DCC feeder wiring question

[Crawfy]

Howdy

Building a DCC layout for the first time, and I've read somewhere (that I can no longer find) that when wiring, you should never create a closed loop with the main bus cable, as-in joining the ends together when doing a loop style layout. Same thing was said about the track, don't make it a electrically closed loop.

My question regarding the feeder cables; should the different fed sections also be isolated from the each other at the rails, because if they were conductive between each other on the track, then effectively you're making lots of small loops?

Not sure if it's an issue at all, just after some thoughts on this please.

 

[Selector]

You will cause a massive hard short if you let/make the two bus wires touch at any point in their journey from the power supply/base station.

However, you CAN loop back each wire on ITSELF. You can make each one, separately, loop back to its terminal. However, it is a waste of wire generally, and entirely unnecessary in all but the most extreme and rare of instances.

If you are relying on some sort of circuitry for short management in a defined area, then the circuitry should be wired in series from the bus to that area, and yes, the rails must also be isolated to be managed strictly by the one in-series circuit protector. Otherwise, your monitor will have to monitor the entire length until whatever gaps actually do cause a boundary.

So, if you really must have a bus due to the lengths and your desire to keep voltage losses to a minimum, there is generally no need to do anything more than to run the bus most of the way along the main track. From places along the bus, you can run sub-buses in series with the circuit protectors cut in. Then you run another short line of that sub bus so that none of the feeders have to be more than about 2 feet long...ideally...if you are using light wire, say 22 gauge.

Does all that make sense to you?

 

[Crawfy]

Thanks mate.

Yeah, I understand that I can't join the positive and negative bus cables together, that at least is obvious to me
What I was getting at, is that supposedly you don't want either the negative or positive bus cables to follow the mainline around the layout, and then connect back onto themselves (ie. positive bus connected to the other end of the positive bus), to ensure that the power is only being fed from one end of each bus cable.

From what you wrote above I guess that's unnecessary though?

 

[Selector]

What most of us do, I think, and it is what I do, is to T the bus. It comes off the power supply or controller output terminals as a short pair, then branches left and right, each wire. Two wires going left and two going right. That way, you effectively feed them for half the length that you would otherwise have to feed them if you started them both going one direction and kept going for about 90% of your main track axis...say your main line.

But, you do not need to loop the wires back onto themselves or back to the terminal from which they come. It is unnecessary. I don't have it square in my mind how far a bus of a given gauge wire should run before you cut in a booster, but for a 14 gauge wire it would be in the range of 30-40 feet. You want to keep voltage losses down, but you also want to keep the signal-to-noise ratio high for the sake of the DCC signal, and while your decoders may still read a weak signal, the short detection circuitry seems to suffer when voltages drop and the signal gets noisy. Hence the 'quarter' test, or a similar sized and composition metal coin is set upon the rails at points further and further away from the controller box to see if it will still detect a short and cut power. On my last layout, I had a fudged staging yard of sorts well off my layout proper accessed by a 1 X 3 lumber seven foot long bridge. The whole yard and bridge were fed on the layout with feeders to the lead on the layout that joined to the bridge via a pair of standard metal joiners. I had perfect decoder reliability with that extreme range, and excellent mechanical results, but a metal screwdriver across the yard rails just produced a high pitched squealing...no short detection. It was dangerous in retrospect. For the decoders, that is.