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OT: Low voltage digital circuit and shielded cable

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  • OT: Low voltage digital circuit and shielded cable

    I've installed some digital devices on a motorcycle and they don't require wire shielding as directed by the manufacturer.
    (With the partial exception of one component they recommend installing in the handlebar.)

    These components plus the LED lights only use 24-26 AWG however, due to my interest of running all the wires in the frame, I'd like to use a multi-conductor cable I have which happens to be shielded.

    Can I
    a) ignore the shielding?
    b) ground just one end?
    c) something else?
    Len

  • #2
    yes you can ignore it
    yes you can terminate 1 end of the shield; this is standard practice in aviation avionics
    no, you cannot ground both ends, this makes a loop antenna and gathers noise as per standard avionics

    it sounds like running wires inside the metal frame, the tubing frame itself becomes a crude shield in and of itself

    Comment


    • #3
      Don't ignore the shielding. Tie it to the frame/ground at one or both ends.

      Depending on how sensitive the electronics are to capacitance, the shield may cause problems, but it will probably be fine. Using a smaller-gauge wire will help offset some of the additional capacitance caused by the shield.

      Comment


      • #4
        Originally posted by andywander View Post
        Don't ignore the shielding. Tie it to the frame/ground at one or both ends.

        Depending on how sensitive the electronics are to capacitance, the shield may cause problems, but it will probably be fine. Using a smaller-gauge wire will help offset some of the additional capacitance caused by the shield.
        In all of my studying of doing VFD installations, a strong point was made while using shielded cable was to only ground one end, not both, especially for the motor wires.

        Sent from my SM-G950U1 using Tapatalk

        Comment


        • #5
          Originally posted by RB211 View Post
          In all of my studying of doing VFD installations, a strong point was made while using shielded cable was to only ground one end, not both, especially for the motor wires.

          Sent from my SM-G950U1 using Tapatalk
          Most fairly low-frequency stuff will work better with the shield tied at only one end, but the reason for this comes from using 50- or 60-Hz AC-powered equipment, where it is easy to set up a ground loop, and induce hum into the signal.

          The shield will be more effective if it is grounded at both ends. If it was a problem to do so, how would your Cable TV work?

          Comment


          • #6
            Every damn wiring diagram I ever worked with (aviation avionics) the engineer drawing stressed shielding, right up to the cannon plug. And yet when you get into repairs in the field, you may see hack-job repairs, ignored shields, broken shields, and so on. However, at no time have I EVER fixed the problem, and come to the conclusion, that the 'bad shield' was the ultimate cause of the original fault. That is only my 25years of avionics point of view.

            But then, all the engineers will tell you that lower hertz A/C power is the absolute WORST offender of noise, therefore requireing the BEST-UTMOST shielding. This is total horsekrap.
            If this was true then none of us could watch TV, Google, YouTube nor anything...........
            Did you ever look behind your entertainment center, desk, or whatever office place you are all setup?
            What do you see? 110v/60hz (unshielded) power all over the place, with power splitters rampant, Tv cables with dubious connections, internet wires with their own dubious connections, and yet, what????
            We are logging onto internet just fine. Watching TV just fine.
            The engineers opinion of shielding and noise is just that,,,,,,,,,a lot of noise.

            Comment


            • #7
              Originally posted by Ringo View Post
              The engineers opinion of shielding and noise is just that,,,,,,,,,a lot of noise.
              Well, as an audio engineer for the last 35 years, i can tell you that I have fixed many problems by correcting the shielding.

              And if I pay attention to it when designing a system, I will almost never have problems.

              Comment


              • #8
                There is a paper out published by Siemens that show that now with the advent or practice of equi-potential bonding, ensuring that there is no difference in potential between one ground point and another, it is possible to ground both ends of a shielded cable.
                As to the output of a VFD, I use metalic conduit, either solid or flexible, where the covering or shielding is ground at both ends.
                In the case of a vehicle, the ground conductor (frame) represents quite a mass which contributes to very little (to non) difference in potential across the ground plane, when several GND points are established..
                Essentially creating equi-potential bonding!
                Max.
                Last edited by MaxHeadRoom; 08-18-2019, 07:04 PM.

                Comment


                • #9
                  Originally posted by RB211 View Post
                  In all of my studying of doing VFD installations, a strong point was made while using shielded cable was to only ground one end, not both, especially for the motor wires.
                  All of the controls, we install, state ground the shield at the device end, not the controller end, except for one company that clearly states, to ground both ends of the shield, and if you don’t there will be operational issues.
                  jack

                  Comment


                  • #10
                    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?

                    Then again, even if I do run separate wires I'd bundle them in shrink tube for protection so I'm thinking...what am I thinking...back to "forget the shielding"?
                    Last edited by QSIMDO; 08-18-2019, 07:48 PM.
                    Len

                    Comment


                    • #11
                      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!

                      Comment


                      • #12
                        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.

                        Comment


                        • #13
                          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.

                          Comment


                          • #14
                            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, 07:59 PM.

                            Comment


                            • #15
                              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

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