Reverse Loops, Polarity Conflicts & DCC

[jtomai]

(please see Diagram)

I have a simple layout that I plan very soon to convert to DCC, I am currently using simple Direct Current (DC), but any day now I am ordering a DCC system.

I have 2 main lines connected by a reverse loop. Both main lines have the (+) rail on the outside and the (-) rail on the inside, so obviously when Main Line #1 takes a turnout that leads into Main Line #2 there is a polarity issue. I have insulated there, with plastic rail joiners. Still, Loco's stop automatically, because the opposite polarity on the other side of the insulated joiner while using DC power tells them to reverse, while the other half of the loco on the other side of the insulation, polarity sends it forward. (See diagram below)

My question is will the wiring scheme in the image below work for DCC? I want trains to be able to travel any direction, say heading forward on Main #1, and then cross over using the reverse loop (with the polarity problem) to Main #2 heading the opposite way. I want my trains to run through the layout looping trough main #1 and then back to Main #2 in any direction. Will DCC magically solve the polarity issue with the reverse loop?

Is there something I am missing in my current wiring scheme, because I am afraid to lay any more track, and wiring for fear that my wiring scheme is not compliant for DCC. Also, will DCC simply use my existing (+) & (-) feeders & bus lines? Or will I have to add another separate DCC bus and connect feeders from that to my track?

Thanks,

John

 

[NH Mike]

The reverse loop needs to be insulated on both rails at both ends after the turnout. The rest of the track as you have drawn it will need both rails polarity reversed while the train is in the reverse loop. All blocks including the reverse loop should have DPDT toggles cross wired to the track feeders so trains can enter the loop from either direction and the mains reversed to agree with polarity while the train is in the loop.

I have no idea how this affects DCC as i run DC but hope this answers part of your question.

 

[jtomai]

Its becoming clear to me that the "looping" section in my sketch is HUGE. All the track in between the insulated joiners would have reverse polarity, therefore in a loop including that much of the rail road you gotta figure there are other trains heading in independent directions. Any of them "inside" the loop would switch directions when the polarity reversed in the loop.

If you look at the sketch above, there is no clearly defined loop, isolated from the rest of the railroad, the loop is "indirectly" created by turnouts joining main lines #1 & #2 -thereby encompassing alot of railroad into the loop, meaning alot of railroad will be interfered with when polarity reverses btween the insulated joiners.

I would be better off to change the design to isolate the looping part of the railroad via a simple branch line off of either main line that terminates with a loop back onto itself .
____________________
/ \
____________/ \
/ \ |
/ \ ___________________ /
/ Branch Loop
/
----------------------------------main line------------->



Right?

 

[CSX Robert]

Its becoming clear to me that the "looping" section in my sketch is HUGE. All the track in between the insulated joiners would have reverse polarity, therefore in a loop including that much of the rail road you gotta figure there are other trains heading in independent directions. Any of them "inside" the loop would switch directions when the polarity reversed in the loop...

To be clear, DCC power is a form af AC, so it has "phase" instead of "polarity." With DCC, the trains direction is independent of the phase, so when a reversing section reverses phase, any trains running in that section will not reverse.

...
If you look at the sketch above, there is no clearly defined loop, isolated from the rest of the railroad, the loop is "indirectly" created by turnouts joining main lines #1 & #2 -thereby encompassing alot of railroad into the loop, meaning alot of railroad will be interfered with when polarity reverses btween the insulated joiners.

I would be better off to change the design to isolate the looping part of the railroad via a simple branch line off of either main line that terminates with a loop back onto itself .
____________________
/ \
____________/ \
/ \ |
/ \ ___________________ /
/ Branch Loop
/
----------------------------------main line------------->



Right?

There is nothing wrong with your original design, except that it needs more insulated rail joiners. I see two possible solutions.

Solution 1: In you first diagram, on the straigh section between turnouts where you have a set of insulated rail joiners, you need to add another set of insulated rail joiners at the other end. This isolated section of track between the insulated rail joiners is your reversing section. You want both of these sets of rail joiners as close to the turnouts as possible, making the reversing section as long as possible(more on that in a minute). You can wire this section through a DPDT switch and reverse it manually(DC or DCC) or through an auto reverser so that it will reverse automatically(DCC only).

Solution 2: Do not add insulated rail joiners to the section mentioned above(but do keep the one set), but instead add two sets, one on each crossover between the two mains. Then one of the mains(can be either one) becomes your reversing section and that entire main would be wired through the DPDT switch or auto reverser.

