[rambodan]

Hi all,
I have a wiring question I was hoping to get some insight on before I started. I have a layout that's DC only common-rail with blocks controlled by spdt switches. I want to add a DCC controller so I can have the choice to operate DC or DCC (never both). Based on some research, I've come up with the following setup.
-Instead of common rail, I've double gapped all existing blocks
-Each blocks' + rail is still wired to the spdt switches
-Each blocks' - rail is wired to a common rail bus. I know this might defeat the double gaps, but I think this allows me to use my switch I describe below to chose between DC and DCC.
-I have a 4pdt switch that I would hook up according to my diagram. If thrown one way with the DC power packs, it behaves the same way my layout currently is. If thrown to the DCC controller, all selector switches are moved to the same side for DCC operation.
If you have any suggestions or see anything wrong, please let me know, thanks!

 

[DC Marvel]

For DC you are slightly over complicating the feeds. It's not wrong but if you think about probable loco movements you can never bring anything out of the orange sidings at the top without the purple point being set straight as there isn't enough length between points, so a feed from anywhere on the rh loop will work for that as well. A feed into the yellow section between the two points at the bottom will be fine for all the lh sidings and the lh green loop section. Similarly a feed to the light purple section will run around the blue loop so you only need a feed to the blue if you want to just shuffle a loco backwards and forwards with a second controller. For DCC just switch them all on at once. For siding isolators rather than seperate feeds just add a line break with a toggle switch.
As you say all returns can be common from all droppers. The only advantage to so many feeds is removing the requirement for points to make or break the contacts.
I slightly over complicated my control panel for my MPD and I spend longer working out which feeds to switch to run from A to B than is practical sometimes. I usually end up switching them all on and see what moves then turn off the unwanted ones.

 

[jamesed]

The starting principle for dcc is that you want to have all the tracks live at once. However, it's not quits as simple as that. For someone starting from scratch it's much easier because you can follow standard advice which involves, decent size power bus, droppers to every track section, live frogs on points erc. Sending a reliable dcc signal around a layout requires a higher degree of wiring integrity than is required for dc operation. It's all about minimising the impedance between any given point on the track back to the control station. Every section of track without its own droppers, every switch in the circuit, every screw terminal join (as opposed to solder joints), every bit of power bus cable that's not at least 1.5mm sq (and preferably 2.5mm sq,) will all contribute to increasing the impedance which may affect reliable dcc running. Of course compromises must be made and nothing will ever be perfect so it's really just a case of getting as much of this right as you can so that the effect of any of these things which are unavoidable are minimised. Sometimes when converting an existing dc layout to dcc all of this can be more challenging and some things which initially don't present as a problem can cause issues later on. If you go into this being aware of all these issues and are confident that you can achieve a high integrity wired dcc system then that's great. Many people have done it and made it work well but equally many others have done it and experienced lots of problems. Good luck!

 

[SPROGman]

If you will only ever connect one booster then the original common rail would be fine (no different to a bus wire). If you want to create multiple "power districts" with their own boosters or district cut-outs then you need to double gap at each district boundary.

 

[34C]

...Each blocks' - rail is wired to a common rail bus. I know this might defeat the double gaps, but I think this allows me to use my switch I describe below to chose between DC and DCC...

You didn't need to double gap the sections! As you correctly observe, the common rail (In the UK we would typically refer to this as 'common return') bus defeats any gapping in this rail.

The corollary to this is that if there is only going to be one DC input, section selection only requires single pole double throw switches: either DC or DCC. The common return can have DC and DCC on it simultaneously with no resulting problem, provided that the key principle of such switched section cab control systems is rigorously observed: that each 'cab' on the system is running on an independent current supply, typically each derived from a separate mains transformer. "The electrons know where to go" as my Physics 'O' level master wryly observed, when explaining how the UK's national grid runs three independent phases with common return: it works there, it will work on our much simpler layouts...

When I went DCC, I had a 'classic arrangement' for DC of a three cab common return wired layout, single rail breaks were all that were required to select the switched sections using single pole switches. The DC controller on Cab 1 was removed, and the DCC system was connected to those track feeds. I could then run the whole layout on DCC, by setting all section switches to Cab 1, so the entire layout was live to DCC; which is the way DCC is designed to be used.

But as required, I could use either or both DC cabs 2 and 3 to operate any or all of the layout on DC, just as had been previously possible. There was no problem using DC and DCC simultaneously, provided the operator sets the section switches correctly to match the route determined by the points' settings; particularly useful to enable test running a new loco on DC on one or two of the track circuits, while continuing operation of the rest of the layout on DCC.

I have maintained this common return switched section by single pole switches principle on the layout I am building, to retain the convenience of a single selectable track circuit for DC test purposes, which can be operated on either DC or DCC as required.