Reversing Wye on the cheap

Andrew_au

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Just checking if this will work. Consider a Y junction on a DCC layout.

Code:
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  \     /
   \   /
   x\ /x
     |
     |

Normal solution would be to insulate at the points marked 'x' and add a reversing module. For DCC, could we get the same effect by just using a DPDT switch attached to the wye point controller (e.g. LGB supplementary switch 12070) and have it reverse the branch polarity every time the switch is thrown.

Caveats:
  • Need to make sure the wiring is right so the polarity matches the switch branch
  • Power will be cut briefly while the switch is thrown, so a stay-alive might be useful
Will this work? It seems to get all the benefits of a reverse loop module without actually needing anything more than a toggle.

Could the same technique work for a conventional reverse loop, as long as you are willing to always throw the points and not rely on push-through?
 
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dunnyrail

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Effectively what the LGB Reverse module does is just what you propose swopping the polarity on the fly. So long as the point motor only is used to change the king point (x) yes it will. You must of course ensure that the full loco or any loco consist is in the wye. When you throw the point the supplementary switch may throw so quick that you may not even slow down the loco. It will also work on a reverse loop. The added benefit is that you can run through the point in either direction and the setup will work but as you say no trailing through otherwise you will create a short. You will also need 4 isolating fishplates at the end of the points into the y or loop. My only worry is how the switch will manage with DCC voltage flashes on the contacts but they should be robust enough to manage.
 

phils2um

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I think it depends on whether the tail of the wye is a dead end. My LGB switch motors are fairly reliable but I would not trust them for such a critical task much less trust myself with a manual switch!:eek: With the overall cost of my DCC system the price of a reverse-loop module is not significant.
 

Gavin Sowry

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DPDT switch is all you need.... less than 5 bucks at Dick Smiths.
And, yes, all 4 rails leading into the turnout (that's the end with 4 rails, not the other end wit only 2) need to insulated.
Effectively, what you are doing is providing power to the tail of the wye (or triangle in proper Colonial speak). So, those power leads will come off the middle lugs on the DPDT switch.
The other lugs are connected to the other two tracks. What the DPDT does is select which of those two tracks is going to feed the tail. I'm not going to give a 'red wire connects to the so and so' lecture, because you might not use red wire :angel: , but for operation, throw the switch so you are connecting the track you are arriving from. When you've stopped in the tail of the wye, flip your turnout AND the DPDT so that you are now connecting the track you are going to. If it don't work, you have put the wrong wire on the lug, swap sides. It works, I've had wyes, triangles, and baloon loops out on my Taita Gorge Railway now, like that, for over 15 years.
 

Andrew_au

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You must of course ensure that the full loco or any loco consist is in the wye.
I was going to comment that this is mechanically required to throw the switch, but it does raise the possibility of adding short circuits to derailments if someone throws the switch while a train is crossing the turnout.

In contrast, a reversing loop module should ensure that the track remains correctly polarised even against faulty switch behaviour.
 

Gavin Sowry

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Search out the thread on Taita Gorge Railway, and go straight to page 14 to see what I've done.......
Scroll back some pages, and you will see how I installed a colour light signal into a balloon loop set up.... I use it as a polarity indicator for trains exiting back onto the main. Red light means polarity is not set right. It works. Done with a couple of diodes, a few bucks for a tab of diodes at Dick's.
 

Paul M

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A mechanically linked switch and point lever seems the best way to ensure the polarity is set correctly. A bit of wire from the point lever to the switch lever
 

chris m01

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I have a triangle like yours but run DC. I have put isolators where you suggest and merely use an LGB point motor which is not attached to a point but is just fitted with an auxiliary switch to change the polarity of the feed to the track at the third point. This point motor is wired with the point motor for the third point so both point and power switching motor flick at the same time. This arrangement has worked fine for me for many years.

