Should DCC bus wires be twisted?

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

  1. David Mitchell-Todd

    David Mitchell-Todd Full Member

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    One of my tasks for this weekend is to run the DCC bus under my baseboard. I read recently that some people recommend twisting the wires to reduce cross-talk, any thoughts on this?

    I am going to use solid copper mains cable, as I have the remains of a drum lying around. It is not the most flexible stuff so I would prefer not to twist it unless I really need to. The total length will only be about 6-7 metres and I was planning a complete loop, not open ended.
     
  2. Jim Freight

    Jim Freight Full Member

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    My busses are not twisted, bare tinned copper wires in pairs held loosely to the frames, enabling easy connections of droppers and modifications, only the droppers are lightly twisted together so avoiding too much of a birds nest.

    There are specific values for twisting but it seems for most of us this is not really an issue, and besides what seems to be overlooked wherever I read about this is that our rails cannot be twisted :giggle:

    Personally I would not have a complete circular bus, but this really depends on the size of the layout, signalling circuits don't always appreciate a loop, in DCC the bus is as much for signalling as power distribution.

    The cross section of the wire needs to be such that at the furthest point on your layout from the DCC booster/control unit it can provide the required current and be such that a short across the rails there causes the DCC booster/control unit to shut off the power.

    You may need to consider how you are going to strip it for connecting the droppers, especially if the wire is UK ring main grade.

    Jim
     
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  3. Kimbo

    Kimbo Staff Member Moderator

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    David,

    I think you will find that on a smaller layout that wire twisting is not essential, but is the norm. It really only causes issues on larger bus runs on large layouts. (Definition of large- well again depends who’s advise you read, some say a bus length over 5m others 10m)

    My advice would be to keep the bus wire as short as possible, so assuming you have a total length of 7m , split the bus into a “T’ and have the control box central to the layout if possible, thus giving to 2 buses 3.5m approx.

    More important to make sure you use good quality multi-stand dropper wires which are correctly soldered to every piece of track work including pointwork. There’s no need to twist the dropper wires together other than to keep them tidy. Again good practice is to solder the droppers to the bus wire, but I’ve used several methods on different layout builds all with good results.

    Re Jim’s point about a complete circle bus wire is I believe a big no no.

    Some of the things I do when planning the bus wire runs:-

    1) keep them as short as possible.
    2) keep aux buses wire for lighting etc at least 100mm away from the main bus wire.
    3) Don’t run mains power cables close to the bus wire.
    4) Droppers on EVERY piece of track, no matter how small in length and solder to the track, not the fish plates.

    kim
     
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  4. Walkingthedog

    Walkingthedog Full Member

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    This style wire stripper are ideal.
    016EB6E9-4419-400E-9484-97FA31F4190A.jpeg
     
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  5. Bonky

    Bonky Full Member

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    Hi David,
    the short answer, I understand is 'yes'. I certainly twist mine.

    BW

    R
     
  6. Bonky

    Bonky Full Member

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    Jim, I believe I read somewhere that IF one wishes to exhibit at a show a bare-wires bus is recognised as a potential fire hazard and won't be allowed.

    Can anyone confirm?

    Rich
     
  7. Walkingthedog

    Walkingthedog Full Member

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    I think it just makes soldering droppers more difficult.
     
  8. David Mitchell-Todd

    David Mitchell-Todd Full Member

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  9. David Mitchell-Todd

    David Mitchell-Todd Full Member

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    Now THAT is a good point! I think I will with untwisted but not the loop. My plan is to have the controller connected to the end of the layout so have two sets of bus lines going down each of the long sides of the baseboard. So more of a Y than a T.
     
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  10. David Mitchell-Todd

    David Mitchell-Todd Full Member

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    I did wonder about this, I wondered if was overkill but thought it wouldn't hurt. I will definitely solder to the track as some of it is so old most of the fishplates could be used as insulated joiners!
     
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  11. Jim Freight

    Jim Freight Full Member

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    Maybe, wouldn't be surprised, however my railway isn't going anywhere, it's built to outlast me and it's surroundings :avatar:
     
  12. Jim Freight

    Jim Freight Full Member

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    I only use the fishplates to align tracks and keep others apart e.g. with insulated fishplates. When I first started building my layout I found that many new fishplates did not grip the rails well enough for DC let alone DCC, many places sell pre-wired fishplates but if you are able to solder sound, shiny joints yourself IMO avoid them.

    Solder as much as possible, every crimp, screw or clamp can loosen, copper compresses with age. A classic example is the solid copper wire used for UK ring main circuits, sooner or later a socket can get a bit warm under heavy load because the copper has crept from being squeezed by a screw terminal, effectively loosening the connection and caused increased resistance, which in turn with current generates heat. Same applies to stranded wire in plugs.

    If the layout is to have a long useful life solder if you can, and I define "useful life" as at least 10 years existence, although model fishplates don't wear as such they are likely to weaken their grip with time. DCC does not like poor connections.

    Jim
     
  13. Walkingthedog

    Walkingthedog Full Member

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    Droppers every section are the way to go. I have done so on my small ish O gauge layout.
     
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  14. Walkingthedog

    Walkingthedog Full Member

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  15. Andy_Sollis

    Andy_Sollis Full Member

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    I’m struggling to understand this? Are we therefore saying if we had a simple oval of track above the board we should have an insulated fishplate installed to stop the full circuit?

    I don’t see the difference between a circle bus below the board cos as soon as it’s connected via any droppers, the one above (track) does the same? :scratchchin:


    And as to twisting, I was never told by my electrics teacher there was ever an issue.. coiling would be the issue as that causes induction if I recall correct, ie starting to form an electromagnet.

    Food for thought? :cheers:
     
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  16. Jim Freight

    Jim Freight Full Member

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    I have multiple busses because I have split it into districts for ease of fault detection, but the longest are probably about 7 to 8m long without issues.

    That's another thing that seems to be overlooked in the theoretical information I have found, the buss and the track are parallel paths connected at intervals making a ladder of wiring, and then you load it with an electrical noise generator called a loco which as it moves varies the electrical paths, remarkably it still works, providing your loco is in good order. :scratchchin:

    Jim
     
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  17. Jim Freight

    Jim Freight Full Member

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    For your thoughts try these nibbles :-

    1) Yes I split my circles of track too.

    2) Twisting the wires causes the wires to induce into each other and nullify any radiation from them to other devices. The number of twists per unit length is based on the frequency of the signals being carried by the wires, apart from noise spikes, it is relatively low frequency.

    3) To stop them being affected by imperfect sources a screen would be wrapped around them which is grounded at one end (not at both creating a loop), typically used in audio cables. In DCC I just leave a few cms (e.g.3 - 5cm) between DCC cables and other potentially noisy wires, e.g. 240VAC mains, 16VAC for points and definitely wires operating electromagnetic point solenoids.

    Jim :cheers:
     
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  18. David Mitchell-Todd

    David Mitchell-Todd Full Member

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    From what I have read elsewhere that is correct. Complete loops are good for DC, but bad for DCC.

    With regards to twisted wire, that is a recognised thing in other applications. Twisted pair wire is commonly used in data transmission. However, I remain to be convinced it is critical on my 2.4 x 1.2m layout.
     
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  19. Jim Freight

    Jim Freight Full Member

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    My droppers are twisted for tidying up wires between rails and buss which is easy as I use a single strand wire for my droppers.

    If we had to twist all the wires what would we do with the rails which run in parallel to the buss?

    Jim
     
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  20. Walkingthedog

    Walkingthedog Full Member

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    I agree with you Andy.
     

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