noise generators, like the ignition system or electric motors. The noise gets into the chassis "ground", and it's potential can vary from place to place.

    Ed
    For just a little more, you can do it yourself!

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