Reverse loop re visited

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Reverse loop again

Good morning all.
Jim I am about to take your advice about moving the cross over. I will also make it quite a bit longer.
If you would look back at the picture of my layout I posted a few days back.
I will run the cross over to the right and parallel to the line at the back of the table, then curve right to meet the line as it exits the tunnel, should be plenty of line space for a decent length train and it won't interfere with the ladder.
Question, do I use insulated rail joiners at BOTH ends of the cross over??
Thanks.
Mac
 
Mac, sounds like moving the crossover will achieve the desired affect. You will need to insulate both ends of the reverse loop. You can use insulated rail joiners but, like all rail joiners, they tend to be untrustworthy. My preference would be to solder regular rail joiners and then gap the rail behind the joiner. You can be sure a gap will always provide insulation. Rail joiners get old, move around, and then you end up with shorts that can be a real problem with reverse loops.
 


Reverse loop

Hi BNSF Fan
No I haven't made any progress in that area, I got side tracked and am re laying my yard.
In fact I keep getting side tracked with something else that needs doing or re doing, but I'll get there eventually.
Mac
 
I hope this doesn't cloud the issue and further than it already is.

I will eventually convert to DCC, but I get confused by how "easy" DCC is supposed to be. As on old-school DC-er, I think in + or- terms. In planning a new layout, I've tried to avoid reverse loops. Going into a reverse loop on DC, the loop polarity is changed to agree with the mainline. Coming out of the loop, the MAINLINE polaity is changed to agree with the loop polarity. That would cause any train on the mainline to reverse direction, too. DCC is supposed to solve that problem, but for the life of me, I couldn't see how it would work differently from DC. If you reverse polarity on the mainline, everything on the mainline will change direction.

So I posted a similar question on another forum. After weeding through several "it's really simple" responses that were anything but simple and didn't really answer my question, I got one that started me thinking and maybe, just maybe, I figured this out.

First, DCC applies an AC signal to the rails, which is why non-decoder engines won't run on DCC (with the exception of "address 0", where the DCC unit also supplies a DC signal?). All engines have DC motors in them, but the decoder intercepts the AC signal, decodes the instructions and sends the proper DC voltage to the motor.

AC voltage changes polarity 60 times a second and DC has fixed polarity. In a reverse loop on DCC, where the two sections come back together, the AC still changes polarity 60 times a second, but those polarities are OUT OF PHASE with each other. When one rail is +, the other is -. And it happens on BOTH rails 60 times a second! Hence, a short circuit! The auto-reverser isn't really changing the polarity but rather changing the phases to agree with each other. The decoder in the engine is only going to send DC voltage to the motor with no phases, so the motor will only see DC with a certain polarity, regardless of the "polarity" (or more properly, the "phase") on the track. As long as the polarity to the motor remains the same, the engine continues in the same direction.

In the reverse loop, when the auto reverser changes the phases of the AC, it does not affect the mainline, except that the phases now agree with each other. Coming out of the reverse loop, the phases IN the loop are changed to agree with the mainline phase. Through the decoder, the engine only sees the correct DC polarity to maintain its direction.

I may not have this exactly correct, but it seems to make sense to me this way. Maybe some of our DCC gurus can come up with a better explanation.

This is one of the reasons why I haven't converted to DCC. There isn't a real explanation of HOW something is working, only that it works. Like magic! I'd prefer to run my trains with something other than MAGIC! LOL

Sorry for the length of this post. To a dyed-in-the-wool DC person, DCC comes off LOOKING like magic!

Darrell, quiet...for now
 
Thanks for that link, Jim! I will be needing it, I am sure!

I've never had a problem wiring DC reversing loops. Somehow, it made sense to me.

DCC has been another story! It's always been touted as "easier than DC!. Just hook up two wires and run multiple trains!" Bah, humbug! Most of what I've read seemed to be written in techno-geek. Over my head. Waaaaay over! LOL Even other modeler's explanations bordered on 'it's magic'. I've rarely run across explanations that didn't have a certain "ethereal" slant to it. And most explanations of DCC reverse loops kept refering to "polarity".

