DCC Bus / Dropper wire Gauges?

[Stephen in Kerry]

Based on all the good advice received here, I will try and fit a bus wire for my new layout, rather than hope one power track will work.

I've found a 16 gauge 12A black/red wire that comes recommended for the main bus wire for model rail layouts.

What I can't find (after hours on YouTube) is what gauge / Amp wire I need for the droppers. I could perhaps use the Peco pre-soldered fish plates, but it could get expensive.

Can someone please advise me on that, and also if the 12A bus wire (flex not single core) will be OK?

Also, if the layout is basically made up of ovals within each other, do I put a pair of droppers on each oval, at roughly the same positions along the track? (Even to me, that last questions sounds like it may be the stupidest thing I've asked so far, but I genuinely can't find the answer).

Thanks again,

Stephen

[morairamike]

For the main buss wires I used 1.5 mm twin and earth cable with the brown and blue stripped out if the sheathing and the earth wire discarded.

For the droppers I use 7/0.2 cable. My layout is a twin loop with a branch line. The buss starts from the controller which located in the centre of the layout and runs clockwise and anti clockwise in two semi circles. The ends are terminated with snubbers. Every piece of track is fed with a dropper at the mid point of each piece soldered underneath. I run 5 or 6 locos at a time. My controller is a Gaugemaster Prodigy2.

[96RAF]

Similar to Mike I used stripped out mains cable (solid core for the main bus as it doesn’t move. Droppers were made from stripped out twin core flex sold for wiring table lamps, etc which is a similar heft to 7/0.2. These are affixed to every track piece.

Pre-wired fishplates are not recommended, because the weak link in any track system is at the rail joints. Better to solder either to the underside of the rails if the track has yet to be laid and you don’t want to see the droppers or solder to the outside face of the rails. This can look fine if you keep your dropper wire holes close to the track.

[Chrissaf]

I concur with MM's suggested solution, particularly the reference to the Snubber BUS terminations. A snubber is a very simple two component circuit that consists of a 0.1 uF ceramic capacitor rated at 50 volts or better in series with 100 Ohm resistor rated at 1 watt or better (I prefer 2 Watts as these resistors will run very hot to the touch). The snubber is wired across the BUS pair at the end of the BUS run. These Snubbers will protect your DCC decoders from damage caused by transient short circuits.

DCC is a digital Bi-polar square wave at a nominal 7Khz frequency. The DCC peak to peak voltage is typically 28.8 volts. When a transient short circuit occurs (for example wheels bridging the DCC track signal on a point) the DCC signal can experience 'Ringing' which is a form of runaway oscillation. This 'Ringing' can generate spike voltages that are up to 3 times the base DCC voltage and can easily exceed 70 volts. This high voltage can break down the insulation in decoder circuit capacitors. Typically, damage will manifest itself as the decoder losing its configuration, but in extreme cases the damage can be fatal. The Snubber circuit is a tuned filter which will shunt these 'Ringing' frequencies and limit the size of the voltage spikes, thus protecting the decoders. Transient short circuits are brief short circuits that may or may not be present long enough to trip the controller. In DCC it is possible to actually hear a transient short circuit as it emits a buzzing noise at 7Khz which is in the audible hearing range.

I am also of the opinion (as others have already indicated) that If someone is going to implement a BUS solution that each and every track piece is given its own dropper. Many consider this over-kill. But this approach completely eliminates the metal track joiners which are the 'weak link in the chain'. The track joiners are then relegated to just performing aligned track joining and serve no electrical purpose. If you implement a full BUS then that also eliminates the need to fit R8232 DCC clips to the points, which are now not needed, but no harm will arise if left in-situ.

IMPORTANT NOTE - test the BUS for a short circuit as you attach each dropper pair. A short circuit will be created if any dropper pair is connected reversed. Trying to find crossed droppers after they have all been wired in is a mammoth and disruptive task. It is far better to test for crossed dropper pairs as you install them. Even if you decide to remove the R8232 DCC point clips, leave them in place whilst installing droppers and testing for short circuits. The point clips will allow any dropper induced short to show up more easily. Once the BUS is fully installed and tested, then the point clips can be safely removed.

[Stephen in Kerry]

@morairamike, @Chrissaf and @96RAF, thanks again guys, I really appreciate the help.

I once wrote a text book in which the average length of words was, in some chapters 4 and 5 syllables. I even used the words 'countertransferential' and 'projective-indentification', which are 7 and 9 syllables, respectively.

But, this is the steepest learning curve I've ever been on. :)

p.s. @Chrissaf I spent 2 hours fitting those points clips, while listening to really bad jazz. They're staying. :)

[AceinOz]

Apologies for dropping into this discussion. Can you provide more detail on the snubber (and a diagram) - I have seen plenty of bus wires on reputable Youtube channels without a snubber.

Also, I don’t quite get why the R8232 point clips are not required if you dropper every piece of track. Any chance of more detail?

