Should DCC bus wires be twisted?

Discussion in 'DCC Control' started by David Mitchell-Todd, Sep 11, 2020.

  1. Sol

    Sol Full Member

    Messages:
    631
    Likes Received:
    146
    Joined:
    Mar 13, 2016
    Now in reply to one of the posts, DCC is perfect for loco control but many in the USA are now going to LCC for signals etc
    https://www.nmra.org/search/node/lcc

    regarding not having a complete loop for the DCC bus - well to confuse everyone, I do have a complete loop 12 x 20 ft layout ,as my signalling circuit requires common return so not only the track power but signal relay 40 v, LED 12 & other 12v relay power ( all separate transformers) use a common ( as did telephone exchanges with different voltages etc )
    Mark Gurries, a reasonably knowledgeable DCC user https://sites.google.com/site/markgurries/home/about-mark-gurries did say to me when I questioned him about a loop and the "story" how a decoder coyuld get confused getting command signals from two directions. he pointed out the the time differnce is almost so small, it would affect a thing - it would need al ayout of over 1mile/ 1.5km in length to give a difference.

    Twisted buss - yes but if not, then keep the two buss wires a a few inches/ cms apart.
     
    Andy_Sollis likes this.
  2. Jim Freight

    Jim Freight Full Member

    Messages:
    1,099
    Likes Received:
    875
    Joined:
    Sep 9, 2019
    Wrong place :hammer:
     
    Last edited: Sep 14, 2020
  3. Jim Freight

    Jim Freight Full Member

    Messages:
    1,099
    Likes Received:
    875
    Joined:
    Sep 9, 2019
    I thought the loop/circuit length would be in that ball park because it is related to the signal wavelength, and at approx 100kHz (I believe) that is almost 3km.

    My parallel buss wires are nominally 3 - 4 cms apart, but the rails are not, oh dear! :facepalm:

    Jim :avatar:
     
    jakesdad13 likes this.
  4. jcm@gwr

    jcm@gwr Full Member

    Messages:
    50
    Likes Received:
    32
    Joined:
    Aug 8, 2020
    You'd be okay with Gauge 1 at 45mm!
     
  5. Jim Freight

    Jim Freight Full Member

    Messages:
    1,099
    Likes Received:
    875
    Joined:
    Sep 9, 2019
    :giggle: If I could build my railway in Gauge 1 it would be steam powered, ride on or RC control. :rolleyes:
     
  6. Jim Freight

    Jim Freight Full Member

    Messages:
    1,099
    Likes Received:
    875
    Joined:
    Sep 9, 2019
    :gday: No apologies necessary Gormo. :)
     
  7. Bernie

    Bernie Full Member

    Messages:
    53
    Likes Received:
    36
    Joined:
    Sep 29, 2020
    One thought about the rails and buss, is that I understand electricity finds the path of least resistance. So does this mean that with every rail soldered to the buss, and the buss being lower resistance copper wire, the buss runs the power around the layout. My assumption is that the track with the loco drawing power is the only piece carrying the current till it gets to the dropper. If so, then this could explain why the rails do not need to be twisted or separated. The track will have the voltage potential but no current, so don't test my theory with your tongue :avatar:. Sorry, does not answer the question of twisting the wires or not.
     
  8. Jim Freight

    Jim Freight Full Member

    Messages:
    1,099
    Likes Received:
    875
    Joined:
    Sep 9, 2019
    Not quite.

    Given two paths, one twice the resistance of the other, e.g. nickel silver rail with dubious joints and a solid piece of copper wire of the same cross-sectional area.

    Both are connected to the power supply at the input end and both connect to the loco at the far end.

    The power supply applies a voltage to the input end and the loco will complete the circuit at the far end and allow current to flow.

    If as an example the rail resistance is twice the wire resistance, 2/3 of the current will flow through the wire and 1/3 through the rail.

    So both will pass current, but the proportion of the total depends on their resistance relative to each other.

    Twisted or not is another issue altogether.

    Ok? :scratchchin:
     
    jakesdad13, Andy_Sollis and Bernie like this.
  9. David Mitchell-Todd

    David Mitchell-Todd Full Member

    Messages:
    88
    Likes Received:
    51
    Joined:
    Sep 9, 2020
    Maybe for DCC there is an argument for having insulated rail joiners between all track sections? That way the data signal is only passing along a single copper wire. Then there is no risk of duplicate signal passing along the tracks and thus confusing the decoder?
     
  10. Keith M

    Keith M Staff Member Moderator

    Messages:
    4,446
    Likes Received:
    2,902
    Joined:
    Dec 16, 2015
    If you had a large enough layout to need it splitting into separate sections electrically for short-circuit protection, then you would use insulated rail joiners and individual circuit breaker protected sections to electrically isolate each section together with separate bus sections. When it comes to running trains however, as the metal stock wheels pass over each insulated rail joint from one section to the next, it will bridge, if only momentarily, two sections together. Not much you can do about that, and the only disadvantage I can see in this extra section protection.
    Keith.
     
    Jim Freight likes this.
  11. Jim Freight

    Jim Freight Full Member

    Messages:
    1,099
    Likes Received:
    875
    Joined:
    Sep 9, 2019
    Hi David, to my knowledge that has never been an issue as the track and wiring lengths involved are nowhere near the the nominal wavelength of DCC signals.

    Poor electrical pickup, causing intermittent disruption of data flow (mangling messages) and electrical noise (high voltage ripples and spikes), is the biggest issue facing users of DCC.

    Jim :scratchchin:
     
  12. Jim Freight

    Jim Freight Full Member

    Messages:
    1,099
    Likes Received:
    875
    Joined:
    Sep 9, 2019
    That will even create more electrical noise, but as you say, not much we can do about that.

    I split my railway into multiple districts for two reasons, (1) it means the maximum current per district can be reduced to safer maximums (reduces chances of burning something), and (2) it is easier to find where a short has occurred when running multiple trains over a larger layout.
     
    BJU and Bernie like this.

Share This Page