# Reversing loop question



## Walman (Dec 18, 2011)

I'm drawing a blank. If I have a reversing loop section I know I electrically isolate it and then use (as one example) a Digitrax PM42 device to control the auto reversing in that section. Here is what I can't remember. Does my whole train need to fit inside that reversing loop or just the engine(s)? This assumes I don't have any cars picking up power for lighting (which I won't). If the whole train does have to be in the reversing loop can you explain for me why?

Thank you!

Walman


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## Brakeman Jake (Mar 8, 2009)

You have it right for the electrical problem.However,you don't need a PM42 for a single loop,though it would do the job fine.An AR-1 will do the same for a much lower cost.


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## RT_Coker (Dec 6, 2012)

Walman said:


> Does my whole train need to fit inside that reversing loop or just the engine(s)? This assumes I don't have any cars picking up power for lighting (which I won't).


Walman,
The key to your question is metal wheels on the non-power cars. I “cheated” and used clear finger nail polish to extend the gaps. Double staggered gaps may also work depending on the various spacing’s involved.
Bob


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## Southern (Nov 17, 2008)

Walman said:


> Does my whole train need to fit inside that reversing loop or just the engine(s)?


Just the engines.



Walman said:


> This assumes I don't have any cars picking up power for lighting (which I won't). If the whole train does have to be in the reversing loop can you explain for me why?
> 
> Thank you!
> 
> Walman


 the lights in the the cars do not know or care about the phase of the curent on the DCC track.


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## waltr (Aug 15, 2011)

The issue to the OPs question is:
you do not want the loop's polarity changing when both ends of the train's wheels cross the gaps at the beginning and end of the 'loop'.

Think of this:
1- the loco enters the 'loop' (crosses the first gap). the auto-reverser detects a short and reverses the polarity in the loop. All in good and the loco pulls the train through the 'loop'.
2- the loco exits the 'loop' (crosses the last gap).the auto-reverser detects a short again and reverses the polarity in the loop.
a) if the entire train is within the 'loop' then no problem at all the the train goes on.
b) if there are still cars with metal wheels outside the 'loop' (they have not yet entered the 'loop'). Then when their wheels cross the entry gaps the auto reverser detect a short and reverses the loop's polarity. Now the train is still exiting the 'loop' so wheels will be crossing the exit gaps causes a short that the auto reverser detects and switches the loop's polarity again. So, as each wheel crosses either the entry gaps or the exit gaps the auto reverse switches the loop's polarity. 

Bob's (RT_Coker) solution works since the Loco's wheels typically are tied together electrically therefore can bridge an extended or staggered gap.
The wheels on car tend not to be tied together electrically (two wheels on a metal truck frame will be, but still shorted than most locos) so do not create a short.

On auto reversers:
DigiTrax AR1 and PM42 use relays to reverse the polarity. Relays are fairly slow to switch and make a sound when switching.
I've tried these and do not like the clicking of the relays and the slower response. I now use Tam Valley Frog Juicer's jumpered to work as auto reverse. Those use solid state (MOSFET) switches to reverse the polarity and are fast and silent.


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## Southern (Nov 17, 2008)

Thanks for posting Walter, I always like to learn more about this great hobby


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## cv_acr (Oct 28, 2011)

Southern said:


> the lights in the the cars do not know or care about the phase of the curent on the DCC track.


If you DO have lighted cars, they may not care about what polarity it is, but they still WILL absolutely cause a short circuit if you bridge both ends of the reversing section at the same time.

So the reversing section must be longer than your *longest set of powered equipment*, whether that just be engines, or included cars with power pickups for lighting or other effects.


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## Walman (Dec 18, 2011)

Very helpful everyone. Thank you!

Walman


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## Hutch (Dec 19, 2012)

If your using a Tortoise or some other switch that includes an extrar DPDT switch, then you won't need either of the above auto switches. I switch mine like this with zero problems. Much cheaper and easier solution.


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