In DC, if two or more trains are on the layout and one train is in a reverse loop, to get the train out of the reverse loop, you have to change the polarity on the layout to agree with the reverse loop. Which means that all trains outside of the reverse loop will reverse direction.

With DCC, you're supposed to be able to run two or more trains on the same track in either direction at the same time. (Whew! What a mouthful that was!) If the "polarity" was being "reversed" on the layout, any trains running on the layout would reverse direction. Until I was pointed in the direction of AC current phases used in DCC, none of this made sense to me.

If I can remember or figure out who was the forum member who pointed me in the right direction, I will make sure I always give him credit in the future! To him, I pledge my eternal gratitude!

Darrell, quiet...for now
 
dgwinup said:
This is one of the reasons why I haven't converted to DCC. There isn't a real explanation of HOW something is working, only that it works. Like magic! I'd prefer to run my trains with something other than MAGIC! LOL
Click to expand...

darrell

http://www.dccwiki.com/Introduction_to_DCC
http://www.nmra.org/standards/DCC/standards_rps/rp912.html

i don't have any experience with DCC. but after reading the above web-sites (and knowing a bit about electronics), i believe this is how it works (and appreciate any corrections).

the basic idea of DCC is to provide full power to the track and communicate to the decoder in the engine the speed and direction you want the engine to move (among other things). the booster provides a DC voltage to the tracks. the decoder controls the voltage and its polarity to the motor (or lamps).

commands are composed of an address for a specific decoder and an instruction encoded digitally as ones and zeros. whenever it transmits a one, it swaps the polarity of the DC power for about 60 us. (micro-seconds). it transmits a zero, by not swapping DC power, for about 100 us. when it is not transmitting a command, you can think of it as transmitting a zero, which is how the decoder determines what the zero polarity is. (you could think of this swapping of polarity for just 100s of us. as AC, but it would hardly be the same as the sinusoidal waveform in your house AC).

the decoder shouldn't care about the track polarity since it needs to be able to maintain the correct polarity across the motor for the commanded direction. in other words, if the track polarity changes, the motor polarity and direction remain unchanged.

as you know, there is a mismatch in track polarity at one end of the reversing section. metal wheels cause a momentary short when they span the insulated gap, and an engine make cause an even longer short circuit when it straddles the gap. current limiting in the booster can protect it from damage, but should be avoided.

i don't think DCC alleviates either of these problems. but it looks like the auto-reverser can get confused. i don't see why a reversing switch couldn't be used if you already have one, but with DCC, i think it can reverse the polarity of reversing section, instead of the main section.

edit: re-reading the standard, i see that a one and a zero are not simply represented by the polarity of the track voltage. both a one and a zero are represented by toggling the voltage - reversing the polarity and then restoring it. a one bit is represented by toggling the polarity for a period of 116us (58us reversed and 58 us normal) a zero bit with a period between very roughly 180us and 12 ms. so both ones and zeroes cause the polarity to reverse, and it is the timing that determines if it is a one or a zero.
 
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Thanks, Greg! I spent some time reading parts of the links you suggested. Ironically, there were a few areas where the DCC process was referred to as "magic"! LOL But there's a lot of information in those links and most of it is written in an easy to understand manner (except for the "magic" references!)

So I was partially right, there are 'phases' in DCC. They just aren't AC current phases. With these "DC phases", it looks like the polarity changes regularly from + to -, similar to AC current, but the duration between the polarity change can be controlled. This variable duration is what determines the DCC commands and what the decoder actually 'decodes'.

In a reversing section on DCC, we not only have polarity change, we also have phase change. It is the auto-reverser's job to match the phases so that the polarity changes are synchronized. The decoder in a loco then 'sees' exactly the same thing it was seeing before the auto-reverser kicked in, matched polarity and phases.

