DCC and Reversing Loops

Discussion in 'FAQs' started by roryglasgow, Nov 2, 2001.

  1. roryglasgow

    roryglasgow Active Member

    I think pupparo asked a similar question in the N/Z Scale forum, but here goes:

    How do reversing loops work with DCC? Does someone have to flip a switch to reverse the track polarity, or does the equipment somehow figure it all out?


  2. shamus

    shamus Registered Member

    Hi Rory,
    reversing loops are still reversing loops whether you are using DCC or not, here is a simple diagram using DPDT switches. There is on the market a reversing module which does the job for you, but you still need to isolate the tracks from one another.

  3. Gary Pfeil

    Gary Pfeil Active Member

    DCC reverse loops can be wired to devices which automatically change the polarity of the loop to match the polarity of the incoming/outgoing track. Both rails must be gapped. The reverse units have four wires, two connect to main line track power (the bus wires for track power) and the others to the rails of the loop. There are several manufacturers, I am only familiar with MRC and Tony's. You can also use a seperate booster, tho this seems unneccassarily expensive to me. Basically, these all work by detecting the difference in polarity when the first wheelsets cross the gaps into or out of the loop. Before a circuit breaker can react and shut down power, the unit reverses polarity to the loop. My experience is that the MRC units are useless for running steam. It seems (just a deduction on my part) that metal wheelsets which are not drawing current (read steam loco pilot wheels) will not cause the MRC unit to operate, so you wind up with a short. Diesels, on the other hand (and I refer to diesels with all wheel power pick up-the only ones worth buying and running-the others will probably exhibit the same performance as steam) will operate the MRC unit, sometimes with a little blip prior to reversing polarity. The units from Tony's are far better. You won't even know any polarity change took place. Check his web site, www.ttx-dcc.com My railroad is pretty much a large circle, with the double track main wired the same polarity so crossovers do not present a "reverse block" problem. The loops at each end are therefore reverse blocks. I have 5 staging tracks at each loop, all now have Tony's units installed and I can run all my steam flawlessly. Hope this helps.


    By the way, I have several of the MRC units which I have taken out of service, I will sell them for $20. if anyone is interested.
  4. roryglasgow

    roryglasgow Active Member


    So basically these little devices switch the polarity of the track when the short occurs, right? I assume that means that when an engine hits the intersection and the polarity is reversed any other engines running elsewhere on the track will suddenly start running backwards, right?

    I've been pondering the Next Big Thing, and I prefer having just one mainline. I'd like to make it continuous running with no human intervention so I can have a train running on the mainline while I work in a yard. There would be times, though, that more than one train would be on the mainline (possibly even heading towards one another, with one heading for a passing siding). I was just wondering how to make such a scenario work, electrically. I might just end up designing it with a double-track mainline, though. There are a number of advantages to that...

  5. Gary Pfeil

    Gary Pfeil Active Member

    Unlike regular DC, locos with DCC decoders run according to which direction you tell them to, forward or reverse. Changing the polarity will have no effect on the direction of travel. So no, there will be no change in direction when the reverser unit changes polarity. This is true of decoder equipped locos only. Analog locos (no decoder) will still need the trusty old dpdt switches to negotiate the reverse loop. To state it another way, the decoder provides power to the motor of whichever polarity is required to make the loco move forward. If you change the polarity on the rails, the decoder simply changes polarity also, and the loco direction remains the same. The reason you still have the potential for a short is at the gap one rail is still minus, the other plus, the wheelset creates a short with either DC or DCC, the reverser unit changes this polarity difference faster than the breaker can trip. Hope this is clear. You may want to check Allen Gartners web page. Start by going to www.digitrax.com and check the links. There are several sites which will be helpful. Also check www.loystoys.com

  6. Gary Pfeil

    Gary Pfeil Active Member

    Regarding the second question in you post, to run trains unattended on a single track main with reverse loops at each end and passing sidings, this can be accomplished with detecting units and either relays (I don't even want to think about it!) or, with DCC, a computer interface. I am no where near computer literate enough to advise you. I am glad my road (NYC) requires double and 4 track mains, because i didn't have to deal with this issue. I am able to run trains unattended by simply having up to four trains run the main at about the same speed (easy with DCC) and follow a set route. I do this while I switch other areas, or while I work on some area. Good luck!

  7. unknwn

    unknwn New Member

    I just registered this morning, and came across the post by
    Gary. I might be interested in your MRC reverse loop modules
    if they are still available.
    Email me at cflinder@prodigy.net

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