DCC Bus Wire

Discuss your experiences with systems, decoders, installations, wiring, control and any general hints & tips you have found.
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Serjt-Dave
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DCC Bus Wire

Postby Serjt-Dave » Mon May 06, 2019 10:36 am

Hi All. Am I right in thinking the if you have a continuous loop of track ie, a oval or circle etc, your bus wires shouldn't go all the way around {in other words the bus wires do not go round in an oval or circle}!

On my layout I have to have a removable section to allow access through the door. The removable section will have to have a feed to power the track etc but do I have to have a feed back out to continue the loop?

All Best

Dave

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grovenor-2685
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Re: DCC Bus Wire

Postby grovenor-2685 » Mon May 06, 2019 10:45 am

This seems to ba an area where misunderstandings often arise.
There is no problem installing the bus wires as a continuous loop, just like the rails in a simple oval.
But its not required, so in your case you can leave a gap in the bus wires at the doorway, whatever suits your setup.
The DCC power and signal is very tolerant of different configurations.
Regards

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Serjt-Dave
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Re: DCC Bus Wire

Postby Serjt-Dave » Mon May 06, 2019 11:19 am

Thanks Keith, that's what I wanted to hear. It means that I only need one connection point for the board rather than two. Happy days :thumb

All Best

Dave

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Paul Townsend
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Re: DCC Bus Wire

Postby Paul Townsend » Mon May 06, 2019 2:04 pm

Having had a long conversation with a Merg member at the Bristol Show this weekend on the difficulty of diagnosing a track short on a complex DCC layout wired according to commonly printed advice, I am stimulated to make some comments designed to guide bods like our Serjt Dave who are about to wire a new DCC layout.. Wherever I have read advice on wiring for DCC I find several things with which I take issue.

1. IMHO for UK typical layouts, there is too much emphasis on a heavy duty wire for the DCC bus. The written advice is more valid for very large USA type layouts. The current draw in the track bus on our layouts will rarely exceed 2 Amps. Thats 3 locos running with modern motors..say 1Amp and allow another Amp for 20 locos simmering. If the bus end to end length is 30ft or say 10M then using a 1 square mm wire is OK. This could be UK lighting mains cable "1mm Twin &E" wires stripped out of the T&E cable or a more flexible wire of 32 x.2mm.

2. Above wiring will have a carrying capacity of just over 10 Amps so will not overheat if a short circuit occurs and your booster can deliver 10A. Most boosters or DCC base stations are set to limit at 3A.

3. Voltage drop on a DCC run of 10M wired as above ( there and back ) will be 0.67V for 2Amp drain. That is the worst case where all the load is at the far end away from the booster. In practice the load will be distributed and so less voltage drop. If you have about 14 to 15v DCC to track from your booster ( normal for a typical 4mm model ) then the drop of 0.5 to 1 volts will not cause decoder troubles.
If your DCC bus wires form a continuous loop ( eg for a roundy-roundy layout ) it is like the ring main in your house and the current rating is nearly doubled while voltage drop will approxiamately halve.

4. The classic recommended coin test is out-dated. This is supposed to check that your booster shuts down if a coin is put across your rails anywhere on the layout. I would always recommend using District CutOuts ( DCO) in your DCC bus which will now be split into a number of districts. I advise as many districts as you are likely to have locos running simultaneously, counting double-heading as one loco. So 3 simultaneous trains suggests three power districts. These could be Up Main, Down Main, Goods Yard to give just one configuration. These DCOs should be set to trip at one Amp or less. Thus a short in one district will shut that district down but the main booster will continue to feed other districts and keep some trains running.

5. If you use DCC accessory decoders for turnouts, signals etc these should be fed off a separate DCC bus from the track and will be unaffected by a DCO shut down of a track district. The commonest cause of a short is running a loco onto the crossing of a trailing point set against it. Given that mistake you want to be able to throw the turnout to clear the short....not possible if the main booster has been shut down.

6. I am surprised that others do not advise using good old-fashioned section switches as you would have done on a conventional DC layout. Normally on a DCC layout these will all be left switched on but they are invaluable for isolating and tracing a short-circuit or other fault. Such switches don't need to be on your pretty mimic control panel. As they are only for fault-finding they are well useful if under the baseboards but near an edge for easy finding by feel.

I hope these notes help Dave and others to an easy DCC experience.

Most of the above have been applied to my large elderly model of Highbridge in its 7 years of DCC experience and certainly help me. The one that has not been applied is the 1mm cross-section wiring as I judged it unnecessary to rewire the old 1980s lighter DC track feeds. As these are many parallel feeds from section switches and not a DCC style bus the max load on any one wire is under half an amp in normal use. On my newer model, DCC from Day1, I am using my recommended 32/ .2 bus.

