Why is polarity important for DCC?


Why is polarity important for DCC?

This is a fundamental electrical question regarding DCC. I have started educating myself and I know if I just follow prescribed wiring rules, everything will be fine. So, this is a question of understanding, rather than one of practical importance. There is a lot of vague information and unabashed misinformation about the electrical characteristics of DCC and it's a rare article that presents clear facts.

Here's what I think I know, so please correct me, if I'm wrong:

  • DCC is alternating current, as opposed to plain old direct current.
  • DCC alternating current doesn't represent smooth sine waves, like household current, but represents square waves of varying widths.
  • Typical locomotive motors are powered by direct current, which is why they must be isolated from the DCC current running through the tracks.
  • Even though DCC is alternating current, the circuit still has a polarity.

Since it is alternating current, can someone educate me about why polarity is important (e.g. requires reversing loops)?

I'd love to see this "in print" as the subject of one of Bruce's columns.

Polarity issues

The need for reversers in loops, wyes, turntables and frogs all arise due to the fact that, although polarity does alternate in among "left" and "right" rails when the wheels bridge a section that has a off phase polarity you create a short circuit.

Let's suppose you have a loco travelling this reverse loop. No matter which rail is + or -, when the loco hits the diverging route on the turnout from whenever direction the leading wheels will be in contact with both the red and black rail polarity. The DCC system will shutdown (at least it is what my NCE PowerCAB does). To avoid that you gap the tails

and feed the yellow and gray sections through a DPDT switch or a auto reverse circuit so once the loco hits the diverging route the yellow/gray rails are in the same polarity phase red/black rails.

Same applies to reversing wyes, turntables and powered frogs.

So in a sentence, phase polarity is important to avoid shorts in these situations.

Hope this helps!

LKandO's picture


For the same reason outlet plugs are polarized in your house (one spade larger than the other) - to prevent short circuits. The devices on the circuit don't care about polarity.


All the details: www.LKOrailroad.com        Just the highlights: MRH blog

When I was a kid... no wait, I still do that. HO, 28x32, double deck, 1969, RailPro

Not to be insulting but you

Not to be insulting but you need to read a basic electrical book. 

Even if it is a c you can't connect the two wires with out gettinq a dead Short

cv_acr's picture

Short Circuit

Whether the current is alternating or not, a short circuit is still a short circuit. AC or DC makes no difference.

This is where I think people are massively overthinking or overcomplicating things when they ask "how do I wire this for DCC?" (Or maybe they just don't know the first thing about basic electrical circuits.) DC or DCC, reversing sections and short circuits have to be handled exactly the same. The advantage of DCC is you can use those electronic auto-reverser devices, but in either DC or DCC you can also come up with reversing solutions using relays or auxiliary contacts to switch the polarity based on a turnout position.

I don't think the following is actually technically accurate, but it may help to just think of it the simply as a direct current that is flipping back and forth 60 times a second. At any given moment the opposite rails are in opposite polarity and that will short out if you bridge the rails. The polarity is constantly switching, but both rails are switching at the same time.

Actually the reason for one

Actually the reason for one outlet part being bigger has nothing in common with dcc wireing. In the ac case it is because in a three prong outlet you have basically one hot wire, and two grounds.  

Now the government and the manufacturer wants to make sure that if something happened in your table lamp for example, and a wire rubs against the metal of the lamp it does not shock you. They do this by making sure the lamp has a good ground.

Now if you have two wires then either of them could be hot or either of them could be the ground. This means there is no way to make sure that the lamp will ground out a short, as you can't predict which wire is hot. And only the hot wire matters in a short.

So they make one blade bigger then the other, and thus they know what wire will be hot and what will be ground so they can wire your lamp in such a way as to minimize the chance of a short in the lamp grounding out though you.

This also helps with modern electronics but that gets to complicated to discuss here.

So in fact your lamp really does not care witch way it is wired nor do you until something goes wrong. And really most lamps don't take advantage of this either, I just used a lamp as an example because most of us know how a lamp works. And if you don't you probably should not be wiring anything even dcc.


As far as dcc goes it is not really ac. It is pulsed dc that alternates the direction of flow very very fast. Kind of like what would happen if you you used a dc power pack and flipped the reverse switch thousands of times a second. This is why a dc loco on Dcc track does not move. Because the motor starts to spin one way then the current reverses and it starts to spin the other way, this happens thousands of times in a short order and thus the engine never moves.

