DC/DCC Wiring

Donstaff Mar 7, 2014

  1. Donstaff

    Donstaff TrainBoard Member

    215
    28
    16
    I'm working on final plans for modifications to a train club N-scale layout and wondered if I could pick your brain before making any big mistakes. The attached file shows the present three-level layout with dotted lines representing the location of proposed grades that will be added to enable movement between levels. At the beginning (lower) point on each level insulated rail joiners will isolate the three separate levels from each other.


    Now the interesting part, where of course, I need some help. The layout must be wired so that each level can be switched to operated on either DC or DCC. I've already wired the lower (outside oval) that way and it works perfectly. With the expanded wiring connecting the three levels, will it work to have a load such as an 12 V. automotive bulb across the insulated track joiners to absorb the extra current draw generated when a locomotive has one set of trucks on one side and the other set on the other side of the junction between levels? And if that works, would this work with a locomotive exiting a level that is running on DC and entering an adjacent level, which is switched to run off of the DCC controller, or visa versa? Finally, on the top (inside loop) level, where a Kato double crossover will replace the existing crossover, will reversing modules be required on either or both sides?

    Thanks in advance for any advice you may have to offer. View attachment N-Scale.pdf
     
  2. PaulBeinert

    PaulBeinert TrainBoard Supporter

    622
    1
    13
    Don,
    I think you could run into some issues crossing from DC to DCC or DCC to DC but hopefully someone with more knowledge can answer that.
    For the double crossover, one of those loops will need to be a reversing loop. If you decide on the left one then make certain that the right side is wired match up with the 3% grade and what it joins to.
     
  3. TwinDad

    TwinDad TrainBoard Member

    1,844
    551
    34
    This sounds like a Bad Idea (tm).

    At the point where the loco bridges the gap, you'd be placing up to a 12-18V DC (depending on your scale and the loco's speed) bias across both the loco's decoder inputs and the booster's drive outputs. And a 12-18v AC bias against the DC throttle's outputs. NMRA standards require the decoders to withstand at least 27V at the rails, so there would be some protection, but I think this would be putting an unnecessary stress on the components, and under certain circumstances you could easily exceed the 27V by a wide margin.

    Another problem would be... while running on the DC part the decoder would be in "Analog Mode". I can't say for sure, but I would not be at all surprised if the loco would need to be restarted/rebooted in order to switch to "DCC mode". Hence, even if you don't fry the decoder inputs, as soon as the loco hit the DCC section, because it's still in DC mode it would zoom off at full speed, instead of starting to listen for DCC commands. In the reverse direction, I'm not sure *what* the loco would do, but it's probably not what you'd intend.

    And then there's the boosters. They should be able to withstand *some* DC bias on their drive outputs, but at best this will unnecessarily wear on the components. At worst, it will fry a very expensive piece of equipment.

    It's generally best to keep DC and DCC sections of the layout completely isolated from each other.

    Might I suggest an alternative if you really need to transit between the two in real time? Have a siding or at least a section of track that the train can stop on, have the DC power removed, then DCC applied (or vice versa in the opposite direction) before resuming travel? You could accomplish this by running the feeders from that siding or track section through a DPDT Center-Off switch with one leg to the DC and the other to DCC. The train pulls up, stops. Click-Click, power is switched to the other system, then the train resumes. Wired correctly at no time are the two power sources crossed, the loco is properly "rebooted" in the right mode, and operationally it's just a brief stop.

    Plop a station and a semaphore down and call it a stop for new "Train Orders". Practically speaking the engineer's going to need to switch throttles anyway when moving to the new system.
     
  4. TwinDad

    TwinDad TrainBoard Member

    1,844
    551
    34
    Your proposed grades would make good track segments to do the isolation trick I suggested. Except for (a) the bit about stopping and restarting a train on a grade, and (b) providing some kind of "interlock" to make sure the operator doesn't forget to flip the power-source switch.
     
  5. Donstaff

    Donstaff TrainBoard Member

    215
    28
    16
    Thanks again TwinDad. Your responses have helped very much. After incorporating several of your suggestions, our layout has the DPDT switched DC/DCC blocks you suggested and they work great. More specifically, we have refined the top level to look like the second image. I would appreciate your critique on whether this will work and what wiring improvements could be made to the top level with the double crossover to make that level more DC-friendly.




    N level 3 RMS.png
     
  6. Eagle2

    Eagle2 Staff Member TrainBoard Supporter

    5,727
    479
    82
    That certainly sounds like a good idea, both from an electrical standpoint and from an operator's point of view.
     
  7. RT_Coker

    RT_Coker TrainBoard Supporter

    516
    32
    13
    I believe that you can have a short when your switches are in the “DC” position and a locomotive bridges a reverse-loop gap. But maybe you are not showing the DC-blocks on this part of the layout.
    Bob
     
  8. Donstaff

    Donstaff TrainBoard Member

    215
    28
    16
    Yes, I agree that I think I would have a short in the "DC" position if a DC loco tries to negotiate a crossover instead of going straight. As currently wired, neither the reverse modules nor the DPDT switches are installed yet. Currently, only the bus drops are installed and the wiring works without shorting only when going "straight". Is there any way to it can be wired to also enable crossovers when running of
     

    Attached Files:

  9. Donstaff

    Donstaff TrainBoard Member

    215
    28
    16
    I've updated and attached the track plan drawing which shows all three levels of which the top level contains the Kato double crossover.
     

