Model Railway Forum banner
1 - 12 of 12 Posts

·
Registered
Joined
·
18 Posts
Discussion Starter · #1 ·
Hi all - first post here and new to model railwaying (well, since I was a kid!)

We've bought a Virgin DCC set for our son for Christmas, plus another second hand DCC engine from his grandparents. Ain't he the lucky one?! Anyway, as is so often the case, space is tight so we've had to be quite creative with the layout.

We decided the best bet was to have a long straight along one wall in his room, which runs into a sort of circular section. The grand plan (or at least part 1 of it) is to be able to run the trains up and back on double track, then go either way round the circle. 6 points in all to allow the train to go around in the circle, or go back and then switch from one straight track on the double track section to the other.

Now, assuming I've managed to describe that coherantly, I'm imagining some of you are already shaking your heads! My initial reading on DCC essentially said "now you can design any layout you like without worry", which is what I did. But it was when I saw a Hornby DCC "reverse loop module" just now that I began to think... hold on, I thought I could do anything?! Why would I need one of those?

So as I can be prepared for the Christmas traumas, I'd love some advice! Will our layout work? Will it need some modifications or extra bits of kit / isolating things? Apologies for being so green!

PS - there looked like a relevent section to cover this in the DCC FAQ, but it was empty?
 

·
Registered
Joined
·
9 Posts
Welcome noiseboy..
I have done the same for my son for Christmas. I bought the Mixed goods set for my son.

The DCC links above your post work fine for me.. Now I am seriously considering upgrading to the NCE Powercab DCC control system in the future. Never knew there was so much to learn about DCC.

Anyway, the Hornby will do me, err , I mean my son for now till I get more track and rolling stock as time goes on !

Happy Railroading !

Peter...
 

·
Just another modeller
Joined
·
9,983 Posts
You will need a reverse loop module if any track doubhles back on itself - the simplest form of that is a single track teardrop shaped loop that the loco enters one way and leaves over the same point in the opposite directon on the single track. A diagonal across a loop has the same sort of problem. If you have any form of reverse loop it creates a short circuit in the track so the special device is needed.

If you draw both rails in a teardrop loop and trace each rail you will see what I mean....

Richard
 

·
Registered
Joined
·
18 Posts
Discussion Starter · #4 ·
Thanks to you both - Richard, yes I do see what you mean. I've found a helpful (if daunting) website at http://www.wiringfordcc.com/track_2.htm#a39. My proposed layout looks similar to figure 2 under the heading "using double crossovers with balloon tracks", about 3/4 of the way down the page (differences are that there will be 2 straight tracks to the end, and the bulbous bit has a optional circular loop built in). I think I understand his solution, but can I just check it with you fine people?!

The train (say) goes up the RHS from bottom to top. It hits the reverse section. First question - what actually happens at this point? The train is going forwards, and presumably still does, it's just the polarity changes, right? Assuming I have that right, round it goes (round and round with my optional circle til eventually...) it comes back down the LHS. At which point the polarity switches again, so now it is the same as when it started, and the points are set so it switches back the RHS. But that is OK... with DCC the train is clever enough to know which way it is heading, and we've broken a potential short circuit. Positive and negative has no effect on train directions, right? It's just you mustn't inadvertantly directly connect one to the other. Perfectly logical.

I note the author says that the reversing section has to be as long as the train. That's OK in this case - I reckon a 2nd radius semi circle is pretty much the same length. However, I don't fully get why. With the Pendolino, it has lights at the rear of the train. Surely as it crosses over, the front of the train is one polarity, but the rear is another, causing problems? Or does current pass through the train somehow?

So if I have all of this right (and I suspect I don't), why do I need a reversing module at all? Wouldn't simply isolating that top section do the same job? Bear with me... I'm slowly getting this, but not quite there yet! Really appreciate the help...
 

·
Registered
Joined
·
2,342 Posts
Hello and welcome to the forum

Part of the job of the reverse loop module is to stop the momentary circuit , created when the wheels pass from one section to the other, from tripping the DCC controllers short circuit protection . The Reverse loop module does this automatically.

The train must be totally in the isolated section for this reason also , as you would have two possible circuits created at each end otherwise

Hope this helps

Regards

zmil
 

·
Registered
Joined
·
18 Posts
Discussion Starter · #6 ·
Thanks zmil - I get the isolation, but still a little confused on what happens when a train passes over - won't the front part have one polarity, and the rear another? And could someone explain why the isolated section must be at least a train length long when clearly the train must be in two zones at once to get there in the first place? Thanks for your continued patience!
 

·
Premium Member
Joined
·
6,201 Posts
Hi noiseboy,

Welcome to the forum


Though a different manufacturer this is also an auto reverse controller hopefully helping in your last posting on the subject , its a PDF and may take a moment to load.

http://www.digitrax.com/ftp/ar1.pdf
 

·
Registered
Joined
·
1,630 Posts
I'll try to explain simply what the reverse loop module does and why it is important that the reversing section it connects to is longer than the longest train. This may or may not be helpful, it's not by nature a simple topic!

