reversing loop question

[fwright]

I know they make automatic reversers for DCC. My question is, does anyone make one for DC?

Actually, you can use a DCC auto-reverser to exit a DC reversing loop.  You hook up the auto-reverser to reverse the polarity of the main when it hits the gaps between the main and the exit of the reversing loop.

What the same auto-reverser cannot do is match the reversing loop to the main when entering the reversing section.

Which means the DCC auto-reverser is only practical in DC if you always use the reversing loop in the same direction.  And is the reason why an auto-reverser for DC is much more difficult to implement if the logic has to accommodate bidirectional operation through the reversing section.  Accounting for all the polarity matching possibilities is more difficult in DC because you cannot reverse the polarity of the section the train is coming from to match.  You can only reverse the polarity of the section the locomotive is going into to achieve matching polarities in order to avoid reversing the train.  In DCC, it doesn't matter much which section of track you reverse the polarity on because reversing polarity does not affect locomotive direction as it does in DC.

hope this makes sense

Fred W

[Texas Zepher]

... We may end up wanting to put in a reversing loop for a variety of reasons. Below is a diagram that may help ask the question.... If we have a loop (perhaps a dogbone with various sidings and passing tracks)

reversing loops in a dog bone can be much more tricky than the simple loop you have drawn in the example.

I believe it's possible to have some trackage split off of the loop and end in a reversing loop.  However, if we do this, I'm assuming that when the train exits the reversing loop, the WHOLE remaining layout reverses?

If the reversing loop wiring is the only special wiring on the WHOLE rest of the layout then Yes.  This is an issue with ANY reversing loop not just the example given.

the WHOLE remaining layout reverses? If so, then it means without dc, only one train could be running at a time?

Do you mean, "without DCC"?  Even if so the answer is no.  This is where one gets into the age old "cab control" or "block control" for DC.

Or if we had two trains running off of two power packs, we're asking for trouble?  I understand that we would use the selector form atlas, for example to help out, we're just confused on what all happens when we flip the switch....

1. No, you are not asking for trouble just all the headaches associated with cab control.   2. Yes, you could use the Atlas selector switches and the Atlas reversing loop switch.  3. You only flip switches when the train is not on the piece of track it is connected to.  That is the managment of cab control.  You flip the switches ahead of the train's arrival into that block.

[Texas Zepher]

I know they make automatic reversers for DCC. My question is, does anyone make one for DC?

Yes. There are many different types out there.

http://www.tchtechnology.com/products/reverse/

http://www.internettrains.com/merchant2/merchant.mvc?Screen=PROD&Store_Code=IT&Product_Code=MNT-RU1-1

http://lighthousehobbies.com/CatalogDetails.aspx?id=15526 

Or you can make your own: 

http://home.cogeco.ca/~rpaisley4/AutoRevCheap.html

[Medina1128]

I know they make automatic reversers for DCC. My question is, does anyone make one for DC?

[dstarr]

Reverse Loops, the Mobius strip of model railroading.   In a reverse loop, the north rail bends around and connects to the south rail, which is a dead short circuit.  This is why Lionel worked on three rail.  To remove the short, you have to cut insulating gaps in the track.  In fact, we cut a pair of gaps at both ends of the reverse loop. 

   We cannot quit with just cutting the gaps.  Once the reverse loop is gapped at both ends it gets no juice, and trains won't run without juice.  So, we connect the isolated reverse loop to the power pack thru a reversing switch.   To get a train to enter the reverse loop, the polarity of the reverse loop MUST match the that of the main line.  If the polarity is backward, the locomotive will stop once its wheels bridge the gap, with half the wheels seeing one polarity and half seeing the other polarity.  Given a reversing switch feeding the loop, we can set loop polarity to match the mainline polarity and the train will enter the reversing loop.  

    Now, draw a reverse loop, draw the gaps at each end, and convince yourself that the polarity at each end of the loop is reversed.  Once you have the loop polarity matching mainline polarity at the left end of the loop, you have a polarity reversal at the other end.  Draw the situation, make little plus and minus signs on the tracks  and get convinced.

