Bill, you're making it too hard

Bill, you're making it too hard.

The bus wires just end. Here's the bus termination on my layout - both bus wires are blue - I put a sticky label on one bus wire to distinguish it when needed.

You don't want the two bus wires to connect or you get a dead short. In a few cases, a really long run of bus wire can pick up radio signal noise from the air and the purity of your DCC signal gets compromised. When that happens, twisting the bus wires can help, as can adding a resistor/capacitor filter between the two bus wires.

The business part of the bus is the feeder drop connections to the rails all along the way. That puts the DCC signal to the rails, so that it can travel up the wheels and into the loco decoder. The decoder listens to the DCC signal for messages targeting that loco number and when the decoder sees a message for that loco number, it does what it is told, be that increase or decrease the voltage to the motor, turn on a headlight or whatever.

Like I said, it's real easy. The two bus wires don't need to connect and in fact never should connect or you will get a dead short. Same as your house wiring - the black and white wires go all through your house and generally will terminate in a plug-in box. If you think about it, it's the same thing - if you haven't got anything plugged into that terminating plug in that's in your house, everything still works fine.  If you stick a paper clip into the two holes on that terminating plug in you will blow the circuit breaker because it creates a dead short.

You don't have to connect the two wires at the end of the run to anything for feeds along the way to work just fine.

To simplify what Joe said even more,

the power to the buss wire comes from the booster to one buss wire, is distributed to one track from that buss wire through the drop wires connected to that buss wire, goes through the decoder in the locomotive and out to the other rail, goes back to the other buss wire through the drop wires connected from that rail to the other buss wire, and returns to the power booster. 

Joe- That one picture gave me

Joe-

That one picture gave me the difinitive answer I couldn't find in 3 different DCC books!!

Thanks for the quick reply.

Bill

Thousand words

I'm first a visual guy - that's why I like to post pictures - worth a thousand words and all that.

It's also why I love the digital rich media format for MRH. It gives us soooo much freedom when it comes to the visual side of presenting how-to information. And we've only scratched the surface of what is possible in this format!

I thought that picture would help the "ah-ha!" light bulbs come on - that's why I took it.

(shameless plug)
It's an example of the kind of stuff I cover in my DCC video. If you use the coupon code that's in the early download bonus extras for issue 4, you can get this $25 video for $10 - talk about a deal!
(end shameless plug)

I Already own all 5

I Already own all 5 volumes!!  Guess I need to go back and rewatch Vol. 3!

Bus wiring

So, I take it that I connect the bus wires to the track feed on my controller, a Roco Z21, instead of to the track, run it round under the layout with dropper wires and then just leave the two ends unconnected? As others say lots of people talk about installing a bus wire  but none - at least in my experience - actually says what wire goes where.

Your posts have shone a light at the end of the (train) tunnel!!!  (Sorry about that)  

By the way I'm in the UK. Any idea what gauge wire I should use for bus/droppers?  

Thanks.

 "this very simple,

The basic function of the bus wire is to allow multiple connections to the rails without running each one all the way from the power supply. Generally running the bus approximately under the rails minimizes the length of the feeder wires. The feeders can be small since they are very short runs. If you are wiring an oval you could follow under the rails but not connect the ends when you get back to the starting point( gap the rails too so they don't complete the bus loop) or you could start at one end and run a bus up the right and left sides and not connect them at the far end. Main thing to watch for is polarity as you go around the circle.It helps to make on bus wire white and one black( or whatever colors you choose) then key them to right and left rails as you go.......DaveB

Rocoman

Correct!  The two conductor DCC main buss wiring will connect to the track power terminals on your command station or booster and follow your track around, underneath the layout, connecting only to the track feeder drops, and terminating in a manner that leaves the two main buss wires electrically insulated from one another and their surroundings.

Buss wire gauge largely depends on length.  In HO, 14 Awg. is generally considered acceptable up to about 50 feet, though some would say no more than 30 feet.  After that 12 Awg. wire seems to be the recommendation.  Feeders are kept short, so they may be of finer gauge wire, 18 Awg. or finer, up to around 22 Awg.

Regards,

does each wire connect to itself ......

I've reread this thread, and still don't find the answer to this part of the OP's query:

does each wire connect to itself (each bus wire forming a connected oval--a red oval and a black oval) ?

On the one hand it feels kind of more tidy to connect both ends of the black wire to one terminal of the booster and both ends of the red bus to the other. On the other hand, maybe this messes up the signal somehow. 

Please clarify.

Thank you,

DrJolS

Do NOT join ends

Per wiringfordcc.com you should not connect the ends of the bus wire to form a loop.  Scroll down to 'General Considerations' and refer to paragraph 1.

