# DCC Reverse Loop



## JohnnyB (Jan 16, 2011)

I understand how a reverse loop is wired in DC, but I am a little fuzzy on how this is accomplished in DCC. I know that with a DCC reverse loop, the loop still needs to be isolated from the main line. I also understand that a DCC reverse loop controller can be installed that senses a short at the point of polarity change and then automatically and instantaneously changes the polarity to keep the train running smooth. 

My question is, if polarity is present in DCC just as it is in DC, why when I change the outputs to the track from my DCC controller, does the train still run forward when I switch the DCC controller to forward. Wouldn't it go in the opposite direction? I guess I need someone to school me on DCC polarity vs DC polarity. 

I hope this isn't a dumb question.

thanks for your input.
JB


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## NIMT (Jan 6, 2011)

I know exactly what your dilemma is!
It's not the Track polarity that is really in question it's the signal "polarity" or phasing. The DCC carrier signal is super imposed on the ((AC) I'll say that loosely) track power that when you loop it around it cancels itself out ! Or at least scrambles it!


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## JohnnyB (Jan 16, 2011)

NIMT.COM said:


> I know exactly what your dilemma is!
> It's not the Track polarity that is really in question it's the signal "polarity" or phasing. The DCC carrier signal is super imposed on the ((AC) I'll say that loosely) track power that when you loop it around it cancels itself out ! Or at least scrambles it!


So in DCC (alternating DC), there still is a + rail and - rail just like in standard DC set ups. That is why you need to isolate the the loop. The direction of the train is determined by the message given from the DCC controller through a coded signal. The DCC reverser does not change the polarity of the track (like a DPDT switch does in standard DC reverse loops) rather it reverses the polarity of the signal being sent from the DCC controller. Does that sound right?


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## tjcruiser (Jan 10, 2010)

Johnny,

I am _not_ a DCC guy, so I'm likely putting my head on the chopping block here, but ...

I think you have most of it right, except that I don't think the reverse controller actually switches the polarity of the DCC signal. The signal still stays as a step-wise (square ware) AC voltage signal (with encoded info), and the instructional sign (+ or -) of the signal stays the same. I think what the reverse controller does is flip (or control, really) which rail is "hot" with the stepwise DCC signal (unchanged in polarity), and which rail serves as the ground back to the transformer.

If one didn't do that, then the stepwise DCC signal would largely cancel itself out when (or if) it joined up to the opposite (ground) rail.

I hope I have that right.

Chop away ...

TJ


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## NIMT (Jan 6, 2011)

Well sort of ???
It's really a complected long winded answer.
Go here and it will give you allot more answers than I can give.
My rule in if it works don't break it and it will still work!:laugh::laugh::laugh:


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## NIMT (Jan 6, 2011)

TJ and JB
An over simplified answer is that you are "bucking" Phases.
The rails are AC with a carrier signal attached and when you cross the wires in a loop you cancel them out causing a Zero volt effect on the rails and Reversal of the DCC or canceling of the carrier signal.
Like I said the complete answer is very complicated.


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## tjcruiser (Jan 10, 2010)

Good info link, Sean!


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## JohnnyB (Jan 16, 2011)

So, basically to really dumb this down, in DC reverse loops the track polarity is changed on the main line while the train is in the loop. With DCC the polarity is changed at the decoder at the instant the train reaches the isolation spot where the loop meets the main line.

One last question, what if you have passenger cars with operating lights? Does this affect anything with a DCC reverse controller? Would all the cars need to be in the loop, before the train reaches the main line at the isolation point?


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## NIMT (Jan 6, 2011)

Thank you!
It's just nice helping to keep the trains rolling along with the least amount of hickups!
No one like's a "Brokey Twain"!


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## NIMT (Jan 6, 2011)

This switch happens not at the decoder but the Auto Reversing Controller.


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## gc53dfgc (Apr 13, 2010)

Just get this and your DCC reverse loop will work fine.

http://www.bachmanntrains.com/home-usa/products.php?act=viewProd&productId=1542

I just want everyone to be clear on this and I can't tell if you are in all the above posts or not.

DC=DC Current

DCC=AC CURRENT! (seems this is one of the biggest things people don't understand with DCC maybe they chould have named it ACC just so people didn't think it was DC current.)


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## NIMT (Jan 6, 2011)

Acc is already taken, accessory power!:thumbsup:
ACC???
Analog Command Control
Humm nope doesn't quite work!:laugh::laugh::laugh:


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## gc53dfgc (Apr 13, 2010)

NIMT.COM said:


> Acc is already taken, accessory power!:thumbsup:
> ACC???
> Analog Command Control
> Humm nope doesn't quite work!:laugh::laugh::laugh:


maybe ACC=Alternating Current Control?:laugh:


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## gc53dfgc (Apr 13, 2010)

Could also always use ACDC=Alternating Current Digital Control.


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