# REVERSE LOOP



## dd1228 (Jan 26, 2015)

is any special wiring required for a reverse loop?


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## OilValleyRy (Oct 3, 2021)

Yes. The section must be insulated from power at both ends, either with plastic rail joiners or gaps in the rails. The power feed to the isolated reversing track must be reversible, that is, polarity needs to be reversible. This can be done in a manual fashion using a toggle switch, or automatic using an Auto-Reverser (sold by Digitax, NCE, and probably others such as Circuitron).

Note that a reversing loop must be long enough to hold all locomotives being reversed AND/OR any lighted equipment such as passenger cars or caboose.


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## JeffHurl (Apr 22, 2021)




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## wvgca (Jan 21, 2013)

you can get several different kinds of automatic reverse loop controllers for DCC. the simple ones [and cheaper] have a mechanical relay, and some run a preset 'short circuit' level .. the better [and more expensive] ones have an electronic switch for polarity [much faster], and a widely adjustable range of short circuit loads that can be selected.. on my layout i ran one of the cheapest ones, [MRC AD520] and it worked fine ..


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## dd1228 (Jan 26, 2015)

Thanks for the reply. I guess I really don't understand the why of this since the dcc voltage applied to the rails is AC which has no polarity. I figured you could just gap the loop and run feeders to the loop. Oh well so much for a "little knowledge is a dangerous thing!"


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## wvgca (Jan 21, 2013)

dd1228 said:


> the dcc voltage applied to the rails is AC which has no polarity.


it's not the 'polarity', it's the 'phase' .......


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## JeffHurl (Apr 22, 2021)

It's not AC like you would have at an outlet. It's more complex than that. Suffice it to say that an AR module resolves shorts that would otherwise trip the automatic shutdown features built into your DCC controller. And it does so automatically so you never have to worry about it.

The DCC decoder doesn't care about phase, but the DCC controller can't handle a short, so we use the term "polarity" since most people understand that concept as it relates to short circuits. And since the decoder doesn't care about phase, the AR module simply makes sure that all pickups (even for lighted cars if there are any) are on the same phase.


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## DonR (Oct 18, 2012)

DD

As I posted in your N Scale thread, to get
correct information for the wiring of a
reverse loop we need your track plan.

DC has Polarity. DCC (AC) has Phase.
In either case if the red wire touches the
black wire you have a short circuit...read this
to include the left and right track rails. 

When you have a loop that begins at a
turnout and winds around back to that same
turnout you'll see the that the 'right' rail
will 'touch' the 'left' rail at the turnout. That
is a short circuit. The formed 'loop' must be
isolated by making gaps (or using insulated
joiners) in both rails at both ends of the 'loop'.
The 'loop' is powered through an automatic
reverse controller. When the loco wheels
cross the 'gap' there is a short. The reverse
controller rapidly matches the 'loop' track
to the main track electrically. It's all so
fast the loco doesn't pause and the light
doesn't blink. When the loco reaches the
'other' end of the loop the wheels again
short when they cross the 'gap'...and again
the reverse controller matches the phase 
of the loop to the main. 

That's the way it works. 

Don


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## JeffHurl (Apr 22, 2021)

Don is spot on as usual. And this is a baeutiful solution since the DCC decoder doesn't care about phase. It just gets power and instructions. It turns the "AC" power into DC output for the motors, and controls polarity of the DC output to the motors to control direction... all based on the instructions... but the inverter can take either phase and conveet it to the proper polarity of DC output in order for the DC electric motor to turn the wheels the right direction and at the right speed.


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## dd1228 (Jan 26, 2015)

Thanks guys. I guess it's not as simple as I thought.


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## JeffHurl (Apr 22, 2021)

The actual wiring is the same if for DC, in that you have to isolate the loop. But the AR solution is much simpler and more elegant than anything you could do with DC. With DC, you have to manually assure the polarity is correct before going into the loop and than also manually prepare for the exit from the loop.