Dave

The best termination is a

The best termination is a terminal block, Crimp a connector to each end and take it to separate poles on a terminal block.  This allows you to continue onwards someday, if you expand.

DCC bus wiring

Thanks for the comments, new and old, which have put me on the right track.(apologies again!) 

I knew about buses and wiring but wasn't clear on some parts as no-one seemed to go into the fine detail. Just shows - ask the right people.

termination of bus wires

Hi Roco Man

I simply use small hooks screwed into my layout and a spring in each bus wire. The spring is then hooked over the hook and keeps the bus wire nice and tight. As you have seen, the other end of the bus wire is connected to the DCC system track out.

Henry 

Bus wiring loop

Hi All,

the idea of the bus wiring forming a loop back to the controller has intrigued me for a while.  Nothing I have ever read has recommended doing it but on the other hand I have never read anything that said "don't ever do it because it will . . . insert dire consequence here . . .

Is there any reason not to do it other than the fact that it is not necessary such as performance issues or potential damage to decoders or the controller ?

Don't loop the DCC buss

There is no benefit at all. In fact, if your layout is long enough, it would degrade the DCC signal. Of course "long enough" is measured in miles at the DCC frequency. This has to do with the waveform canceling itself out as it circles the loop at the speed of electricity (not as fast as the speed of light, but up there.) Incidentally, the advice to terminate the buss is based on the same principle-- some percentage of the signal reflects back from an un-terminated wire end. Again, that only matters with extremely long wires. Folks who deal with high frequency systems like data transfer and video have to worry about this all the time. So does the power compnay, with a very low frequency, but immense distances.

pqe

edit: I did the math after breakfast.-- "long enough" is 5 to 7 miles depending on some details of the installation.

Wow!

I was the original poster of this question some six years ago.  I'm surprised to see it pop back up again.  My subject post is "Wow" because I just recently returned to working on my layout.  I had made a mental note to go back and look for this thread to make sure I continue the wiring process correctly.  

I think I did it right: 

The above pictures show the bus for the lower level.  My construction has resumed with work on the upper level (which had been about 2/3s completed).  I had run a second bus on the upper level...but I can't remember why.  I THINK it was perhaps that I was considering each level as its own power district.  Would that make sense?   

(While progress has been made and things have changed on my layout, my dislike and knowledge of wiring and DCC obviously has not!)

 "on the other hand I have

As PQE says  it needs to be a long run before it causes problems. I've had layouts wired for DC with no twisted wires and bus wires forming a loop and they worked fine when DCC was added to them. I doubt I'd ever build a layout big enough to worry about these things. It's not too much extra trouble to twist the wires a bit or not wire up bus loops so probably worth it just to eliminate doubts( although twisted wires are harder to solder feeders to).....DaveB

Yeah, I couldn't really see

Yeah, I couldn't really see how wiring the bus wires back to the controller could do any damage unless of course they were NOT wired back to their originating terminal thereby creating a short.

With regards to the DCC signal, I guess by the time the bus length causes a problem you would already have split the layout into separate power districts with boosters anyway.

Nooo, don't

"don't ever do it because it will . . . insert dire consequence here . .

It will surely rent asunder the space-time continuum.  It will mean the end of civilization as we know it!

It is not recommended to loop the bus

It is not recommended to provide electricity more than one path to flow; unless it is intentional, as in the case of a electrical circuits or parallel lights.  Instead of providing redundancy, more than one path for electricity to flow (such as a loop) sets up a parallel circuit that can cause an unpredictable and constantly changing voltage reference point.  I am sure many have experienced this with audio equipment when a 'ground loop' gets inadvertently created.  You end up with hiss and distortion in your audio.  If this is done with DCC you can get that same distortion which will affect the data being sent to the locomotive.  Another good example of a 'ground loop' is when more than one DCC booster are connected to the command net, plug into the same electrical power source and not grounded together.  This 'ground loop' allows the voltage reference point for the booster to fluctuate and can corrupt the data transmitted over the command net.  

My recommendation would be to never loop the BUS and to minimize the number of track feeders (connects the BUS to the rails of the track) that power a particular electrical block of track.  Do this, not by limiting your track feeders, but by breaking you track work into electrical blocks that only require one or two track feeders to power that block. Remember the rule of thumb is that one track feeder can be placed every 6-9 feet.