As for the needing a reverse unit. Well at any give one thousandth of a second rail one is positive while rail two at that same time is negative. So if rail one ever makes contact with rail two you will get a dead short. Standard wireing issue weather we are talking a c or d c.

Now you could leave a small gap between the two rails like you do with block gaps, bu when a metal wheel crossed the gap it would connect them and you would have a short.

If leave a dead section of rail in between then a wheel won't short the track, but the current flow in The engine would as it was in both sections at once. Now if we make the unpowered section longer then your longest engine the track would not short, but the engine would stop and you would have to push the engine across the unpowered section. Not much fun.

Now if you used a pusher engine at the rear of the train that may work. As the pusher would have power and push the front engine across the unpowered section, and the the front engine would pull the pusher engine across the unpowered section. But that is a bigger pain then it is worth.

Remember in DCC the engine does not care witch way the polarity of the track is. But as per basic electrical theory, you can not allow two sides of a circuit to touch.

Please note that the above examples are massively over simplified. But the basics are correct and anything more accurate would have made an already to long post into a small book.

So to the original poster, I suggest that you get a basic electrical book for beginners, as well as a basic model railroad wiring book, and then after reading those you get a basic dcc book. Because, while dc and dcc as used on a layout, will probably not hurt you if it is messed up, it could cost you a fortune as it does your decoders, or in the worst case, burns down your house.

LKandO's picture

A lesson in context

In the ac case it is because in a three prong outlet you have basically one hot wire, and two grounds.

You are of course absolutely correct. I picked my words poorly because my mind was thinking short circuit - the answer to the OP's question. I used the outlet reference in an attempt, although obviously a poorly chosen one, to compare DCC polarity wiring to something common the OP would be familiar with.



All the details: www.LKOrailroad.com        Just the highlights: MRH blog

When I was a kid... no wait, I still do that. HO, 28x32, double deck, 1969, RailPro

DCC is not AC


First DCC is not AC.  AC  starts at Zero go to a positive peak, then returns to Zero and then goes to a negative peak equal in value to the positive peak and then returns back to zero and repeats this cycle, with each cycle being the same.

DCC is Pulsed Wave at goes to a positive peak value and then returns to zero and then to the positive peak and returns to zero.  Each of these pulses are different by the amount of time spent at positive peak and at zero. A combination of these pulses in a time period is the DCC signal which the decoder uses to control the locomotive.

DCC wiring is made up of two wires, signal goes out on one wire and returns on the other. The connection between these two wires via the decoder either powers the electric motor in the loco or the lights in the loco.

If during wiring the layout, a section of rail is connected to the wrong wire you will have a "short circuit".

Hope this clears things up.


Re: Polarity issues

Thanks. That makes sense now and you got to the meaning behind my question. I was thinking of only the signal instead of the whole circuit.

So, in a nutshell, without isolating the track segments and making a reversing section, you would end up with a direct connection from one rail to the other.

A summary for me to keep this straight:

  • Without gapping any rails in the loop, the rails themselves would make a complete (short) circuit.
  • With only one gap, a wheel bridging the gap would cause a short circuit.
  • With gaps on both ends of the reversing section, the polarity of one section can be switched (manually or by an automatic reversing device) to make the polarities of the main and reversing sections match, and fix the short.

Very nice explanation. I appreciate everyone's input.

LKandO's picture


DCC is square wave AC on the rails. The decoder rectifies AC taken from the track to PWM DC supplied to the motor.


All the details: www.LKOrailroad.com        Just the highlights: MRH blog

When I was a kid... no wait, I still do that. HO, 28x32, double deck, 1969, RailPro


Thanks for your input Doug. I know DCC is not household AC. From all that I've read and understand from other sources, the voltages for DCC alternate between equal positive and negative levels, which is AC by definition.

From the NMRA Standard S9.1:

The NMRA baseline digital command control signal consists of a stream of transitions between two equal voltage levels that have opposite polarity.

It's not simply pulsed DC. That is logical, since the decoders rectify those voltages into DC to power the circuit board. If it were pulsed DC, as you suggested, then the decoder would be unpowered (zero volts) half the time.

The fundamental flaw in my thinking was considering the signal as separate from the circuit. Even then, had I continued my line of thought, the "short circuit" would mean the signal would cancel itself out. Same outcome: train stops.

I appreciate your willingness to help.

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