This video shows the triangle in use. I was doing a bit of shunting around with battery powered locos but it does work very well with track power. The yellow LGB rail insulators can be seen by the third point. Just behind the track you can see a rather manky looking LGB point motor all by itself which does the work of polarity switching. Skip to 4:30 to see the point with the insulators. In the background you will see the point motor to the left of a set of lights. The top two lights indicate which way this third point on the triangle is set. This is very important to know because the point has to be set correctly for any train entering what is in my case the storage sidings. If the point is not set to the correct entry track before the train arrives it will cause a direct short as soon as the first set of current collecting wheels touch the rails of the point. The bottom four lights show which storage road is selected. Again all done with auxiliary switches attached to the point motors.

 
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dunnyrail

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I have a triangle like yours but run DC. I have put isolators where you suggest and merely use an LGB point motor which is not attached to a point but is just fitted with an auxiliary switch to change the polarity of the feed to the track at the third point. This point motor is wired with the point motor for the third point so both point and power switching motor flick at the same time. This arrangement has worked fine for me for many years.

This video shows the triangle in use. I was doing a bit of shunting around with battery powered locos but it does work very well with track power. The yellow LGB rail insulators can be seen by the third point. Just behind the track you can see a rather manky looking LGB point motor all by itself which does the work of polarity switching. Skip to 4:30 to see the point with the insulators. In the background you will see the point motor to the left of a set of lights. The top two lights indicate which way this third point on the triangle is set. This is very important to know because the point has to be set correctly for any train entering what is in my case the storage sidings. If the point is not set to the correct entry track before the train arrives it will cause a direct short as soon as the first set of current collecting wheels touch the rails of the point. The bottom four lights show which storage road is selected. Again all done with auxiliary switches attached to the point motors.

Nice to see some complex maneuvers being undertaken, just 3 trains with 1 a reversal in 7 minutes. That is how to have a fun operation. Notice the train with tank never swopped ends with the caboose, another maneuver that could have added to the fun. But perhaps your line represents a tourist line where such things do not always occur.
 

chris m01

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Nice to see some complex maneuvers being undertaken, just 3 trains with 1 a reversal in 7 minutes. That is how to have a fun operation. Notice the train with tank never swopped ends with the caboose, another maneuver that could have added to the fun. But perhaps your line represents a tourist line where such things do not always occur.
I generally run my open car tourist trains with a caboose at each end. Not only does that save time at the end of the line but I can also charge the passengers extra for the experience of riding in a caboose. :)
 

dunnyrail

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Interesting that Gaugemaster have today put out some useful info on their letter today. 3 parts to the article. Links:-

Analogue Link
Part 1: Reverse Loops

DCC Link

More information with details about Turntables as well.
 

Andrew_au

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On the Gaugemaster articles:


Possible criticism of the analogue (DC) article. I was under the impression that reversing loops for DC come with some additional complications not shared by DCC.

In DCC, the absolute polarities of the various sections don't matter (since the locos rectify the current and choose the direction of travel based on their own facing). You can even flip the polarity of a DCC track section while a train is moving along it. All that matters is that relative polarity matches whenever a train passes over a transition between sections.

For DC, polarity determines direction of travel. Assuming you don't want to stop trains and reverse the throttle, this means that:
  • any track section that will support trains travelling in both directions needs to have a mechanism to reverse polarity (manual or automatic)
  • polarity must be reversed while the train is not on that section (or else bad things will happen to your train)
  • you can have a fixed polarity loop and switchable mainline, but this requires that trains always travel around the loop in a particular direction. The alternative is to sync the ingress of the loop to the mainline based on the current turnout setting, and then while the train is in the loop flip the mainline to match the egress of the loop.
I didn't see any of this mentioned in the first article.

(Thought: DC reversing sections are sort-of like frog juicers, but for entire track sections)


Why would some command stations not work with their reverse loop module? Is this because they are highly sensitive to short-currents and their safety circuits trip before the reverse loop module can flip the current?

Their reverse loop module documentation is very short on details. Greg points out that it doesn't even mention supported current, but there are other details such as relays vs solid state, trip current, etc that could be relevant but for which I couldn't see documentation.