The reversing section has both rails isolated at each end. Its rails are connected to the outputs of the reverse loop module instead of directly to the command station. I will refer to the bus wires as red and black.

What it does is if it detects a short circuit it reverses the power very quickly in the reversing section, too quickly for the command station cutout to operate. If the short circuit still persists then the command station will cut the power as normal.

Consider a train running along the normal section of the track, where red is connected to the left rail. Assume that the reversing section is currently set so red is connected to the right rail. When the first wheel on the left hand side the train passes over the insulated joiners, then it will be connecting the red rail before the reversing section to the black rail within the reversing section and therefore causes a short circuit. The reverser then flips the power to the reversing section so that its left rail is connected to red. This cures the short circuit and the train can continue.

However the normal track beyond the far end of the reversing section has the red connected to the right rail. When the first wheel of the train reaches here then there is another short circuit and the reverser flips the reversing section again so its right rail is connected to red. Reversing the power under the train doesn't matter as it is DCC and the train direction does not depend on rail polarity. The interruption in the power when this happens is too short to matter.

The problem arises if any part of the train is still on its way into the reversing section, as any wheels bridging over this join will now cause another short circuit and if the reverser flips again then it will just move the problem to the exit of the section. You may get away with it if it is just a metal wheel, especially if you create a very short dead section at both ends of the reversing section, but any lighted coaches or trains with locos both ends are likely to cause a short circuit if they are longer than the reversing section.

The same applies if the reversing section includes points. Any path through the reversing section which links sections where the rails are of oppsite polarity (red to left rail, red to right rail) needs to be longer than the longest train using it.
 

·
Just another modeller
Joined
·
9,983 Posts
*** Edwin has explained it well I think.

When you buy an autoreverser, avoid ANY which use a relay - that includes lenz, Digitrax and MRC or Gaugemaster... - they are simply too slow to react in some circumstances, often "chattering" as relay based products can and eventually, the relay always fails.

You need one that is totally solid state/electronic to get best reliability and these are readily available, often for less money too. The OG-AR from DCCspecialties in USA is a very good choice as it also has inbuilt overload protection.

I think John at bromsgrove models stocks these in the UK

Regards

Richard
 

·
Just another modeller
Joined
·
9,983 Posts
QUOTE (noiseboy @ 15 Dec 2008, 18:00) <{POST_SNAPBACK}>Thanks zmil - I get the isolation, but still a little confused on what happens when a train passes over - won't the front part have one polarity, and the rear another? And could someone explain why the isolated section must be at least a train length long when clearly the train must be in two zones at once to get there in the first place? Thanks for your continued patience!

*** A specific answer to part of this (athough Edwin covered most of it well).

But... its important to really understand the basics, and each explaination helps many others who haven't yet asked the question - so....

What happens and why does the section need to be at the very least full train long?

As the wheels touch the gap, the circuitry will change the polarity of the reversing loop section the train is in and leaving to match the section of track it is entering. It will do this in about 1/10,000th of a second with an electronic reverser, so you see no reaction... it just happens.

If the section is short and the train spans entry section, rev loop section and exit section, then its clearly impossible for all 3 to be at the same polarity.

The conflict will very likely cause a short which may be repeated every time metal wheels at each end of the loop pass the gaps... clearly not something desirable!

Richard
 

·
Trainee pensioner
Joined
·
212 Posts
I don't want to repeat what others have said, but sometimes, the same information given in a different way can help. So I hope this helps!
When you create a reverse loop you effectively connect the "positive" and "negative" (or left and right) rails together, forming a short circuit. To prevent the short circuit occurring, insulating fishplates are fitted at both ends of the reverse loop (creating an isolated section) and the output of the reverse loop module (RLM) is connected to the isolated section. The input of the RLM is connected to the main track circuit bus or controller output.

The RLM will supply the power to the isolated section and when the loco enters the section it will set the polarity of the rails in the isolated section correctly to match the polarity of the input track so that no short circuit occurs.

When the loco leaves the isolated section at the other end of the loop, it will re-set the polarity to match that of the exit track

The isolated section should be longer than the longest train if metal wheels are fitted because they will cause a temporary short circuit as they pass over the track joints, even with insulated fishplates. Plastic wheels on the coaches/wagons won't cause this problem, so in this case the isolated section can be as short as the longest loco. (but plastic wheels are best avoided)
 

·
Registered
Joined
·
18 Posts
Discussion Starter · #12 ·
Everyone - thank you! I think I've finally got it!

Now I just have to figure out how to explain this to my 9 year old!

Is the Hornby reverser ok?
 
1 - 12 of 12 Posts
Top