For those interested in more information, I provide a simplified example below:

One thing that many people don't think about is that our electrical situation consistently changes because our locomotives are in motion.  As everyone knows the track we use has a significant amount of resistance per length.  Therefore as the locomotive moves away from the power source the resistance between the power source and locomotive increases.  If the locomotive is on a continuous loop of track with the power feed on one side, then the locomotive is seeing an increase in resistance on the side traveling away from the power source and a decrease in resistance on the side traveling towards the power source.  Now imagine a un-soldered rail joiner in that loop that is a little loose or corroded.  The resistance of that rail joiner could be anywhere from 0 resistance (as it should) to infinite resistance (open).  All this creates a constantly changing voltage reference point.  Not a big problem with a single DC locomotive on a loop of track, however, with multiple, DCC locomotives you may experience a problem.  The solution is to electrically gap the loop of track opposite the power source so the locomotive is only being feed power from on side at a time.  The solution is so simple it is not worth taking the chance of having a problem.

Putting a BUS under tha loop and connecting track feeders to reduce the voltage loss is a great idea.  But from an electrical loops point of view it gets slightly more complicated.  First, do not make a loop out of the BUS for the same reasons for not making an electrical loop out of the track.  Second, as soon as you connect two track feeders to the track you have created another electrical loop.  Fortunately, the solution for this is simple; insert some more isolated rail joiners.  If you plan to use 4 track feeders to power the loop then you should divide the loop into four blocks and use 1 track feeder per block.

Of course the examples I stated above is based on the theatrical world and is the ideal way to do it.   I doubt that having two track feeder wires 6 feet apart on two pieces of track that are soldered together will be a problem. On the other hand I would not build a whole layout that is one big block with multiple track feeds and un-soldered rail jointers.  People who experience decoders that reset and 'weird', and intermittent electrical issues; this could be the problem.  Once again I would never loop the BUS and I would minimize the number of track feeders to no more than two per electrical block. 

Sorry this was so long.  I will take my electrical engineering hat off now.

Art

If you have a fairly large

If you have a fairly large loop of tracks, all connected for continuity, and feed it at opposite ends of the loop, either with DC or DCC, what damage or unwanted behaviour happens?  In my reading, nothing.  Who knows definitively?

Thank you for the detailed

Thank you for the detailed explanation Art. 

You have also addressed what would have been my next question regarding multiple feeders creating loops unless the track is isolated between feeders.

A fairly large loop

Selector-

It all depends on how you define a fairly large loop.  It is also depends on whether you are speaking ideal or reality. In reality, if you have a loop around a Christmas tree, whether DC or DCC, I don't think you will run into a problem with a continuous loop and one power source.  Ideally, however you should even gap the Christmas tree loop.  However, if you have an around the wall layout in a 12' X15' room that results in a continuous loop of track, all connected for continuity, and feed it from opposite ends of the room, ideal becomes reality, and I think you have a good chance of experiencing a problem.  This would be even more true if two or more people are operating at the same time at different location on the loop (assuming DCC) with multiple locomotives.  All the different voltage levels could introduce 'weird' behavior caused by corrupted data.  Remember we are not just powering our locomotives but, also sending them data.

The solution to this possible problem is so simple it does not make a lot of sense not address it.  All that is required is to use a few isolated rail joiners in the right places.  It also makes sense to do this from the beginning so it can be done while laying track.  I would just electrically block out your layout for power districts and/or block detection; even if you don't plan to do either.  All it is really going to cost you is a little planning and some isolated rail joiners because you were already going to put in a few track feeders from a BUS to avoid power loss through the track.  The added benefit is that if you change your mind about power districts and block detection, the track work is done.     

Art

Don't confuse power loops and ground loops

EEs are taught at an early stage that all points on a wire are the same, and that no matter how complex the shape, the potential is equally distributed. Later, we learn that that only applies for DC or AC on the small scale, and only if there is a single source of potential. The scale issue is what I described earlier--basically, when the size of a wire approaches a quarter of the wavelength of the signal you get strange interactions. The model railroad world is safe from this problem, because the wavelength of DCC is immense compared to the size of the most ambitious layout.

It is the second issue, multiple sources of potential that Art has been talking about. Audio engineers strive to avoid loops of wire, because a loop is a circuit, and it is amazingly easy to get small currents flowing in one, driven by incidental magnetic fields-- these can come from wall wart transformers, radio stations, even starlight. Since analog audio requires a lot of amplification, we are fighting a constant war to eliminate hum, static, and chatter from passing taxicabs. The solution to the problem is digital audio. Digital circuits deal only with relatively high voltages, with no amplification, and basically only respond to on and off. A couple of simple tricks, such as differential signals and twisting the wire is all the protection needed. Digital systems are still analog at the ends, but these small components are easily protected.

So don't loose any sleep over the loops made by buss, feeders and track. It does no harm to loop a buss, but there is no need to, and doing so may complicate future expansion. Breaking a layout into small power districts is a good idea, even if all are powered from the same booster, because it helps in tracking down problems.

pqe