   Once you believe that the two ends of the loop have opposite polarity with respect to the main line, the sequence of getting a train thru the reverse loop should be clear.  Set reverse loop polarity to match the mainline at the entrance to the loop.  Run the train into the loop.  The reverse the polarity of  the main line (use the reverse switch on the power pack) and run the train out the other end of the loop back onto the main line. 

   The dead short circuit provided by a reverse loop will short out DCC as well as DC.  One other thing to keep in mind.  As a train crosses a track gap, metal wheels bridge the gap, so if the polarity is reversed across the gap, shorts will happen.  DCC is fussy and usually shuts down immediately when even a brief short occurs.  

   You can connect lamps or bicolor LEDS to show green when the track polarity is correct across a gap into the reverse loop and red when it is reversed.

 

 

[Medina1128]

Pick up a copy of Andy Sperandeo's Easy Model Railroad Wiring. It explains how to do it quite easily, with easy to understand diagrams. I used Atlas' selector switches on my first layout, but they take up quite a bit of space, compared to toggle switches. I am using DC with two control cabs, and his book covers that, as well.

[Autobus Prime]

DC and DCC are fundamentally different in the way they handle track polarity for reverse loops.

In DC, you must first match the polarity of the main and the loop as the train enters the loop.  (I'm simplifying this to "main" and "loop" here, ignoring multiple cabs, blocks, etc.)  Depending on how the turnouts are set, and what the polarity of the loop was the previous time you ran a train through, you may or may not have to flip the polarity of the loop before the train gets there.  Then, as the train exits the loop, you must reverse the polarity of the main, because the train will be going the opposite direction.

MrB:
Right. You don't have to do it as it exits, though. You can even flip the main as soon as the locos have left and entered the reversing section. The important thing is to have it ready when it gets there. Also, I suggest that the operator shout "REVERSING POLARITY!" while doing that, for that Star Trek atmosphere.

[MisterBeasley]

DC and DCC are fundamentally different in the way they handle track polarity for reverse loops.

In DC, you must first match the polarity of the main and the loop as the train enters the loop.  (I'm simplifying this to "main" and "loop" here, ignoring multiple cabs, blocks, etc.)  Depending on how the turnouts are set, and what the polarity of the loop was the previous time you ran a train through, you may or may not have to flip the polarity of the loop before the train gets there.  Then, as the train exits the loop, you must reverse the polarity of the main, because the train will be going the opposite direction.

In DCC, on the other hand, we typically reverse the polarity of the loop, both when the train enters the loop, if necessary, and then again as the train exits the loop.  The polarity of the main, on the other hand, does not change.

[dehusman]

You don't change the polarity of the layout, you change the polarity of the piece of track the train is about to enter (before it enters the track).

If you have multiple trains on a DC layout then you must break the track into "blocks" with insulated joints in one or both rails.  Then you will have switches that control which cab/power supply is connected to which block.  When you reverse directions or use a reversing track, on the blocks that have been selected for that throttle are affected by the reversing switch.  The blocks that are being powered by the other throttle are not affected.  I would suggest buying a book on wiring your layout, that will make it much clearer.

In DCC you don't have to worry about the seprate blocks since they aren't required.  All you have to do is connect one side of the reversing unit to the two wires for the main  and the other two wires to the track for the reversing section and you are done.

Dave H.

[dehusman]

The gapping and identification of the reverse loops is exactly the same whether it is DC or DCC.  The only difference between DC and DCC is how you control the reversng section. 

With DC the polarity has to match the polarity of the direction the train is going.  If the engine is going forward when the north rail is positive, then to keep the train going forward the north rail in the reversing section has to be positive. 

With DCC the polarity just has to match the boundary the train or engine is passing over.  If the north rail on the layout is positive, then when the train enters or exits the reversing section the north rail at the location of the train has to be positive.

From a track diagram there is no difference between a DC or DCC layout.  The only difference is how many gaps you need to have on the layout overall (DCC needs way less) and how you control the reversal of polarity in the reversing section (DCC typically does it with an automatic switching circuit and DC tends to do it with a manual switch.)

