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!

AC

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.

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

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.

A lesson in context

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.

DCC is not AC

Suncat2000

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.

Doug

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.

DCC is AC

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

DCC is AC

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.

Sorry

I was wrong in my description of the DCC wave.  It is series of Bipolar DC pulses (see Alan drawing), with the length of each pulse determining if it is an "one" or "zero".  An "one" is roughly half the length of the "zero" pulse.  But where it fails to be AC is that the pattern is not repeating.  The pattern is changing as the command station sends a random combination of "one" and "zero" to complete the required message. ie 11001010001. 

Yes this is splitting hairs, but if I said DCC was AC, my electrical profs would be rolling over in the graves.

Doug W

Can I ask

this question, because I'm curious and think it may have bearing on polarity of rails-

Is DCC 'restricted' by the motors of locos?  Is it possible to create a motor that can go on the pulse of a single signal, and separate things internally/electrically/digitally if needed?  Could a motor be designed that takes a signal, and in that signal there is a A and a B polarity embedded (if even needed), and it goes from there?  DCC evolved from the motors already out there, so just asking.

Please forgive my Star Trek thinking...maybe too much SyFi channel!

my electrical profs would be rolling over in the graves

Not this one- AC signals are not limited to repeating waveforms. Many do repeat, but it's not required. It's actually like MIDI and RS323 serial communications, except with a lot more current available. 

pqe

AC Signal vs AC Power

I see your point Pelsea, but my electrical Profs worked in the world of  AC Power and not in communications.

So it depends on one's background how these simple terms apply.  Is it AC Power no, an AC signal, I will take you at your word that it is, since it is not may background.

Doug W

Steve, the DCC signal is

Steve, the DCC signal is filtered by the decoder, which uses the digital signal (00111000111001001...) to control the outputs of the decoder. The motor and the other functions (mainly lights) receive a DC signal that can be voltage-constant but turned on and off (as in a beacon light) or voltage variable (motor control, dimmable headlights) or PWM (also motor control). So, a DCC engine is a DC engine controlled by a "computer" (the command station) through a "cable" (the rails) and the signal is turned from computer-speak to eletric connections (light functions on/off, motor control, triggered sounds as horns and bells in a sond decoder) by the dcc decoder.

So the DCC signal comes out of the booster, through the rails, into the decoder (through the track power pick-ups) and from then on everything is normal DC.

Not to take this thread down

Not to take this thread down a different rabbit hole, but if the Decoder is basically acting as a rectifier for the signal, does it always send full voltage to the motor and just quick pulses (fast pulses=faster speed, fewer pulses=slower speed), or does it also vary the voltage and send the motor a continuous appropriate voltage for the speed chosen? 

I assume the latter, but don't have one to test at the moment to know for sure...

EDIT:  Ahh Never mind.  I just reread the previous post and saw my answer....

-Stephen

As long as we know how to hook it up

why does it matter if it uses DC or AC? Doesn't all this stuff come prebuilt and ready to plug together? In the old days cars had generators then they switched to alternators but they still drove the same way.  ....DaveBranum

It does matter

A AC motor is a different beast than a DC motor. In Germany ther is a manufacturer of locos (märklin) that uses AC current in the tracks and internal wiring. If you try to convert one of these engines to DCC you have at least to change the magnet of the eletric motor.

Decoders include a computer

All basic decoders include three hardware elements; a DC power supply, a microcomputer and a motor driver. The DC power supply section includes a rectifier to convert the track pulsed DCC to a DC voltage to power the decoder and the motor. The micro detects and decodes the DCC signals and generates signals to drive the motor and the various functions. The motor drive signals generated by the micro drive the full bridge motor driver. Additional circuits are added to support other functions. There is also the software in the microcomputer, but that is another topic.

For a representative circuit check out dec11sch.pdf designed by Mike Bolton for the MERG crew. On the left of the schematic is the rectifier comprised of diodes D1 - D4. The microcomputer to the right is labeled U1. In the center is the H bridge motor driver circuit with the power transistors labeled Q12 and Q13. On the far right are five transistors (Q3 - Q7) for driving external functions (lights).

Stephen, To answer your questions regarding the motor drive voltage. Except for some Large Scale and special AC decoders, I have not seen one decoder that does not switch the full track voltage directly to the motor. Again referring to dec11sch you can trace from the Track inputs through the rectifier diodes directly to the switching transistors driving the motor. There is no “regulation” of the voltage, so decoders do not normally* send a continuous appropriate voltage (ie. continuous DC Voltage) to the motor. To control the speed of the motor the decoder Pulse Width Modulates (PWM) the signal to the motor. In a PWM system the relative pulse width determines the power supplied to the motor not the frequency of the pulses. The signal on the left of the following illustration will result in slower motor drive with the one on the right running faster.

* In the case of 100% PWM the switches are always on and the full available voltage is continuously applied to the motor. But how often do we run our locomotive at full throttle.

Check out my early columns, starting in Oct 2011!

DCC

The decoders send PWM to the motor. Positive or negative 12 to 14 volt pulses. A meter will see this as a DC voltage. A Scope will show you the pulses. Wider for more speed, narrower for less speed. Zero speed, no pulses.

Rich

Re: early MRH columns

Thanks, Bruce,

I learned about MRH only a few months ago, so I'll check out the early issues. The ones I have read were very readable and easy to understand. You're a beacon of knowledge. I can always get something useful out of what you have to say. Thank you.

--Ed

Electrically Challenged, ....looking for auto-reversing wiring

This was an interesting intro to DCC. I'll have to search for more, but I am not interested in learning all of the theory/details,....just want to make use of DCC to run my trains.