# How to set up a reverse loop?



## jimtyp (Jan 2, 2008)

There is something I don't understand about a reverse loop. My understanding is the loop is made up of a switch with the track looping from the straight path of a switch back around to the divergent path of the same switch. Thus the train enters and exits the loop via the same switch. My understanding also, is that the track inside the loop is in a separate isolated block.


The problem I see is that at some point as the train is either entering or exiting the loop, the track polarity must change. And when this polarity change occurs, at some point if the loco has multiple pickups, the first power pickup will be getting, let's say, positive on one side but the next wheel or pickup will be getting positive from the other side? Won't this cause issues with a loco that has multiple pickups that are wired together?


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## Semper Vaporo (Jan 2, 2008)

Picture you are sitting on the straight track about to enter the loop. Assume the loop goes to the left and back to the track you are sitting on. It does not matter which side is the diverging route on the switch. The Right hand rail you are sitting on is the outside rail of the loop as it goes to the left and when it comes back it is now the left hand rail. Inside the loop the left hand rail stays the left hand rail and connects back to the switch to the same rail at the frog... Kind of a tear-drop shape.

The whole track set inside the loop is isolated from the straight section that feeds it.

When the train enters the loop the loop itself is connected to the straight section on the side of the switch the train is to take to make the loop the same polarity as the straight section feeding it AND the other end of the loop is NOT connected to the straight section at the switch so you don't get the left and right rails shorted together at the switch.

Once the train is wholly inside the loop (the power requiring cars) THEN the loop needs to be disconnect from the straight section at the place where it entered and the other end of the loop is connected to the straight section to make its polarity the same as what is in the loop.

It is the straight section that is feeding the loop that gets the polarity reversal WHILE the train is within the loop itself.


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## jimtyp (Jan 2, 2008)

Semper, so you are saying the polarity is switched on the fly once the train is inside the loop? So one nanosecond the polarity is one way then it switches to the next? 

So how does an automatic reverser work? It has to know when the end of the train has passed so it can change the polarity? 

Is it possible to have a reverse loop without an automatic reverser?


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## Semper Vaporo (Jan 2, 2008)

The train doesn't even see a glitch in the power.

There needs to be a sensor somewhere inside the loop to detect the presence of the train and trip the relay that swaps the polarity of the track outside of the loop. A photo cell positioned somewhere will do. MicroMark sells some specific boards that tie to their reversing loop board to do just that.

To me, all "reversers" are "automatic"... the only way I can think of that is not automatice would require you to stand there and throw a DPDT switch at the appropriate time to keep from blowing the fuse on your power supply. If you forget, the train will short the non-reversed main track to the loop when the train enters the switch to leave the loop.

I suppose that if your power pickups are small enough to skip over the insulator at the switch and if your train is going fast enough it could roll right through the switch and since the polarity is now reversed from what it was when it came in (because the train is now facing the other way) then it would immediately go into reverse, spin the drivers wildly, slide to a halt, start going backwards, get up enough speed to roll right through the insulator again and go around the loop the other way... assuming the switch got thrown somehow so the train doesn't get split in the switch. (the kind of engineer I am, this sounds like a fun way to run my trains!







... but then I run Live Steam and don't need a track power reverser at all.)

As for the placement of the detector to know when the train is in the clear so the track that it has just left can change polarity to be ready for the train to re-enter it going the other direction... well, it needs to be far enough away from the switch in the direction the train is going to not reverse the polarity while some car after the leading engine that needs the power to be the same as the leading cars is still on the switched track section... AND far enough from the switch the train is headed toward, to get the power reversed before the leading power car enters it and needs power in the correct polarity... granted, flipping a relay to reverse the polarity can be just a millisecond or so, but physically moving the points of the switch can take more time, depending on whether they are moved by a solenoid (fast, snap action) or by some geared down electric motor that takes a few seconds to move them.


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## Dwight Ennis (Jan 2, 2008)

There needs to be a sensor somewhere inside the loop to detect the presence of the train and trip the relay that swaps the polarity of the track outside of the loop. A photo cell positioned somewhere will do.
I could be wrong here, but I think DCC auto reversers work differently - i.e. when the train is either entering or exiting the loop they sense the short if the polarity is set the wrong way and almost instantaneously reverse the polarity in the loop itself. Since a DCC equipped loco doesn't care about track polarity in determining direction, and since the reverser acts far more quickly than the DCC system's short detector, the polarity switch is seamless from the train's perspective. 
Is it possible to have a reverse loop without an automatic reverser?
DCC auto reversers only work if one is running DCC (which I suspect you are since you're asking this in the DCC forum). The old way of doing things in the smaller scales using track power/cab control was to wire track power to the reverse loop through a manual toggle switch that would reverse polarity within the isolated loop. This required that one stop their train within the loop, switch the loop polarity toggle AND switch the direction switch on their cab/power pack, and then start the train again and proceed normally.