For turntables, is it a valid option to simply power all the adjoining tracks and power the moving track from the adjoining tracks?
  • should avoid any polarity issues as long as tracks that are directly lined up have the same polarity
  • means that loco is unpowered while turntable is moving
  • requires special pick-ups to ensure that the moving track gets a good electrical join once lined up
 

dunnyrail

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On the Gaugemaster articles:

I was under the impression that reversing loops for DC come with some additional complications not shared by DCC.
yes you are correct in this assumption. You need to have Doides and go round one way, but see further.
In DCC, the absolute polarities of the various sections don't matter (since the locos rectify the current and choose the direction of travel based on their own facing). You can even flip the polarity of a DCC track section while a train is moving along it. All that matters is that relative polarity matches whenever a train passes over a transition between sections.
In effect a DCC reverse module does just this. See later Re turntables as well.
For DC, polarity determines direction of travel. Assuming you don't want to stop trains and reverse the throttle, this means that:
  • any track section that will support trains travelling in both directions needs to have a mechanism to reverse polarity (manual or automatic)
true
  • polarity must be reversed while the train is not on that section (or else bad things will happen to your train)
true
  • you can have a fixed polarity loop and switchable mainline, but this requires that trains always travel around the loop in a particular direction. The alternative is to sync the ingress of the loop to the mainline based on the current turnout setting, and then while the train is in the loop flip the mainline to match the egress of the loop.
not quite true, if the polarity on the main line is reversed while a train is in the loop you can go round the loop either way, the diagram below shows what I think you mean on a dog bone dodgy drawing.
I didn't see any of this mentioned in the first article.
possible that there have been developments since that may make some of the assumptions out of date, never said that the articles were perfect just useful. At lest they have helped get the grey matter on the go.
(Thought: DC reversing sections are sort-of like frog juicers, but for entire track sections)
true

Why would some command stations not work with their reverse loop module? Is this because they are highly sensitive to short-currents and their safety circuits trip before the reverse loop module can flip the current?
This could well be so, when we first got a Massoth DCC system on the Ruschbahn we found that it was tripping far too often, there is the ability to reduce the sensitivity to cut out and this resolved matters. Please do not ask me how to do this, was over a decade ago now!
Their reverse loop module documentation is very short on details. Greg points out that it doesn't even mention supported current, but there are other details such as relays vs solid state, trip current, etc that could be relevant but for which I couldn't see documentation.
I think you will find that the documentation is for the 00 system that Gaugemaster supply, they do not use the large power supply in terms of volts or amps that we need for our bigger trains. Multiple G scale lash ups would likely burn out a Gaugemaster 00 reverse loop module.

For turntables, is it a valid option to simply power all the adjoining tracks and power the moving track from the adjoining tracks?
  • should avoid any polarity issues as long as tracks that are directly lined up have the same polarity
  • means that loco is unpowered while turntable is moving
  • requires special pick-ups to ensure that the moving track gets a good electrical join once lined up
We had a powered Turntable on the Ruschbahn for a while. I know Andy was keen to keep sound on the TT but the issues were a bit of a pain as once the TT has spun 180 degrees you are effectively having a reverce loop on the turntable so would need a reverse loop module on the TT.

588D19D7-DFD0-41D7-BC9D-47D7D4F3C7FA.png
 

Andrew_au

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not quite true, if the polarity on the main line is reversed while a train is in the loop you can go round the loop either way, the diagram below shows what I think you mean on a dog bone dodgy drawing.

View attachment 300768
I agree with that diagram, but my claim/question is slightly stronger.

If only the central section ("main line") is reversible, then loops 1 & 2 are "fixed" and thus you always need to travel the same way around the loop (for a given polarity of the throttle). I'm not sure what it would take to allow the polarity on the side loops to automatically follow the mainline and turnouts. You can't just make the loop polarity follow the turnout / mainline polarity - the loop needs to follow the mainline on ingress and then the mainline follow the loop on egress. Basically, whatever section the train will enter next needs to match polarity with whatever section the train is in currently.