    Attached Files:

  10. RT_Coker

    RT_Coker TrainBoard Supporter

    516
    32
    13
    It sounds like you do not have switches that allow you to reverse the dc polarity to the track sections. If you did, you could set them so that a DC loco could negotiate the crossover. If a DC loco can not negotiate the crossover, then a DCC loco will also not negotiate the crossover.
    If you what a little, I suspect that a real expert will post with better help than I can provide.
    Bob
     
  11. Donstaff

    Donstaff TrainBoard Member

    215
    28
    16
    CCFhrrns.2.jpg N level 3 RMS.png
    Bob,
    The crudely drawn schematic on the left illustrates how I switch between DC and DCC in the layout. The drawing on the right, of just the top level which contains the double crossover, also shows how a reversing module will be wired into each lobe of the dog bone. The DPDT switches are intended to allow switching the isolated sections between direct power from the bus, for use in DC operations, and power through the reversing modules for use in DCC operations. When switched for DCC operations, the reversing modules should allow crossing over. I believe that a DC engine, upon encountering this would cause the RM to fire and cause the DC engine to reverse direction. Hopefully, someone will tell me whether I have that part right or not.

    Looking at the scenario where I have this level running on DC rather than DCC, I'm wondering how I could wire additional switches so that when the double crossover is switched to cross over rather than to go straight, said switches would reverse the DC polarity and allow the engine to crossover and continue in the same direction.
     
  12. RT_Coker

    RT_Coker TrainBoard Supporter

    516
    32
    13
    It looks like the “experts” do not want to advice someone on a combined DC/DCC wiring (can’t say that I disagree). I am not the right person to check you wiring plain.

    But the fact: the DC wiring works without shorting only when going "straight", indicates a big problem! It indicates that the DC & DCC connects are such that the two outside-rails of the crossover are connected to the same-polarity of the power source. This is why the DC loco will not make it across the crossover without creating a short. It is also the same reason that a DCC loco will not make it across the crossover without creating a short. For your DCC configuration of the layout and auto-reverse units, the crossover needs to be connected so that the two outside-rails are connected to OPPOSITE-polarities of the DCC power source.

    Hope this helps.
    Bob
     
  13. Donstaff

    Donstaff TrainBoard Member

    215
    28
    16
    Thanks Bob,
    Isn't what the reversing module does - sense the short produced by the reverse polarity and reverse it? I understand that the EM will noe work for DC powering, but why would it not work for DCC?
     
  14. RT_Coker

    RT_Coker TrainBoard Supporter

    516
    32
    13
    Unless I am misreading your layout your auto-reversing units are not changing the polarity of the crossover. They are reversing the two outside loops.
    Bob
     
  15. Donstaff

    Donstaff TrainBoard Member

    215
    28
    16
    Bob,
    I am getting evev more confused. What I am trying to figure out is how to get through the crossover. In operating in both DC and DCC, it al works perfectly as long as I go straight. If I flip the double crossover to cross over, the engine stops. If I'm running I. DC, and physically pull the engine forward a few inches and release it, it goes in reverse; lf I reverse the power surpply dirsction the engine goes forward. The same thing happens when running on DCC, except that when it stops at the crosskver point, it will continue going forward after it is pulled across the crossover point. Can the reversing modules be wired to make the engine automatically continue
    in the same way that they perform the electrical switch on reverse loops?
     
  16. PaulBeinert

    PaulBeinert TrainBoard Supporter

    622
    1
    13
    I have not given any thought to DC mode at this point but if you wire just the double crossover (plus about 12" of track on each end) to the auto-reversing module, this should work just fine, unless you are using metal wheels and then we have to consider the maximum length of your trains.
     
  17. Donstaff

    Donstaff TrainBoard Member

    215
    28
    16
    Paul,

    If you'd look at the attached drawing, the Kato double crossover is divided at the crossing point. The left side is totally isolated from the right side. Does that mean that if one set of the leads from a reversing module is connected 10 or so inches to the left side of the double crossover's center divide, with the other set of leads bridging across the insulated rail joiners to the left of these insulated joiners, then only one reversing module is needed? (I'm assuming that that would mean that when the engine bridges the insulated gap on the crossover, it would trigger the reversing module.) Or, are you saying that the reversing module should be connected across the crossover, bridging the gap in the double crossover? Could you provide a sketch?

    Don
     

    Attached Files:

  18. PaulBeinert

    PaulBeinert TrainBoard Supporter

    622
    1
    13
    Don,
    I am not at my computer to do a diagram but based on a information on trainee.com, I would change my approach.

    the top and bottom rails run all the way thru. Call the top rail A and the bottom rail D, now label the other two rails as B and C (top to bottom).

    Rails A and C need to be the same polarity on both ends and then rails B and D need to be the same polarity.

    is this how you are wired?
     
  19. RT_Coker

    RT_Coker TrainBoard Supporter

    516
    32
    13
    The auto-reverse unit on the presently wired crossover will not work, because of the way the crossover is connected to the power. The wires on either the top or bottom of the crossover need to be changed so that the top and bottom rails of the crossover have opposite connections to the power source. A volt-meter or something that will show the presents of a voltage should be used to check that the DC voltage is across the top and bottom rails of the cross-over.

    ------------------------------------------------------ <-- Top-rail
    |||||||||||||||||||||||||||||||||||||||||||||||||||||
    ------------------------------------------------------ <-- needs to be same connection as Bottom-rail
    ------------------------------------------------------ <-- needs to be same connection as Top-rail
    |||||||||||||||||||||||||||||||||||||||||||||||||||||
    ------------------------------------------------------ <-- Bottom-rail
    When crossing over the loco connects the Top-rail to the rail above the Bottom-rail and the Bottom-rail to the rail below the Top-rail!

    References:
    http://www.wiringfordcc.com/intro2dcc.htm
    https://sites.google.com/site/markgurries/home/dcc-with-dc
    Bob
     
  20. Donstaff

    Donstaff TrainBoard Member

    215
    28
    16
    Yes, this is how it is wired.
     

Share This Page