I had automated reversing loops at both ends of my old track-powered SCLCo that were operated via LGB track magnets and sensors. Since there were loops at both ends, I kept the polarity of the loops constant and switched the polarity of the connecting single main line. Actual polarity was handled through DPDT switch machine contacts. Details of my old system can be found here[/b][/b]. 

While a similar system would probably work in DCC, it's far easier to just use a DCC auto reverser module.


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## Greg Elmassian (Jan 3, 2008)

They sure do Dwight... my jaw was dropping as I read this thread. 

Since this is in the DCC forum, I will assume that the topic is a DCC autoreverser. 

Basically DCC is AC, but we will assume we are looking at small increments in time so there is a "polarity" to the rails. 

The way an autoreverser works is that if it senses a short as a loco or car goes from the mainline to the isolated "reversing loop", it will reverse polarity so the transition into the loop has the polarity matched, no short. 

Now it STAYS this way until the train tries to exit, which will again cause a momentary short circuit, and the reverser changes the polarity of the loop again to match the transition from the loop to the main line. 

This all happens so quickly, the loco does not see any glitches. 

Also, since DCC locos go "forwards" when commanded IRRESPECTIVE of the way they are placed on the track, reversing polarity "underneath" a train does not cause any problems. 

This is part of the designed-in "beauty" of DCC, makes it simple. 

Now, what you must do on a DC layout, or a DCS layout is another can of worms... 

Regards, Greg


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## George Schreyer (Jan 16, 2009)

DCC autoreversers will work with DCC or any waveform that has transitions. It will work with PWC as long as the throttle is not set to max where it makes DC. 

For more general reversing info see 

http://www.girr.org/girr/tips/tips3/reversing_tips.html


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## Greg Elmassian (Jan 3, 2008)

I did not know that George. I would think that it would work if the voltage went to zero at some point. I have never tested this, and when I ask this type of question to the autoreverser companies, they give me the equivalent of a blank stare. 

Obviously you got better information, or did you actually try this out on PWM DC? 

Regards, Greg


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## George Schreyer (Jan 16, 2009)

I asked the DCC Specialists folks, that is what they told me about their autoreverser. It may or may not hold for other kinds. Basically, the short circuit detection needs to work which it won't for DC and a transformer coupled sensor


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## jimtyp (Jan 2, 2008)

Thanks Gents! I think I'm understanding how a reverse loops works with DCC, but I still have more questions. How do the points flop back and forth? I assume the points need to flip automatically after the train is through the switch but before it gets back to the switch on it's way back out of the loop? Does the PSX-AR reverser do this in conjunction with a switch motor? I believe I would not need a decoder for a reverse loop, is that right? OR can I simply use a spring switch?


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## Greg Elmassian (Jan 3, 2008)

Forget about the switch, the autoreverser basically makes your electrical route right no matter what the polarity of the reverse loop was. 

When you talk about the switch, you are probably thinking of some "automated" system on DC that was "programmed" to always enter one way, then change polarity and have you exit one way.... forget all of that stuff... leave the switch in any position you want.... this is why it's called an AUTOreverser.... it is automatic. 

You could use a spring switch if you wanted... remember, the autoreverser handles whatever polarity you need... hook it up and forget it's a reverse loop. 

No you do not need a decoder for a reverse loop, unless you want to remotely control something.... 

Regards, Greg


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## jimtyp (Jan 2, 2008)

Thanks Greg. Somehow the switch points need to move as whatever direction the points were when the train entered the loop the points will need to be the opposite when exiting. So it's either a spring switch (to hold it in place), or free wheeling, or the auto reverser connects to a switch motor control?


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## Greg Elmassian (Jan 3, 2008)

It's up to you... the easy way is a spring switch, cheap, easy, no electrics. 

Most turnouts can be run through from behind and will snap over... if you don't care which route on in and out, then this is the easiest solution. 

If you want remote control, then do that, but you have the line the switch yourself. 

If you want the switch to change electrically, one of the PSX-AR series has switch motor outputs, but not sure if you can relate it to the autoreverse function. Read the manuals on the site... if you run DCC, then you definitely will have to program it, so you definitely will need to understand the manual. 

Regards, Greg 

Regards, Greg


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## Trains West (Oct 4, 2008)

the PSX-AR will also run a tortoise machine by hooking it up to J5 pin 1&2 


when the short is senced the polarty will switch and so will the tortoise machine 


or you can throw the machine with dcc and it will also change the polarty so it is right 



and yes the tortoise will work with a g-scale turnout ..... I have a guy here in town that only uses the tortoise .... just put it in a building to protect it from the weather


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