Extending decoder wire

[Ron Ron Ron]

James, you’re assuming that the end of the bus wires is the end of the bus.

It isn’t.

Issues with excessive inductance, ringing and voltage spikes are said to be related to….

Poor wiring

Length of the Power Bus (long runs)

Badly designed Booster output

Excessive load on the layout (e.g. too many locos and other devices)

 

For most average to large sized UK layouts, there shouldn’t be a problem with bus lengths.

 

 

 

 

 

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Edited February 2, 2023 by Ron Ron Ron

[ISW]

On 31/01/2023 at 20:29, Tomathee said:

Cheers, I've found something in B&Q to pick up next time I'm passing, £8 something for 10 metres, at least half the price of the Rails example I linked earlier.

If it helps, I used this 'speaker' wire from ScrewFix (other establishments may also stock it) for my layout without any problems. At the time is was less than £3 for 25m. But to clarify, my layout has several bus wires for specific 'areas' of the layout. The whole layout is not on a single DCC bus. 

 

 

Ian

[jamesed]

5 hours ago, Ron Ron Ron said:

James, you’re assuming that the end of the bus wires is the end of the bus.

It isn’t.

Issues with excessive inductance, ringing and voltage spikes are said to be related to….

Poor wiring

Length of the Power Bus (long runs)

Badly designed Booster output

Excessive load on the layout (e.g. too many locos and other devices)

 

For most average to large sized UK layouts, there shouldn’t be a problem with bus lengths.

 

Errr, sorry I don't understand what you are getting at.  By definition the ends of the bus wire must either be the start or the end of the bus.  If you've got spurs (and I've already explained why it's better to avoid them if possible from the design stage) then stick another snubber on the end of them too.  Adding more snubbers won't do any harm but won't necessarily do any good either and certainly won't make up for poor wiring.   

 

I agree with you about bus lengths but the answer to that is in making sure you use an appropriate sized cable.  Whilst a 20m long bus might be too long for 1.5mm sq cable it won't be too long for 2.5mm sq. cable. On the other hand 20m long bus wired as a ring circuit would probably be fine with 1.5mm sq. cable.   Theoretically you can use 4mm sq. cable for a bus if you are concerned about the length/impedance but very few of us would ever find that to be necessary. ( I wonder what size bus cable they use on Pete Waterman's Making Tracks layout in Chester Cathedral?   It's all about the overall impedance and choosing the right cable, minimising joins, soldering droppers all of which contribute to that. It isn't helpful to say that long power bus runs are bad because you have to know and understand the other factors to define what is too long. 

 

 

 

[Ron Ron Ron]

52 minutes ago, jamesed said:

Errr, sorry I don't understand what you are getting at.  By definition the ends of the bus wire must either be the start or the end of the bus…..

 

What I’m getting at, is that unlike in most applications of data networks and bus wiring, the application in DCC isn’t restricted to the wires we are talking about here.

 

In effect, the DCC track Power Bus commonly extends beyond the end of the 2.5mm cabling (or whatever is used), out along the length of the solid wires we know as the metal rails.
 

The track bus, droppers and rails are all part of the same bus circuit. 
As such, there are normally branches and T’ees going off in different directions and provided it’s all correctly wired, with adequate cable, there shouldn’t be any adverse effects because it’s not linear.

[jamesed]

1 hour ago, Ron Ron Ron said:

What I’m getting at, is that unlike in most applications of data networks and bus wiring, the application in DCC isn’t restricted to the wires we are talking about here.

 

In effect, the DCC track Power Bus commonly extends beyond the end of the 2.5mm cabling (or whatever is used), out along the length of the solid wires we know as the metal rails.
 

The track bus, droppers and rails are all part of the same bus circuit. 
As such, there are normally branches and T’ees going off in different directions and provided it’s all correctly wired, with adequate cable, there shouldn’t be any adverse effects because it’s not linear.

 

Hmm I think there is a danger here that we might both be getting a bit pedantic about this!  In that vein all I will add is that the "bus" is the bus wires, the "droppers" are the droppers and the "track" is the track. Whilst I agree that they all form part of the same circuit, in this instance I was most clearly referring specifically to the "bus".

 

Don't forget that the track is very different from the bus wire. Usually made of nickel silver these days it has a higher resistance/impedance to copper wire. This adds to the impedance between the loco decoder and the controller.  That's why it's recommended that we install a dropper to each section of track directly to the bus rather than looping wires from one bit of track to another. The contact between the wheels of the loco and the track will also increase the load to source impedance. Many of these things we cannot do anything about which is why it's important that where we can make a difference (low impedance bus wiring, droppers to each section of track, minimising bus joins, clean track etc.) it's well worth doing the best we can.  I seem to remember recently reading somewhere that on some exhibition layouts they now install two pairs of droppers to each track length as standard. I can see the logic of that for portable layouts.