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Polarity With Reverse Loops

Craig W sent in this tip:

“With a DPDT switch you can alter the polarity manually when on a reverse loop. It can be done with the track before the reverse loop. You would need to double insulate each end of the rail loops.

DCC requires insulating each rail at both ends as mentioned previously and having an auto-reversing circuit mounted to feed the reverse loop. You won’t notice the engine slow down whereas with DC you would likely stop the engine, change polarity to the track, reverse power to that track to proceed. However, when using DCC the reversing circuit is the way to go for this.”

If you would like to comment on what Craig said, then post your thoughts below.


9 Responses to Polarity With Reverse Loops

  • Kevin Chingsays:

    On DCC i use a digitrax AR1 they are the best and most reliable

  • Craigsays:

    Kevin…. agree digitrax AR1 is popular

  • Frank Bsays:

    Think again on DC ! If the loop is isolated both ends, there is no need to stop the train, just flick your switch to change polarity on the main line while the train is progressing round the loop.

    • Bill Suarezsays:

      Yes, but while the train is in the loop you need to simultaneously through the DPDT AND the reverse switch on the power source to then align the rail polarity for the train to exit the loop.

      • Herveysays:

        No Bill. Just change the polarity of the block the train will be going into when it comes out of the reversing section. No stopping of train required but you must be paying attention so that the polarity is the same when it comes out of the reversing section as to what it had at the end of the section. Sounds basic but it is just a matter of time until you forget and you have a short.

    • John Ogdensays:

      Won’t the Loco go into reverse if you switch polarity while it’s still moving!

  • David Krausesays:

    Seems that when the LOCO exits the main line to enter the loop and travels the loop, the main line polarity would need to be reversed just before the LOCO re-enters the main line…should work.

  • Craigsays:

    Thanks guys I’ll take it all onboard.

  • Herveysays:

    David has it correct. You don’t change the polarity of the reversing loop while the train is in it. You change the polarity of the block the train is moving into when it leaves the reversing loop while the train is in the loop. If you get it correct the train never stops and continues on at the same speed.
    That is the theory, unfortunately it is just a matter of time until the DPDT switch doesn’t get thrown and everything stops as you will get a short coming out of the reversing loop.

    Circuitron has an auto reverser (AR-1) that can be used on DC systems only. I have no experience with it but for those still using DC it may be the solution you are looking for.

    DCC has a number of options for dealing with this issue. The basic method is the same as for DC (DPDT switch). There are also electronic solutions of which the AR1 from Digitrax is one. This works adequately in most cases but is “old” technology. They use a mechanical relay to switch polarity. It works but it is slow and DCC is very sensitive to shorts. To use an AR1 on a DCC system you generally have to reduce the speed which the system or breakers shut down if there is a short.

    This allows you to go through the reversing loop and come out the other side without your DCC system shutting down. You may here a click as the polarity is thrown if you are running quiet. Unfortunately all your decoders are experiencing the short and overtime the electronics of the decoders will not react well to this shorting. The better route to go is with a solid state auto reverser, they will sense a short and flip the polarity quicker than the DCC system will sense the short and reduce the risk of damage to your other decoders.

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