Reversing with DCC

[SteveJ]

I am asking my first question today but have been a steady reader and know that many of you can assist me if I can word my question correctly.

(Note added 9-19-07 where I wrote polarity in following paragraph think phase as it is DCC not DC).

Here goes, I am planning to add two reversing segments to my current track that currently has none. My current layout is DCC, has a DCS50 command station/booster and the track is wired as 2 rail not common rail.   I am planning to purchase a Digitrax PM42 for the two reversing segments. I can’t use a clean ‘loop configuration or a simple short reversing segment without part of the main run. I am unclear on how the segments need to be isolated for my particular application. I have read a lot of great information on the internet and the following information I have found and think I understand of what happens with DCC current except for when the train leaves the isolated section as follows:
1.   When an engine enters the isolated section, the polarity may be matched or mismatched. If the polarity is matched, nothing special happens.
2.   If the polarity is mismatched, both booster and reversing module think that they are shorted and both shut down for a few milliseconds.
3.   The regular booster comes back on line with the same polarity as before.
4.   The auto reversing module guesses that the problem might have been a reversing episode and comes back on with reversed polarity.
5.   Now the boosters match and the train proceeds without even a flicker of the headlights.
6.   If it was a real short circuit, the boosters will continue to retry applying power for a short time and shutting down again until the short clears. The autoreversing module will be switching polarity every time it retries.
7.   The same thing will happen as the train leaves the isolated section.

Finally here is where I need help; as both my reversing segments are similar I will only discuss one.  If I isolate/gap part of the main run (isolate at both ends) with the branch (isolate at turnout end) in my reversing segment, will a train passing through the main run isolated section not short out then it leaves, as per my understanding of points 1 thru 7?  In other words does the reverser module change the phase regardless of the entering phase so will short as the train leaves or not?
I hope this is clear without diagrams.

[Hunt]

If properly configured and installed the Digitrax PM42 should preempt booster shutdown when a short circuit is detected.

To use PM42 with Zephyr (DCS50) read http://www.digitrax.com/kb/index.php?a=15

PM42 Configuration for Reverse Loop Control read http://www.digitrax.com/kb/index.php?a=944

[SteveJ]

Thank you Hunt for the links they are most informative.   

[Jake]

Shouldn't you use an AR-1 for auto reversing? Since it is an auto reversing unit.

[SteveJ]

Jake,

Yes I could however I need two and I can get a PM42 for a good price.

[SteveJ]

I have added the PM42 reversing module and taken Hunts advice to adjust the trip ampage for the 2.5 amp DC50 command station and it all worked perfectly. I thought I could test my assumptions in this forum and get good advice.  Hunt gave that and thanks Mr Bachmann for allowing us to do this freely.

I didn't originally plan in reversing segments as quite frankly the books for DC wiring seemed to make it appear a little daunting.  However after I converted to DCC it was quite straight forward although I never found it clearly explained in any book or magazine only alluded to. 

I have a good link that explains how to that may assist others like me.   http://www.digitrax.com/kb/index.php?a=722>

 

[Frankford el car]

This is why I use hot overhead wire, and both rails grounded together, for my trolleys and el'. Whether straight DC, or DCC, reverse loop wiring is a non-issue on a traction layout, when the rails are bonded together, and the feed is through "hot" trolley wire, or third rail.

[Jim Banner]

When going around a reverse loop using 2-rail dc, the supply rail (right hand rail) going into the loop becomes the return rail (left hand rail) leaving the loop.   Some means is required to reverse the functions of the rails to avoid a short.

With centre rail pickup or overhead pickup, the centre rail or overhead always remain the supply and the running rails together are always the return, even when you go around a loop.

For "supply" and "return" you can substitute "positive" and "negative" if you wish.  With DCC, we cannot talk about polarity as it keeps changing, but we still have to worry about phase, that is, whether the rails both go positive at the same time ("in phase") or whether one rail is always the opposite polarity compared to the other rail ("out of phase.")

[SteveJ]

Good catch Jim my original message quoted polarity not phasing through out.  Every day I learn or relearn a little more.