[Chrissaf]

BUS and Snubber example:

Close up

Note that my original post stated 120 Ohms for the resistor value, but 100 Ohms is the correct value. My original reply is now edited to change 120 to 100 Ohms.

The R8232 DCC point clips are used to circumvent the inbuilt electrical isolation that is designed into Hornby points. For DC Analogue layouts, Hornby points act as electrical switches to route power into the route that they are switched to. In DCC ALL routes are powered permanently and thus the isolation switching function of the Hornby points needs to be eliminated. The clips in essence connect all the left rails together and all the right rails together across all three point inputs and output routes. Thus all left rails are connected [bonded] electrically together and all right rails are electrically connected [bonded] together. If you place a track power dropper in a DCC BUS distribution system on each side of all three routes of a point, then that is performing exactly the same function as the R8232 clips, thus the clips do not then need to be included. The power on the droppers are feeding power INTO the three point input / outputs to electrically bond together the same rails as the clips do.

My definition of YouTube - a video service for the 'ill informed' guiding others. Thus propagating misinformation.

Snubbers and their benefits are not understood by the majority of YouTube information producers.

However, to give your YouTubers the benefit of the doubt, particularly as you haven't specified specific YouTube channels. Although not a critical 'must have' element of a DCC BUS design, the protection they offer is worth implementing as an 'Insurance Policy' given the insignificant low cost to do so.

More on Snubber theory can be read here. Mark Gurries [author] is a member of the NMRA that defined the DCC Standards.

Snubber/RC Filter - Mark Gurries

[AceinOz]

Thats great, very clear. With the snubber/RC filter, one end of the resistor goes in the terminal block (+ red/pos), I solder the other end to one end of the capacitor, the other end of the capacitor goes into the terminal block (- black/neg) - is that right. Apologies for seeming simple but I just want to get it right.

Again, many thanks for the clear explanation. Much appreciated.

[Fishmanoz]

While droppers to every track piece can be considered best practice, you don’t need them to every piece to avoid the need for DCC Point Clips. All you need is that there is a dropper that can be traced via a track piece/s directly to all 3 directions of all points without going via any other point.

And thanks Chris for mentioning Mark Gurries, I’d been trying to remember his name for ages. He is the only one I’ve come across who applies proper Transmission Line Theory to DCC layouts. He also covers the need to twist your bus pairs to further eliminate transients along with snubbers on larger layouts (think 2 car spaces in the garage) in particular. Twisting has not been mentioned in threads for some time but has been covered extensively in the past.

You are also unlikely to find a YouTuber showing you an exact incidence of a derailment causing a short followed by the whole layout being taken out by the resultant high voltage ringing without snubbers. Whole layout is extreme, more likely more subtle faults on some decoders such as ID resetting. Some decoders might be terminal, others recoverable by reset. Even these will be more likely to be terminal next time around.

Simple advice - if you are going to the extent of fitting a bus, fit snubbers as part of it to avoid tears before bedtime.

[96RAF]

A picture of a soldered snubber which is the same electrically as Chris’ choc block method.

[AceinOz]

Noting Chrisaf's excellent advice in regard to wiring Hornby points, I note the Peco insulfrog set track points seem to be already wired. Does this mean I don't have to wire up all the input/output points? I assume the Hornby points are not similarly wired.

[Chrissaf]

@adam (post on page 1)

PECO Set Track points which have product numbers that start with ST are identical to Hornby points and have exactly the same wiring requirements.

[96RAF]

The ultimate aim of a power bus is to solder droppers to each and every piece of track and attach those droppers to the main bus wires.

I have used the insulated wires stripped out from ex-ring main cable, which is single core but as it will not be flexing it is OK.

My droppers for economy are the insulated wires stripped out from a reel of 3A twin core mains flex such as is used for table lamps or other low wattage appliances. This is multi-strand as it may flex under the layout.

How you join these dropper wires to the bus wires is up to you, solder, suitcase fittings (insulation displacement fittings), Wago connectors, choc-block strips, dedicated junction boards, etc.

Pre-wired fishplates apart from being an expensive way of doing things are in fact using the most unreliable part of a track system to pass your power - i.e. the fishplates, which can and will work loose. Best avoided in my opinion.

[AceinOz]

I am about to start rebuilding my layout (as soon as Covid restrictions here in Australia allow me to get to the hardware store). This advice is great and will really help me build a good layout. Many thanks to all.

[Fishmanoz]

Hope you are in NSW so can go next week Adam. Vic another matter, could be another month I’m thinking.

New premier worries me for NSW though. He’s scratching around for political advantage at the moment rather than giving confidence in the COVID Roadmap. And if he thinks he’ll get federal help, that’s unlikely given a PM with the leadership skills of a lemming.

[AceinOz]

I’m in NSW but my closest Bunnings is in the ACT. I have a Bunnings gift card from the kids that I want to use to buy the timber etc for the layout. Not good at waiting - driving me nuts.