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Serjt-Dave
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Re: DCC Bus Wire

Postby Serjt-Dave » Wed May 08, 2019 9:46 am

Tanks Paul. I hadn't thought about making districts etc, so will have to look into doing this once I have enough track made up and ready to fit. I plan to have a second bus wire for the points and signals and possibly a third for lighting. My points and signals will not be DCC.

All Best

Dave

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Paul Townsend
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Re: DCC Bus Wire

Postby Paul Townsend » Wed May 08, 2019 2:35 pm

I understand that several manufacturers offer DCOs. I only have experience of the Merg kit at about £8....you do need to assemble it.

The Merg one is cunning. If set to cutout at say 1Amp and seeing a load greater than this at switch-on it limits the current for a while to allow for capacitors in eg Sound decoders to charge up. If overload persists beyond a preselected short time it trips and lights an led and buzzes.

It has been tested with many commercial DCC rigs as well as Merg's own.
Merg only sells to members but you can usually find a chum who......

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Serjt-Dave
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Re: DCC Bus Wire

Postby Serjt-Dave » Thu May 09, 2019 6:46 am

Again Thanks Paul. I am a member of MERG and have built some of their circuits before, I'll take a shifty at their DCO's.

Dave

davebradwell
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Re: DCC Bus Wire

Postby davebradwell » Thu May 09, 2019 8:22 am

If it's good practice to avoid a loop, then surely this is the thing to do, especially as it doesn't cost anything. It isn't quite the same as a ring main in a house as that wasn't designed to carry data although they sometimes do now, of course, and at a higher frequency. Whatever, why not do it correctly? On the other hand I have plenty of thin wire on my layout installed for dc operation but wouldn't encourage it. It's also worth remembering a warning which dates back to C. J. Freezer and avoid using wire intended for mains use as one day, somewhere, someone is going to put a plug on it.

However, the single change that most improved reliability for us was using better, ie more expensive, decoders and in particular the Zimo, although we were also tidying up wiring and eliminating any loose connectors. These decoders also brought wonderful motor control which is far more sophisticated than anything available elsewhere. Watch out for cvs mysteriously changing when things aren't right.

I found the change to DCC to be very rewarding and would use it even on a one loco layout just for the motor control. Perhaps don't try to do it on the cheap.

DaveB

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grovenor-2685
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Re: DCC Bus Wire

Postby grovenor-2685 » Thu May 09, 2019 12:43 pm

If it's good practice to avoid a loop, then surely this is the thing to do, especially as it doesn't cost anything

The DCC system is agnostic on a loop in the bus, if the layout has a loop in the track, then a loop in the bus has the benefit of reducing voltage drop in the bus. But if there is a good reason to break the loop, eg for a lifting section then that is OK.
If possible keep the supply point of a gapped bus near the centre of the bus. If the two ends are of very different length there will be a step in voltage when a train crosses the gap.
Regards

nigelcliffe
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Re: DCC Bus Wire

Postby nigelcliffe » Thu May 09, 2019 1:14 pm

One small detail on the MERG DCO - if you have a RailCom based command station, you need the alternative firmware in the DCO's processor, otherwise it doesn't work with RailCom being present. You can turn off RailCom in the command station, but then loose the features it offers.

With the alternative firmware loaded into the MERG DCO, it seems to be fine with RailCom based systems. Loading the firmware means either asking someone to do it for you (shouldn't be an issue for MERG members, MERG is a pretty friendly place), or owning a PIC programming device.

RailCom tends to be present in European brands of Command Station.


There are a lot of commercial DCO's around. The NCE ones are fairly cheap and seem quite effective.


- Nigel

davebradwell
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Re: DCC Bus Wire

Postby davebradwell » Tue May 14, 2019 9:20 pm

In my working days, signal wiring was never in a loop in order to minimise pick-up of interference - or radiation of it. Volt drop was dealt with by using an appropriate gauge of wire in power circuits which also avoided loops. Now, the better DCC systems are very good at rejecting interference so it's probably no big deal if you do it right or not but I prefer to do things correctly just in case and especially as I can no longer borrow test equipment to track down faults.

Now the thread is turning to short circuit protection, traditionalists who insist on the interlocking of points and signals will find that they get very few shorts because they can't run into a wrongly set point except possibly in a yard.

Finally, feeding districts via 21W bulbs does very well at preventing total system shut down, is cheap and won't require a software re-build.

DaveB


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