Now, concerning reversing section length and why you might consider solution 2 over solution 1: If you electrically bridge both ends of the reversing at the same time you will create a short. This means that if you are running metal wheels the reversing section needs to be longer than your longest train. There are some exceptions and workarounds, but generally speaking, this is the rule. If you are running metal wheels and you plan to run trains longer than the reversing section you would get with solution 1, then I would suggest solution 2. With soultion 2, if you add any more crossovers, each crossover will have to have a pair of insulated railjoiners. Also, you cannot have a train traverse a crossover at the same time one is crossing the original set of insulated rail joiners.

 

[fast_ian]

Its becoming clear to me that the "looping" section in my sketch is HUGE. All the track in between the insulated joiners would have reverse polarity, therefore in a loop including that much of the rail road you gotta figure there are other trains heading in independent directions. Any of them "inside" the loop would switch directions when the polarity reversed in the loop.

Not true - You're confusing polarity in DC & phase in DCC - While the polarity of the track determines the direction a loco goes in DC, in DCC it is the decoder that determines the direction, so changing the polarity "right under the engine" will NOT affect the direction.

In DC the track voltage is applied directly to the DC motor.

In DCC, the track voltage is AC at approximately 6KHz - This is applied to a Bridge Rectifier in the decoder that rectifies this A.C. voltage into DC and the decoder's microprocessor determines the polarity of this DC voltage, to the DC motor as set by the digital data in the DCC signal.

In short (no pun intended!), you're worrying unnecessarily The magic of DCC (plus an AR) will handle it all for you......

HTH, cheers,
Ian

 

[fast_ian]

One more comment that will hopefully make this clearer:

- It doesn't matter which way you wire your DCC power to the track - Commanding the loco to go "forward" will make it go forward regardless of which way round the track is connected.

There's some really good explanations here: http://www.wiringfordcc.com/intro2dcc.htm

Also, I may have appeared to "recommend" the Digitrax AR1 (in the other thread) - Current consensus is that the DCC Specialties PSX-AR is a superior solution - Particularly if you're running sound equipped locos.

Final comment (hopefully!) - The "gotcha" with huge reversing loops is while you can have as many locos *in* the section two "entering" (or one entering & one leaving) AT THE SAME TIME will cause a short.

HTH, cheers,
Ian

 

[jtomai]

Thanks everybody for such great answers and clear logic. Regarding the independent direction of Locomotives in DCC -thats what I thought initially until some idiot at my LHS told me otherwise. Anycase, I see now that it is possible to incorporate a reversing section into the design as you all have shown. And learned alot about how they work. Im all set, thanks!

However, based on the design, the looping section would be really, really long, or way to short. So, last night I took out the turnouts on the diagonal bridge between lines #1 & #2, in favor of adding a new independent branch line off Main #1 further down the line for an isolated section with a simple old fashion loop.

A big reason for this is that I dont want a significant part of my RR caught in the Loop and there is a YARD in my real layout not shown in the diagram for simplicity purposes that would be inside the loop too.

In hindsight, Ive learned that reversing sections should be isolated.

Thanks All!

 

[fast_ian]

...Im all set, thanks!

Very good!

However, based on the design, the looping section would be really, really long, or way to short. So, last night I took out the turnouts on the diagonal bridge between lines #1 & #2, in favor of adding a new independent branch line off Main #1 further down the line for an isolated section with a simple old fashion loop.

A big reason for this is that I dont want a significant part of my RR caught in the Loop and there is a YARD in my real layout not shown in the diagram for simplicity purposes that would be inside the loop too.

In hindsight, Ive learned that reversing sections should be isolated.

Whoa! Hang on - Wasn't this all working as it should with DC control? [OK, you have to flip the switch, but from what you said this "huge" reversing loop was operational, and included your yard area, no?]

If it was OK, I fail to see why you want to change it (?) - There's nothing wrong with having 50% (or more!) of your RR in a "reversing section" - As I said above, it sounds like you should simply replace the reversing switch with an AR unit of some kind and you're home free......

Cheers,
Ian

 

[jtomai]

well I already took it out and I think its actually improved the layout. There is a big yard (multiple passing lanes built off of main #1 at the top of the diagram) and all that was "caught" in the loop. As you can imagine, a yard is a heavy traffic area and given that others have told me that if you have two trains entering/exiting the loop at the same time it can cause a short, then a loop is not a good idea if it includes a yard.

Anyhow, to simplify things greatly, I moved the loop to an isolated section of the RR by putting the loop on its own independent branch line.

IMO I think that was the lesson here and is the best way to handle reversing loops.

 

[fast_ian]

well I already took it out and I think its actually improved the layout.

OK - Good for you! I wasn't trying to criticize your plans BTW, just trying to understand where you were at. Glad you like it more now!

Anyhow, to simplify things greatly, I moved the loop to an isolated section of the RR by putting the loop on its own independent branch line.

IMO I think that was the lesson here and is the best way to handle reversing loops.

Fair enough! Good luck,

Cheers,
Ian