At least, that's my understanding of what I just read! LOL (Does anyone feel like imitating Rex Harrison singing "By George, he's GOT it" from "My Fair Lady"? ROFL)

Gee, this would be so much simpler if I could just accept "magic" as a solution! Must be my hard-headedness! Had the same problems with computers many years ago in wanting to know HOW it worked rather than that it just works. (Plug A into B, B into C and it works "like magic". LOL)

Okay. I'm going to go off and do some more reading now. Thanks for helping me understand a bit more, Greg!

Darrell, quiet...for now
 
dgwinup said:
With these "DC phases", it looks like the polarity changes regularly from + to -, similar to AC current, but the duration between the polarity change can be controlled. This variable duration is what determines the DCC commands and what the decoder actually 'decodes'.
Click to expand...

the phase/polarity changes are not periodic as in AC, they only occur when a command is sent and last for less than a second.

dgwinup said:
In a reversing section on DCC, we not only have polarity change, we also have phase change. It is the auto-reverser's job to match the phases so that the polarity changes are synchronized. The decoder in a loco then 'sees' exactly the same thing it was seeing before the auto-reverser kicked in, matched polarity and phases.
Click to expand...

in the reversing section, you only need to worry about matching the track polarity at the ends. label the two wires from the booster to the track as A and B. the reverser (auto or person) just needs to make sure that tracks at the reversing section end match: A-A and B-B.
 


And another $.02 worth. A DPDT manual switch can be used until an AR is purchased. Just have to remember to throw it.

Moving from DC to DCC, I too was confused because I wired DC with inside/outside rail. And on an oval with a cross over, the cross over must be isolated. When I read about wiring with a front/back rail and isolate the ends of the loop (dogbone, balloon) for reversing, then it made sense. On all my track, the front rail is the "red" and the back is the "black" (or is it the other way). None of the crossovers are isolated. Only the two loop ends are on ARs. I ran DPDT manual switches for several months. I still use a DPDT for the manual TT.

If you are still debating the DC vs DCC - visit a DCC layout. It really is much easier than DC after the initial "what is DCC". If you are in N Florida - stop by, would love to give you a tour of my layout.
 
dgwinup said:
DCC has been another story! It's always been touted as "easier than DC!. Just hook up two wires and run multiple trains!" Bah, humbug! Most of what I've read seemed to be written in techno-geek. Over my head. Waaaaay over! LOL Even other modeler's explanations bordered on 'it's magic'. I've rarely run across explanations that didn't have a certain "ethereal" slant to it. And most explanations of DCC reverse loops kept refering to "polarity".
Click to expand...

Y'all are over thinking this whole thing. You wire it just like a DC layout as far as keeping the rails separated. In a reversing track, you put the insulating joints in just like a DC layout, but instead of a DPDT switch, you put a auto reverser.

How does it work? Beats the heck out of me. Don't have the foggiest idea. Doesn't matter either. I have built two layouts and helped others build a half dozen more with DCC and they all worked fine. Wiring a DCC system IS extremely simple. Engineering a DCC system isn't. I don't need to engineer a DCC system to wire my layout.
 
My thanks to all who have provided additional information on DCC. I think I have a much better understanding now and I have several good links to more information.

I find it ironic that DCC, in reality, does NOT use AC current, yet I am continually encountering information stating that DCC uses AC current. DCC is relatively new and you'd think that those who designed the systems in the first place would have coined NEW terms rather than use existing, and misleading, terminology.

dave1905 said:
How does it work? Beats the heck out of me. Don't have the foggiest idea. Doesn't matter either...... Wiring a DCC system IS extremely simple. Engineering a DCC system isn't. I don't need to engineer a DCC system to wire my layout.
Click to expand...

Dave, you're absolutely right! I had difficulty learning to use computers, especially before Microsoft came along. Having had a hobby background in electronics, I tried to understand HOW the computer worked. It wasn't until I read a story about an inventor who used "black boxes" to control his machinery. A reporter questioned him on how the computer controlled things. His response was enlightening: "How do I know how it does it? I just know that when I hook it up a certain way, it does what I want. If it doesn't, I replace it with something that does!"

I haven't had too much trouble with computers ever since! LOL

Darrell, quiet...for now
 
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