I read that if you have a long track you should have a power "feed" every few feet because of the voltage drop over a length of track, particularly for DCC. They say to install a "bus" i.e. a piece of wire or copper tape from which the feeds are taken. But why doesn't the "bus" suffer from the same voltage drop as the track? Is nickel silver a worse conductor? Or is the difference because of the rail joints? And if so why isn't it sufficient to just solder a piece of wire across the joints?
The voltage drop occurs at the fishplates (rail joiners) because the metal surfaces tarnish causing resistance.
Because the "bus" is made of thick wire. In my case I use household mains cable, suitably treated so you know it's not carrying 240v.
Rail joints - see above.
It is sufficient. Always assuming your soldering is perfect every time. And always assuming the wire you use across the joints is heavy enough to carry the required current.
See the other replies, but yes, copper is a better conductor than nickel-silver. Pure silver is an even better conductor than copper, but is rather expensive to use on model railways (and it would wear too quickly if used for rails.)
If you melt the sleepers, you're doing it wrong because you are getting heat in the wrong place. What I do is to tin the wire first, then scrape the rail with a needle file where the wire is to be soldered. Then I apply some flux (I use Fluxite, but other fluxes could be used) to the rail and tin the rail with a small amount of solder. Then I bring the wire to the rail and apply the iron, which causes the solder on both rail and wire to melt and fuse.
Periodically I check all soldered joints of wire to rail, as I have seen a blackbird really enjoying himself trying to pull a wire out of the baseboard and away from the rail, obviously thinking it was a tasty worm!
I tried soldering wires to the fishplates, but it didn't work very well, as the rail-to-fishplate friction joint soon deteriorated as a result of the usual effects of the outdoors on the metal surfaces.
Incidentally, I use a 40-watt iron with a screwdriver-shape blade, which cost me very little in Homebase. Smaller irons do not generate enough heat to get in there, do the job, then get out again quickly.
We had a bit of a discussion about this a while ago. Basically, you want to solder your wire directly to the rail itself since you can get resistance between the fishplate and the rail giving you a lower voltage at the track.
To avoid melting sleepers when soldering directly to the track, you need to make sure that the rail and the wire are clean at the place you want to make the solder joint. Next, you want a soldering iron which is up to the job. 25 watt and upwards will usually deal with Code 100 rail and smaller.
Apart from the wattage of the iron, the bit you use on the iron will also have an effect on how the joint is made. You want to transfer as much heat as possible in the shortest time, so a tip that resembles a screwdriver blade will allow this if you place the tip in the web of the rail and get a large contact area. If you have a pointed style of tip, or one with an angled flat on a circular bit, then you will not really get a large contact area on a rail and it will take time for the rail to get up to soldering heat, if at all. It is in cases like this that the heat has time to travel along the rail and start to melt the plastic fittings close to the join. If you use a good hot iron with a screwdriver/chisel type of tip, then you should get a fast joint and heat will not have time to conduct itself along the rail to the plastic fittings. What also helps is a small smear of suitable flux on the rail and wire. You can use liquid flux such as that supplied by Carrs (now C&L), or you can use paste fluxes like Fluxite or Templers Telux. Make sure you get a flux for non-ferrous metals which won't cause long term oxidisation problems. The paste fluxes I mentions are fine for this purpose.
Also, to help get a fast joint, break the soldering rule and carry solder on the tip of the iron to the joint. The solder on the tip increases the heat transfer area and ensures a quick joint. You need to use flux on the joint if you use this method.
It's worth getting a bit of scrap track and trying out some soldering on it until you get the feel for doing fast joints. It can be done and all you need is the experience to prove it to yourself. :-)
OK John, but the thick wire is still thinner than rail, is it not?
Don't these points apply equally to the feeder wires (heaviness of) and the (imperfect) soldering of them to the track and to the rail? I still don't understand why the bus is better. Is it basically just down to the tarnished rail joiners?
ISTR that Nickel silver which is what most model track is made from actually has a fairly high internal resistance compared with the copper used in UK mains cable. The brain cell holding this old fact also remembers something about nickel silver having a low contact resistance which I have always taken to mean it lets the current out easily. Not being an electrician or a metallurgist i may have got my wires crossed.
Talking about track material the MOD may have some phosphor bronze rail for sale soon.Dean Hill ammunition store is closing its narrow gauge system. Anybody want to build a large scale set with 2 rail electrification?
The point about feeder wires from a bus is that they are short and carry the current only for the section of rail they feed. So, even if they are on the thin side, the voltage drop (being proportional to the length and current) will not be that great.
Wires across rail joints (with no bus) carry all the current for the downstream track. Again, each wire is short and the voltage drop will be small (possibly greater than a feeder due to the higher current) but the drop is cumulative as you reach the furthest points of the track.
It's more critical for DCC where the whole layout may be one electrical section with much higher current than a block DC system with multiple lower current feeds.
I found when I was a kid in the days a having a nice oval loop with just one power feed (all tacked onto a nice piece of chip-board covered in cheap and nasty plastic buildings - oh those were the days!), on the opposite side of the board I had poor running, and eventually found that this was directly because of my fish plates.
Later layouts that I started constructing, using various set-tracks, I found it best to locate all the track on the board, but not tack it down, and then lift and up each piece of track individually solder wires to the underside of each and every rail. It was painstaking to a 12 year old, but did the trick. It was then all fishplated and tacked down.
I too suffered the problem of melting sleepings, so I used a nice needle file to reshape the head of my soldering iron from the sharp point it came with to a flattened out screw-driver shape, about 3mm wide. This allowed a much better control of heat, and using a good multi-core solder I could "tin" my rail first, then the wire (giving time for heat on the rail to disperse), then join the two together quickly.
More recently, I use PCB track, but use the same method - finding it easier to hide a wire connection to the underside of the actual rail rather than on the side of it, or on top of the sleeper.
Ronnie
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