DCC reverse loop length

Discussion in 'DCC & Electronics' started by TrainNut, Jan 25, 2008.

  1. TrainNut

    TrainNut Ditat Deus

    In setting up a reverse loop, I seem to remember something about how the length of the reverse loop should be longer than your longest train. What happens to the circuit if it's not? The electrical portion of my reverse loop will be five feet but I will have two trains for sure that are a bit longer than that.
  2. pgandw

    pgandw Active Member

    With DCC it's almost but not quite as critical to have the reversing section longer than the longest train using it.

    In conventional DC, while the train is in the reversing section, you change the polarity of the main track (outside the reversing section). This means the polarity of the track leading into the reversing loop and the reversing loop are now opposite. If part of the train hasn't entered the reversing loop yet, every metal wheel that crosses the gap between the 2 sections is going to cause a short circuit.

    In DCC, you change the polarity of the reversing loop while the train is in the reversing loop. Since track polarity does not control direction in DCC, this works fine. But the entry track outside the reversing loop and the reversing loop are still opposite polarities when the change is made. Metal wheels crossing the gap will still cause short circuits.

    But if you use an auto-reverser in DCC, the auto-reverser senses the short circuit and changes the polarity of the reversing loop before the circuit breaker can shut things down. In fact, the auto-reverser does not even act until the locomotive causes a short at the exit end of the reversing loop. If part of the train is still entering the reversing loop when the polarity is swapped, then metal wheels will cause the auto-reverser to reverse again. Then metal wheels at the exit end will again cause a short and cause the auto-reverser to switch the polarity back. The auto-reverser will keep flopping the polarity until there are no more shorts at the entry end. The bad news comes if metal wheels are bridging the gaps at both ends of the reversing loop at the same time. The auto-reverser can't save the day by switching polarities anymore, and the DCC system will shut down with a short circuit indication. And it's possible that the auto-reverser may prematurely release its magic smoke in disgust with your treatment of it.:eek:

    Solutions: No metal wheels or lighted cars in the section of train that is longer than the reversing loop (and don't ever forget to do this!). Or extend the reversing section to accommodate your longest train. Or run trains that will fit in the reversing loop entirely.

    Hope the explanation made sense
  3. baldwinjl

    baldwinjl Member

    To simplify Fred's totally correct explanation...
    A short a both ends of the reverse section at the same time is bad. And all that takes is metal wheels crossing the gap. So, the safest thing is to have the section longer than your longest train. Otherwise, you will have to be careful about what you run through the section.

  4. TrainNut

    TrainNut Ditat Deus

    Hmmm, sounds like I could chance it but it'd be like playing Russian roulette.
  5. rogerw

    rogerw Active Member

    Train Nut What about the physical length of the loop. If you have a reverse loop the train can only be as long as the loop minus a couple of inches where they meet back at the switch(where the gaps would be). The tail end has to be out of the switch when the engine comes back around. Does that make sense, did I explain it ok?
  6. TrainNut

    TrainNut Ditat Deus

    Yes, if it were a simple loop, that would be the case, however my loop does not immediately close back on itself removing the physical aspect of that hinderance. To further explain this, look at my arrival/departure track... the design has been updated here...
    As it passes around the roundhouse, between the two switches is where I will wire in the modulator... now a distance of about 7'.

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