Author Topic: Reversing track for main to opposing main (different polarities)  (Read 266 times)

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OldEastRR

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My layout has a double track main for most of it but because of the routing each main has the opposite DCC bus hookup from the other. (because it's a 3x around one loop mainline).  But after operating a while I'd found I need crossovers between the mains to make passing sections or change track routes. However because of the opposite wiring I'm going to need a DCC reversing module. Or do I need more than one?

Here's the situation:



The red line has buss leads opposite to the blue line. I'd like to add crossovers (the green lines) but obviously that would short the circuit. However, where do I attach the reverser module? Section 1 only? Or do I also need reversing modules for sections 2,3,4 or 5? I assume whatever the answer is would apply if the crossovers were opposite to shown.
Also I'm not certain how a DCC reverser circuit works. Does it switch the polarity only once, when the train enters a conflicting block? But does it also switch the polarity again when the train leaves the other end of the reversing section to the other polarity? This only when the train is completely within the reversing circuit block, obviously.
« Last Edit: October 25, 2019, 11:04:16 PM by OldEastRR »

jagged ben

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Re: Reversing track for main to opposing main (different polarities)
« Reply #1 on: October 26, 2019, 12:10:15 AM »
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I would probably do the entirety of section 1, to right past the frogs of the inner turnouts.  If I saw the rest of your track plan I might possibly have other ideas, but for what you've presented section 1 makes sense.

A DCC reverser watches for the short circuit between the reversing section and the main bus, and it switches the reversing section when it sees that, thus eliminating the short by aligning the polarities.  Whether it switches once or twice just depends on how it was aligned to begin with.  Or put another way, it only has to reverse polarity when entering if the polarity was already opposite.  It will always reverse polarity when exiting if you run straight through, because the exit will require opposite polarity from the entrace.   Regardless, you can not have a train with metal wheels, or especially powered units (including lighted passenger cars) crossing over both ends of a reversing section simultaneously.  So your reversing section needs to be longer than your longest powered consist, at the very least.  Just doing the green sections is not practical, and the simplest solution is to connection them with your section 1.

davefoxx

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Re: Reversing track for main to opposing main (different polarities)
« Reply #2 on: October 26, 2019, 08:11:38 AM »
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So your reversing section needs to be longer than your longest powered consist, at the very least.

^This.  And if you run lighted passenger cars, then it is highly recommended to make the reversing section longer than the entire train.

DFF

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carlso

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Re: Reversing track for main to opposing main (different polarities)
« Reply #3 on: October 26, 2019, 10:35:28 AM »
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DFF,

If he is running metal wheels on freight cars would the reversing section need to be as long as the train also?

Carl
Carl Sowell
El Paso, Texas
Southern New Mexico N Scalers, Las Cruces, New Mexico

conrad

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Re: Reversing track for main to opposing main (different polarities)
« Reply #4 on: October 26, 2019, 10:59:08 AM »
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"If he is running metal wheels on freight cars would the reversing section need to be as long as the train also?"

I say yes because if a metal wheel has stopped across a gap it will cause a short.  Better safe than sorry.

Conrad

jagged ben

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Re: Reversing track for main to opposing main (different polarities)
« Reply #5 on: October 26, 2019, 11:10:02 AM »
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DFF,

If he is running metal wheels on freight cars would the reversing section need to be as long as the train also?

Carl

Probably.  This is the sort of thing where DCC is less forgiving than DC.  When you have metal wheels that aren't drawing power and aren't connected to anything else, they will often (not always) roll past the boundary, thus removing the short and allowing the train to continue.   With DC it might make the train a bit sluggish but you might not even notice. (Note this doesn't apply f you have something like Kato passenger cars which come with all the wheels connected electrically together on each side for power pickup, it doesn't matter if you've actually installed a lighting kit.  You still have to treat them like they're lighted or like locomotives. ) With DCC the reverser action is so fast you likely will get a loss in track power that causes the train to stop or jerk.  If metal wheels end up stopped on the gap you won't be able to keep going.


Maletrain

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Re: Reversing track for main to opposing main (different polarities)
« Reply #6 on: October 26, 2019, 02:09:23 PM »
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Not knowing your total track plan, all I can do is suggest something for you to think about.

If you are going to have a lot of crossovers where two mains are parallel, it would probably be best to make them the same polarity, so that none of the crossovers need reversers.  That means that there needs to be a section that does reverse where one main transitions to the other parallel main.  That location is probably long enough to contain a whole train, so the reversing strategy would be easiest there.  Think of it as a single track with reversing loops at both ends, except that the trackage between the loops is doubled, but both tracks wired the same way (and not reversing.

OldEastRR

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Re: Reversing track for main to opposing main (different polarities)
« Reply #7 on: October 27, 2019, 12:38:45 AM »
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OK, so I can assume 1) if the polarity of the track that the reversing DCC circuit is wired to is different from the polarity of the track feeding into it, the circuit automatically changes it to match both. Right?
And 2) if the polarity of the track the DCC reversing track is feeding into is different from the reversing section  then the DCC circuit will automatically change the reversing section to match the track it's feeding INTO. Right?
Which makes sense -- the only section of track which can have its polarity reversed is the one controlled by the DCC reverser .... obviously the rest of the layout's polarity always remain unchanged. Right?
And yes, the circuit will extend through the frogs of the crossovers, with insulated gaps where the turnouts meet. Like wiring for DC blocks.
So far I'mlooking at only one of these crossover sections. And it will be over 6 feet long so no problems with overlap of the train at either end.

Maletrain

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Re: Reversing track for main to opposing main (different polarities)
« Reply #8 on: October 27, 2019, 09:33:06 AM »
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The way you have it drawn, you could use a single reverser on section 1 and it will work properly. 

But, if you have cross-overs elsewhere, particularly if they allow a train on the inner loop to use the 180-degree turn of the outer loop, then you will need to reverse some part of the outer loop, also. 

Because you initially posted "it's a 3x around one loop mainline" and "after operating a while I'd found I need crossovers between the mains to make passing sections or change track routes", I am wondering if you will be making additional mods to your track plan, later.  If so, having the two directions of your "double-track" mainline in opposite polarity (actually "phase" in DCC) will make the logic for adding more reversers for more cross-overs much more complex that it really needs to be.  My club went through a similar issue with a layout that was initially just a very complex folded and twisted "dog bone" that appeared to be a double-tracked mainline.  That layout was designed long ago with the intent of continuously running trains around the loop for display, so it really did not matter that the mainline tracks were wired with mismatched phase.  But, when the idea of doing more realistic ops on the layout was adopted, and things like crossovers were found to be necessary at several locations to support that, the easiest way to deal with the reversing issue was to change the long runs of mainline to make the phase match, and put reversers on the sections where one mainline track made the 180-degree turn to become the other mainline track.  That way, we only needed reversers at two points on the mainline, no matter how many crossovers we added.  And, there was no problem with the trains being longer than the length available for a reverser section somewhere in the middle of a run.