Dave H.

[Autobus Prime]

We're still working on the designcof new n-scale design for the new room. We may end up wanting to put in a reversing loop for a variety of reasons.

Below is a diagram that may help ask the question...it is NOT our layout- it's just here for explanation purposes.

Let's assume we're in dc. If we have a loop (perhaps a dogbone with various sidings and passing tracks), I believe it's possible to have some trackage split off of the loop and end in a reversing loop. However, if we do this, I'm assuming that when the train exits the reversing loop, the WHOLE remaining layout reverses?

If so, then it means without dc, only one train could be running at a time? Or if we had two trains running off of two power packs, we're asking for trouble? I understand that we would use the selector form atlas, for example to help out, we're just confused on what all happens when we flip the switch....

nnd:

The Atlas "Selector" is a set of four SPDT switches for common-rail cab control. They select the cabs. The Atlas "Controller" is the reverse loop unit. They probably have good instructions. What I am calling the "East-West Switch" they are calling the "X-Y Switch", I think.

While I have no doubt that DCC is a good system, this situation is no problem with DC, actually.

Break the whole layout into blocks with toggles for cab control. Add one extra toggle DPDT for each cab. Call this the "East-West" switch. It's wired like a reversing switch.

Now wire your cabs directly to all block toggles for reversing sections. Wire them to all other toggles through the "East-West" switch.

When your train enters the reversing section, flip the East-West switch to match the train's new travel direction when it leaves the reversing section. This will switch all controlled blocks, but since the whole point of cab control is that you separate train control between cabs, no other train is affected.

Under this system, your throttle's reversing switch is always right. Forward is always cowcatcher-first, Reverse is always caboose-first. The E-W switch takes care of travel direction.

If your reversing loops have bidirectional traffic, add another DPDT toggle for each bidirectional loop, the "Loop Direction" switch. The purpose is to make sure the throttle's reversing switch stays consistent. Insert this switch between the reversing sections block toggle and the track, and flip it in the direction of travel. I. E. on your layout shown, you'd flip one way for clockwise, the other for CCW, around the reversing loop.

Only complicated for people who stick their fingers in their ears and say LA LA LA DCC. [>:D]



With common rail, the difference is that you have a common return, so from each E-W switch, one wire would go to that return, and the other would go to all non-reverse block selectors. One wire from the Loop Direction switch would go to the loop's block selector; the other would go to the common return.

If I am just contributing to confusion with all these words, Atlas sells a wiring guide to their products, and this guy has info on his page:

http://alines.home.att.net/

Click the "Wiring" tab. He uses Atlas components.

[HarryHotspur]

This is not a technical answer, but it might help. This applies to DC only.

Think of two separate layouts, with no interconncected tracks and a separate power supply for each. If you put an electrical gap in the siding between your outer loop and the reversing loop, and connected one power supply to the outer loop and the other to the reversing loop, you would have two separate layouts (electrically speaking) which you could operate independently.

When a train goes from the inner loop to the outer loop, the polarity will have to match; meaning, in effect yes, the whole layout will have to be reversed. However, you can use switches (usually toggles) to switch power supplies to different (electrically isolated) sections of your layout, but others can explain this far better than I can.

Perhaps this begins to illustrate why DCC makes sense, even for small layouts. BTW, I'm not familar with the Atlas switch you mentioned, so I have no idea what it would do. 

[nik_n_dad]

We're still working on the designcof new n-scale design for the new room. We may end up wanting to put in a reversing loop for a variety of reasons.

Below is a diagram that may help ask the question...it is NOT our layout- it's just here for explanation purposes.

Let's assume we're in dc. If we have a loop (perhaps a dogbone with various sidings and passing tracks), I believe it's possible to have some trackage split off of the loop and end in a reversing loop. However, if we do this, I'm assuming that when the train exits the reversing loop, the WHOLE remaining layout reverses?

If so, then it means without dc, only one train could be running at a time? Or if we had two trains running off of two power packs, we're asking for trouble? I understand that we would use the selector form atlas, for example to help out, we're just confused on what all happens when we flip the switch....