It's really not that complex. Just do a little more research. it's pretty straight forward once you see someone else do it. A lot of conversation comes up about all the various complexities that an AR module can resolve. BUt in reality, a simple reverse loop is not difficult to wire at all.


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## OilValleyRy (Oct 3, 2021)

From what I understand DCC signals are a Square Wave DC, which imitates AC but isn’t true AC. I’m unsure why they opted for that, perhaps one of our resident electrical engineers can confirm the reason. I suspect it is easier to convert Square wave DC to DC than converting AC to DC, which likely matters in close quarters of decoders and locomotive internal space; but that’s just an guesstimate.


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## Severn (May 13, 2016)

I would buy one of the "auto reverser" products ... In fact my intent is to do just that for my own, although I haven't quite gotten there yet.

But I don't know which one ... I would want instantaneous ... all digital then. Last I looked there's several in this category.


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## OilValleyRy (Oct 3, 2021)

Severn said:


> I would buy one of the "auto reverser" products ... In fact my intent is to do just that for my own, although I haven't quite gotten there yet.
> 
> But I don't know which one ... I would want instantaneous ... all digital then. Last I looked there's several in this category.


I’d suggest the NCE one. It’s solid stateI believe, no delays or interruptions.
I cannot confirm that the Digitrax AR-1 was updated, but it used to have a momentary pause. It also had a mechanical tuner for sensitivity. 
I’ll be using the NCE product.

Roy Smith (UP Evanston Division) on youtube did a side by side comparison, including the operational differences being demonstrated. That vid settled the debate for me.


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## JeffHurl (Apr 22, 2021)

I have a Digitrax AR-1. It appears to me to be solid state... I don't see anything on it that resembles a mechanical relay. I believe the older ones do have mechanical relays.

If I ever need a second one, I'll try the NCE version. The Digitrax one works, but the input and output terminals are really small. I couldn't get anything bigger than 18 AWG in them.


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## mesenteria (Oct 29, 2015)

OilValleyRy said:


> From what I understand DCC signals are a Square Wave DC, which imitates AC but isn’t true AC. I’m unsure why they opted for that, perhaps one of our resident electrical engineers can confirm the reason. I suspect it is easier to convert Square wave DC to DC than converting AC to DC, which likely matters in close quarters of decoders and locomotive internal space; but that’s just an guesstimate.


It absolutely is AC current. It's just the wave form and the 'zero stretching' components that make it 'digital' in its application for our hobby's purposes. 

They opted for it because the electronics require it to function the way we need them to function. And no, there's no practical difference in rectifying household AC and digital or 'square wave' AC to direct current (DC), at least none that I am aware of. Obviously there IS a difference, but practically is alla same, alla same.


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## mesenteria (Oct 29, 2015)

dd1228 said:


> Thanks guys. I guess it's not as simple as I thought.


It isn't complicated, but it does help if you see the light. Understanding why it all matters, and how it is managed, will help you in managing all of your train's electrical requirements and pitfalls. If you draw each rail of the loop, including those in the turnout affording access to the loop, with two different ink colours, and make sure you don't cross them up accidentally, you'll soon see that there's a confound across the gaps at some point, depending on how the approach tracks are wired for phase/polarity, and how the loop is wired. There will be a conflict on one of the two ends of the loop. The digital reverser senses the conflict as soon as the first metal tire bridges the first gap that is conflicted, at which it reverses the phase to BOTH rails immediately, usually within a few milliseconds. The decoder doesn't care what's happening below it, provided it doesn't get conflicted power for any significant length of time. It will continue to feed the motor and lights and amplifier even as the polarity or phase changes below it at the rails. 

However, your command station will mind unless the changeover happens inside of the time it takes for its own circuitry to trip to protect itself.


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## wvgca (Jan 21, 2013)

the MRC auto reverse has four wires that are maybe three inches long coming out of the wrapped package ... no screw terminals, just the loose colour codes wires


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## OilValleyRy (Oct 3, 2021)

I guess NCE is incorrect when they say their power supply is 13.5V DC. That’s what I was going by.


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