The reversing loop problem...

Discussion in 'DCC & Electronics' started by Chesticus, Mar 19, 2008.

  1. Chesticus

    Chesticus New Member

    I have had and continue to have problems with the reversing loop. The seems to be a consensus that the legnth of the train (cars and engine) should be less than the section that has been blocked. the reason for this is that the metal wheels of the cars can set off the reversing loop switch. But this is not practical on many layouts due to space and size concerns.

    Further, I have read that the section should only be as long as the longest set (consist) of engines you plan to run.

    I have also seen problems with the reversing loops where the blocked off section is too close to the switch, and there is a short that occurs at the frog. The short can occur as the engine (longer engines) hit the frog and the tender or rear wheels (truck) are still in the blocked off section.

    I know there are always "Bugs" to get out of the system, but I have never seen so many problems and solutions to a particular question.

    Any comments would be greatly appreciated.

  2. Nomad

    Nomad Active Member

    With all the problems I have been reading about, I think if my next layout has a reverse loop, I will stick to a good old fashion double pole double throw switch wired accordingly.

  3. Chesticus

    Chesticus New Member

    Yeah i know what you mean. I have 4 reversing sections on my layout, and the trouble is with only one of them. My others work exactly the way they are supposed to.

    I know that i will fix it as it really is nice to have the engine just go through.

    So I will keep trying.

  4. Nomad

    Nomad Active Member

    Good luck! Please let us know what the problem was when you get it fixed.

  5. pgandw

    pgandw Active Member

    The latter is only true if all of your cars have plastic wheels. As soon as you have any metal wheels in the rest of your train, you now have the ability to span both sets of gaps simultaneously if the train is longer than the reversing section.

    With DCC, the chance of both sets of gaps being spanned simultaneously is actually not that high, so you can often get away with a train longer than the reversing loop. What happens is the auto-reverser is continually flipping the polarity back and forth as first the exiting gaps are spanned and then the entering gaps are spanned by a long train with metal wheels. Since changing polarity does not change direction in DCC, no harm is done as long as the entering and exiting gaps do not get spanned at the same time.

    This problem has nothing to with the reversing loops. The turnout must be thrown correctly for a powered frog to match polarity with the chosen diverging path. The gaps for the reversing loops can be the same gaps that isolate the frog, but have to be in both rails. However, as you point out, a short circuit will occur when wheels of the approaching train span the gaps isolating the frog while the turnout is set the against the train.

    Unfortunately, there is a lot of misinformation bandied about on wiring, especially reversing sections and turnouts. I do recommend a good book on model railroad wiring - with lots of diagrams - if you don't understand electrical circuits pretty thoroughly. A good wiring book is this one: Kalmbach Publishing: Model Railroading: Books: Wiring / Electronics: Easy Model Railroad Wiring, 2nd Edition.

    I have no way of proving to you in advance that what I'm saying is more accurate than anybody else's (mis)information. I may not know much about airbrush painting; or using foam for scenery; or building, painting, and weathering structures. All I can say is trust me, I do know my wiring inside and out. Learning to wire model railroads as a kid is what got my interested in my career as an electrical engineer.

    hope this helps
  6. Squidbait

    Squidbait Recovering ALCO-holic

    Which is why you should stagger your insulated joints so they're not opposite each other, and you'll eliminate any chance of freightcar wheelsets shorting out. Locos are another matter... :)
  7. pgandw

    pgandw Active Member

    NO! wall1

    A reversing short occurs because the same rail on both sides of the gap has different polarity, and is bridged by a metal wheel (not a wheel set!). Staggering gaps does nothing to prevent this short.

    The auto-reverser detects this short circuit. And before the DCC system circuit breaker can react to the short circuit, the auto-reverser flips the polarity of the reversing section so that the polarity is matched on both sides of the wheel gap. Because polarity does not affect locomotive direction in DCC, the polarity flipping goes unnoticed by the human eye.

    The reason for slightly staggering gaps on reversing loops - about 1/8" is recommended - is to prevent a "double" short. Wheels on both rails bridging the gaps of opposite polarity at the same time could possibly confuse the auto-reverser. A slight staggering of the gaps allows only one wheel to cause a short at any given moment. The auto-reverser reacts correctly, and flips the polarity before the other wheel spans its gap.

    hope this helps reduce confusion
  8. Squidbait

    Squidbait Recovering ALCO-holic

    Ah... good point. I'm still on my first cuppa, so I'm not thinking straight yet. :D
  9. Nomad

    Nomad Active Member

    So, it sounds like the best thing to do is make the gap big enough so that the metal wheel does not connect the two gapped rails at the same time, correct?

