In the previous three articles, we have reviewed the components of DCC. This article will focus on configuring the system to run trains. In this article, I will use the Digitrax DCS100 command station as the example; however, the lessons of this article apply to most command systems.
This article and the others on this site provide basic information to educate DCC consumers. DCC by Design offers the service of designing a DCC system for you. As part of our Design Services we will work with you to examine your needs, address the issues below and many, many more. Then we convert your preferences, layout design, and budget into a complete DCC system.
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Most Model railroaders start with an MRC power pack or the power pack that comes with their first train set. As one learns how the power pack works and how to connect it to track, one begins to learn about Direct Current (DC) and how to power the layout using it. Thankfully, most of those lessons also apply to Digital Command & Control (DCC). The reason the lessons are transferable is because the problems are transferable. Many of the same problems with DC (reversing loops, turnout frogs, etc) still apply for DCC. Still there are differences. This article will cover practical differences, not the electrical engineering.
The DCS100 and most other command stations have at least four inputs for wires, two for power inputs and two for track power. On the DCS100, track power is labeled Rail A & Rail B. These two connections provide power to the layout.
In the past, some articles said that DCC could power a layout using just two wires. While theoretically correct, such concepts are not good real world advice. Using DC, one could not power a large loop with just two wires because of voltage drop, loose rail jointers, etc. The same still applies with DCC.
In fact, the best advice is this. If the item didn’t work well as DC, then it won’t work with DCC. This applies to locomotives, track, wire connections, and most anything else related to powering the layout.
If you have an existing layout, which runs well under DC, then you have a good start on your DCC Layout. The first step is determining power needs. If you have already read Article #1, then you should have a good sense of how many boosters you will need. If not, please read Determining Your Power Needs Section in Article #1. Keep in mind that most boosters provide 5 amps of power.
For the purposes of this article, any layout that can be operated by a single booster is defined as a small layout. Such a layout would require only one power district which simplifies wiring.
The next question to consider is whether you plan to implement block detection. Block detection provides the ability to determine which blocks are occupied by a train and which ones are not. Block detection is the basic building block for CTC, signaling, crossing logic, and other functions. If you have a DC block system, then you can use your current blocks for block detection. IF you do not have an existing block system, then you will need to isolate the blocks and add new wiring for each block.
Figure 1 shows a basic track element where single track becomes double track or a passing siding. It is wired much like a first layout would be with two wires directly from the cab control to the track at one point. Such a wiring method works for only the most basic layout DC or DCC.
Wire Size Considerations
In the event of a short circuit, DCC layout wiring an track is exposed to substantially more current than it would see from DC. The result is that wires generally must be larger for DCC layouts. Bus wires typically are 12 gauge (AWG #12). Feeder wires should be 18 gauge and kept short, generally less than one foot. Thus for existing layouts converting from DC to DCC generally either new wire is needed or more advanced power management techniques. Power Management will be the topic of the next article.
Figure 2 shows a typical way of wiring sections for DC blocks using Atlas Selector. Two Cabs are provided. Cab A is shown controlling the mainline and Cab B controls the siding. Note the presence of an insulator to separate the blocks. The positive wires (shown in blue) run from the selector to the block of track which they control. The negative wire (shown in pink) runs from both cabs to a shared bus wire. Feeder wires are run from the bus wire to the track every so many feet.
Figure 3 shows the system from Figure 2 converted to use DCC. Note that the Atlas selector and wiring to the track remains the same. Also note that the two cabs were replaced with a single DCC system, now the locomotive decoder controls the speed, not the blocks.
Figure 4 shows the system from Figure 3 upgraded for block detection. The block detection unit has replaced the Atlas Selector. Also, a second insulator has been added, creating a third block. This way the block detection system can distinguish when a train occupies the single main line (and cannot be passed) versus occupying the double track main (and can be passed on the siding).
For a large layout, i.e. one which requires more than one booster, the diagrams are similar. The key difference is the common bus. DCC systems require Direct Home bus system, which DC systems typically use Common bus system. In layouts with only one booster there is no difference. In multiple booster systems, figure 5 below applies. Not that the old DC negative (pink wire) has been divided. The result is that each booster has its own common, independent of the other booster. Also, note that both tracks have been insulated between the two boosters, this practice is called double-gapping and is required to separate boosters in DCC.
The name Direct Home stems from the pink wire is a direct route back to its booster, not a common route shared by all boosters.
Wiring for new layouts is much simpler, because one is starting with a blank sheet of paper. The first thing to consider is whether you want block detection, today, tomorrow … or in 10 years. It is much similar to wire the layout for block detection at the start of construction, even if it is not implemented from the start. The next consideration is whether you plan to use multiple boosters, or a single booster.
A quick note on Reversing Loops
Reversing loops must be insulated from the rest of the layout. If you have an existing DC layout, then it is likely you have used a Atlas Controller or similar unit. For DCC Operation, simply replace your existing controller with the automatic reversing unit of your choice, such as the Digitrax AR1. If you are designing a new layout, treat the reversing track as though it is one booster power district in a multiple booster layout. Instead of using the more expensive booster, use an automatic reversing unit such as the AR1.
Below is a table of the four possibilities and the applicable figures.
Single Booster, No Block Detection
|Single Booster, Block Detection
Figure #4 above
Multiple Boosters, No Block Detection
|Multiple Boosters, Block Detection
Figure #5 above
Figure #6 shows a single booster system without block detection. The layout is wired with a bus wire for Rail A and a separate bus for Rail B. Feeder wires are provide at a frequent intervals (ideally at every piece of flex track) from the bus wires to their respective rails.
Figure #7 shows a multiple booster system without block detection. The layout is wired with a bus wire for Rail A and a separate bus for Rail B for each of the boosters. Feeder wires are provide at a frequent intervals (ideally at every piece of flex track) from the bus wires to their respective rails.
Wiring for detection now… implementing later
The best way to wire for block detection to be used at a later date is to install the wiring and insulating gaps during construction. Run all of the wires to a central location (where the block detection unit shall be mounted). Then install a terminal stripe. Each wire for the track is installed on one side of the terminal strip. On the other, jumper wires are installed so that the single wire from the booster is connected to each row of the terminal strip.