dcc wiring (bus wire)

[rile42]

I am doing alot of investigating before designing and building my first dcc layout.  However, I am not confident with my knowledge of the bus wire.  For my purposes, I've decided that 14 or 16 AWG wire would be adequate for the bus wire.
My question is this: am I correct that the bus wire should NOT form a loop nor should the track itself form a powered loop.  My understanding is that somewhere the track connectors should be plastic to stop the power from being in a loop. 
I am waiting on my Digitrax Zephyr controller to arrive so I am not sure of the connections to be made at the controller itself but, I want to make sure I understand the bus wiring from there.  How to I wire from the controller to the bus making sure a loop isn't created.
I know some other questions will arise as I proceed and want to thank board members in advance for all your help.

[Doneldon]

rile-

It's perfectly okay to have a continuous loop of track. You don't need to use separate sections or electrically block a closed loop though there are some reasons you might want to have electrically separate track sections. That's especially important when you are running DC and want to be able to operate more than one locomotive at a time. DCC obviates the need for separated blocks but you might still want to have at least some sidings or an engine terminal which are electrically isolated. For example, a DC locomotive sitting idle on a DCC system will slowly fry to death. And, having several idle DCC locomotives sitting on live tracks will use some of the power capacity of your DCC system, capacity which you might want for running locos, lights in passenger cars or sound systems.

Sixteen gauge wire, like the stranded wire in a household extension cord, makes a good buss wire for small- to medium-size model railroads. Use 14 ga. for larger pikes or if you will be running a high-power system like for large scale trains. The stranded wire is easy to work with, an asset. Use short (6-8") 20 or 22 gauge stranded wires for the feeders which run up to your tracks. Ending your buss wires in a loop isn't a problem but there is no reason to do it. Essentially, you'll just be using a bit more wire than you need at some small expense.

Welcome to model railroading and this board. I know you'll find this to be an excellent resource and I'm confident that you'll find model railroading to be an interesting activity with endless variations and lots of new skills to learn.
                                                                                                                                                         -- D

[jward]

i use the zephyr, and on 2 di've used 18 guage wire for both different layouts i have successfully used 18 guage wire for both bus and feeders. it is large enough to do the job, but small enough to solder to code 83 rail. also, i am not sure you could insert 14 guage wire into the terminal block of the zephyr.

will you be using the jump ports to add additional controllers to your system?

[mabloodhound]

You are wise to do your research in advance.   You are correct....You DO NOT want the bus wires to form a loop!   The bus should terminate at both ends.   Ideally, the power pack should be in the middle of the bus wires.   Yo also want to twist the bus wires as you are installing them ( if I recall, about 6 loose twists for every foot).
Do read up some more on this from the experts.
This is just one of many helpful DCC websites http://www.wiringfordcc.com/dcc_waveforms.htm .

[Jim Banner]

Both Doneldon and Mabloodhound are correct, depending on the size and shape of the layout.  You can run continuous loops on small layouts, say 1 to 2 sheets of plywood in size.  But large layouts, particularly around the wall types that enclose a large area, can resonate at harmonics of the DCC frequencies, causing large voltage spikes that can interfere with decoders or even damage them.

The DCC buses and the tracks form large coils while the capacitance between the rails and the wires form a distributed capacitance in parallel with the coils.  Capacitors and coils in parallel form tuned circuits.  If the capacitors and coils are small, as on a small layout, the resonant frequency is high, so high in fact that the harmonics of the DCC fundamental frequency are virtually nonexistent.  As the coils grow larger and the capacitance across them increases, the resonant frequency drops.  When the harmonics are powerful enough and the frequency resonates with the DCC wiring, the large spikes mentioned above occur.

The first sign of problems is usually loss of control at points far away from the command station or booster.  Some decoders are more sensitive than others to the voltage spikes and usually lose control first.

We can go into more detail if anyone wishes.

Jim 

[rile42]

OK, from what I've gathered then, on my 5'x9' layout with a few sidings and maybe one ladder track, I could use a loop for the bus but it would probably be better if I didn't.  As for the track forming a loop, it appears the same thing is true.
So, here is my plan.  As to the bus, I'll start at the controller, wind it all around under the track and at the return to the controller, use the last feed wire as the end of the bus.  For the track, opposite the controller, I'll install two insulating track connectors.
Does that plan agree with what I've been given here?

[Jim Banner]

What you propose should work on a layout that size.  It is more common, however, to put the gaps in the rails at the point farthest from the command station and have your bus make a loop that is interrupted under the gaps.  The command station then sits in the middle.  Think of your two arms held out in front of you in a big circle, fingers almost but not quite touching.  That is your bus.  The command station sits in the middle of your chest, sending out power both ways.

When a locomotive crosses the gaps at the far side (your finger tips,) it momentarily connects the tracks together.  However the tracks are electrical equals at that point so no problem.  When the gap is close to the command station, the track are not electrically equal - one is only a short distance from the command station and the other is a very long way from the command station (as much as 20 feet on a 9 x 5 layout.)  This may results in problems when a locomotive crosses the gaps.

As far as I am concerned, transmission line and antenna theory, especially with poorly defined transmissions lines like the wiring under a model railroad and the tracks above it, is one of the black arts of electronics.  You follow the rules and it will probably work.  In the rare instance it does not, you fiddle until it does.  Problems usually start rearing their ugly heads when the buses exceed about 30' in length.  Then there are things you add along the buses or at the ends of buses or you can add a booster every 60 feet or so along the layout.  Don't worry about these problems until the time comes that you have to. 

Jim

[rile42]

I intended to wire the bus using the chest/arms analogy.  However, and I know this must be very elemental, I have had difficulties envisioning how the wiring would procede from the contoller.  Would I run a line from the controller to two lines, one to the left, one to the right and splice them all together?  Or, would I just run two lines from the controller one to the right and one to the left for each polarity?  I'm kind of guessing that there must be some gadget I could use to distribute the power from the controller to the bus in both directions.

[Jim Banner]

Just the run the heavy bus around the table ("finger tip to finger tip, the long way.)  Then tap into its center with some lighter wires, say 18 gauge.  Run these wires to your command station.  The power will flow from your command station, through the lighter wires, to the bus.  How much is drawn in each direction will depend on the loads.  That will take care of itself.  If all the power is needed on one side of the table, that is where it will go.

For the bus wires, I often use 14 gauge Romex type house wire.  I split the outer casing and take out the three wires - a black, a white and a bare one.  I use the white wire for the outside rail and the black for the inside rail - no special reason for the colours but after wiring a number of layouts, it is easier to remember if the colour coding is consistent on all of them.  The bare wire I often string down the middle of the table as a common return wire for lighting and switch motors.  I don't know the prices in your area but where I live, 14 gauge house wire is cheaper than 18 gauge lamp cord or bell wire - go figure.

Jim

[rile42]

Thanks for all the help.  I'm pretty sure I now have the mental picture of how to wire the power bus around my layout.  Now, if my hardware would just get here.

[Ed Kunkel]

I have two independent tracks - mostly parallel - one controlled by Digitrax in a continuous loop with feeders.  It has been working great since it was finished a year ago.  The second track had no bus/feeders until I was inspired by the above writers to add an "extended arms" bus/feeder line with an E-Z
Command controller.  Now neither system works though there is power going to the controllers. 
Can there be a "harmonic interference" condition since one track has a loop and the other has extended arms with fingers ten feet apart?
We are fortunate to have electric/physics wizards available to address these issues.
Thank you in advance for your response.
Ed