  10. pgandw

    pgandw Active Member

    If you make the gap big enough that a wheel can't span it, it sure ain't going to be smooth running.

    Guys - it's really pretty simple. Don't run trains longer than your reversing section, and there will be no problems. DC or DCC.

    If you need to run longer trains, lengthen your reversing section. On a reversing loop, both train length and reversing section length are limited by the length of track between the 2 paths of the turnout. On a wye, the length of the shortest tail track limits both the train length and the reversing section length. In all cases, you can make the reversing section as long as the longest train you can physically reverse. It may take a little more effort and study, and some extra gaps and wires, but it can be done.

    Either that, or go to 3 rail or overhead electric where there are no wiring issues for reversing sections. :mrgreen:
  11. Chesticus

    Chesticus New Member

    This has been a great discussion. So here is where I am at with this problem. Insted of going longer witht he section ( I do not have the room) I went shorter. Even though this goes against the wisdom that has been stated here, I did it anyway. My reversing loop module is not the problem. The engine is going throgh the section just fine with the shorter blocked of track. But I did find a problem with power getting to the track between the blocked of section and the switch. It is not a short like I thought it was. My mistake. So I am going to drop some feeder wires to these sections of track and see if this cures my problem.

    Now I am not disregarding anything that has been said here. I am little disapointed about the reversing section needing to be longer than my entire train. I do not have the room for this. I will when I build my next layout, but not now. I have not really had any trouble with the reversing sections of my track until I went from Bachmann to MRC. There are issues with my 5 amp booster seeing the short before the MRC can switch (I have read that they are slow), so I went with my 1 amp power supply. It is tough as all of my engines are DCC and sound. But I am finally seeing a little light at the end of the tunnel. And I am gaining a great deal of experience for my next project.

    I have all of the books "pgandw" and beyond. But they are just the beginning. That is why forums like this are so handy. You can get input from "electrical engineers" that clarify everything.

    let the debate continue, I am learning a great deal.

    I will post on what happens next.

  12. baldwinjl

    baldwinjl Member

    One of the tricks with reversing sections is that with some creativity you can "move" the section controlled by the reverser to a location that doesn't seem to be where the reversing section is. Sort of like reversing the main, rather than the loop in DC block/cab control. I agree with the earlier statement, there is alway a way to make the reversing section as long as anything that you can physically reverse, it just might not be obvious!
  13. Gary Pfeil

    Gary Pfeil Active Member

    This is just what I was trying to say, in the other thread about reverse sections. DCC reverse modules really require a different "mind set" about deciding on boundaries of a reverse block than dc. You can include trackage that can also be used without ever requiring reversing, just make your block as long as required. The only caveat I can think of is to keep in mind you want to avoid having more than one train crossing gaps at any point on the reverse block, at the same time. For the same reason the reverse block needs to be longer than a train, which is to prevent metal wheels from bridging a gap at both ends at the same time. I tried running a train longer than I degined for, to see if I would actually have a problem, it seemed unlikely wheels would cross at the exact same time. Well, it was far from dependable! So follow the advise, and remember to forget how you did it with dc.
  14. baldwinjl

    baldwinjl Member

    Yes, that's the other side of the coin. The basic point is that you don't want to be crossing multiple boundaries of the "reversing" section at the some time, with the same train, or with different trains. One thing which gets lost in this discussion is that exactly the same rules exist in DC operation. A short is still a short.
  15. hickstmj

    hickstmj Marcie

    I thought I knew what a reverse loop is but after reading this I am confused. If the train is longer than the reversing loop won't the engine run into the rest of the train when it exits the loop?

    Thank you for any explanation you can give.
  16. baldwinjl

    baldwinjl Member

    A reversing section does not have to be the classic "balloon". It can be a cutoff across an oval (though that one sort of turns into a balloon, if you look at it right), or any other means that gets the train reversed (such as a wye). Point being, it doens't have to look like a loop.
  17. MasonJar

    MasonJar It's not rocket surgery

    As above, it doesn't have to look like a loop or balloon to be reversing - a turntable is also a reversing loop (at least electrically).

    Basically, if you can end up back on the same piece of track, but pointing in the opposite direction from when you started, you have gone through a reversing loop at some point in your travels.

  18. hickstmj

    hickstmj Marcie

    Thank you both for your explanations.

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