DCC is simpler. Whenever you detect a mismatch one side still needs to flip, but with DCC it doesn't matter which side.


Side question: one advantage of DCC when it comes to reversing is that flipping the polarity of an active section has basically zero effect on the loco & train - the main risk is shorts across the insulated gaps (at which point - smart autoreversers notwithstanding - the booster's circuit breaker should kick in and everything goes unpowered and slides to a halt). In contrast, what happens if I spontaneously flip the polarity of a DC track section when the loco is moving at speed? There's obviously a risk of derailment of rolling stock due to the shock of the sudden reversal. Is there also a risk of motor damage or are the motors usually robust enough to handle this?
 

dunnyrail

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I agree with that diagram, but my claim/question is slightly stronger.

If only the central section ("main line") is reversible, then loops 1 & 2 are "fixed" and thus you always need to travel the same way around the loop (for a given polarity of the throttle). I'm not sure what it would take to allow the polarity on the side loops to automatically follow the mainline and turnouts. You can't just make the loop polarity follow the turnout / mainline polarity - the loop needs to follow the mainline on ingress and then the mainline follow the loop on egress. Basically, whatever section the train will enter next needs to match polarity with whatever section the train is in currently.
Analogue Only. No the main is flipped while the train perambulates around the loop in either direction. of course this only works if one train is running.
DCC is simpler. Whenever you detect a mismatch one side still needs to flip, but with DCC it doesn't matter which side.


Side question: one advantage of DCC when it comes to reversing is that flipping the polarity of an active section has basically zero effect on the loco & train - the main risk is shorts across the insulated gaps (at which point - smart autoreversers notwithstanding - the booster's circuit breaker should kick in and everything goes unpowered and slides to a halt).

In contrast, what happens if I spontaneously flip the polarity of a DC track section when the loco is moving at speed? There's obviously a risk of derailment of rolling stock due to the shock of the sudden reversal. Is there also a risk of motor damage or are the motors usually robust enough to handle this?
yes that is so, but if you have the loop with diodes as the early LGB Analogue reverse loop packs did the you must go round the loop one way and flip the controller from say forwards to reverse. Better if that can be done with a switch and at speed does not matter, however not sure what prolonged high speed flipping would do to the controller long term. On my setup that used this system I ran the train into the loop forwards (clockwise running only) stopped it then drove the train out with the controller reversed.
 

Greg Elmassian

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Side question: one advantage of DCC when it comes to reversing is that flipping the polarity of an active section has basically zero effect on the loco & train - the main risk is shorts across the insulated gaps (at which point - smart autoreversers notwithstanding - the booster's circuit breaker should kick in and everything goes unpowered and slides to a halt). In contrast, what happens if I spontaneously flip the polarity of a DC track section when the loco is moving at speed? There's obviously a risk of derailment of rolling stock due to the shock of the sudden reversal. Is there also a risk of motor damage or are the motors usually robust enough to handle this?

reversing DC polarity on a DC train while under speed? Well at a minimum, wear and damage to drive train, brushes, etc.

When locomotives become sentient, they may rise up and bitch-slap you!

You are really asking if there is a risk of damage? o_O
 

PhilP

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If you accidentally reverse the polarity of DC track, whilst a loco is running, then yes you will very likely cause damage to the drive-chain. - Normally, the worm, will chew the teeth off the worm-gear.

This is more likely, at speed, and with a heavy train, where there is more momentum to overcome.

You really do not want to do this.

The logic of switching section polarity, can get quite complex, quite quickly..
Reverse loop modules, can simplify this for you.
Only you can balance their cost, against (the probably cheaper) switches and wiring, for a manual system.

PhilP
 

ntpntpntp

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I used to help operate a DC powered 009 layout back in the 70s and 80s. It had a reverse loop. We developed the simple technique of setting up the loop to allow the complete train to enter, change the point at the neck ready to exit, then simultaneously change the loop DPDT switch and the controller direction. The train continued and exited without stopping. Easy.
None of my own layouts have reverse loops so it's not something I do nowadays.