Wiring standards part 4: Auto Reverse and gapping

I was going to start this post with a description of Auto Reverse (AR) sections -- what they are and how they work. Then it occurred to me that someone else probably already had done that and, sure enough, you can find such a description at Wiring for DCC.

My layout will have 6 standard AR sections, 5 for the return track in each of the staging areas, and one for the wye at Union Station. There will also be one crossing with live frogs on the upper deck, and that requires a special application of an AR device. I'll address that topic in a later post, probably not until upper deck construction is underway.

Before getting to the wiring standards, there are a couple of AR section design standards to mention. First, all of the 6 standard AR sections will consist only of a single track, no switches or crossings. This not only simplifies installation, it also greatly reduces the possibility of accidentally triggering the AR device through electrical shorts (more on that later in this post). Second, standard AR sections will be at least one train length long -- some possibly fitting two or even three shorter trains in a pinch. In general you want AR sections to be at least as long as your longest train. This is pretty obvious if you think of every car as potentially carrying electrical current in the wheels. It's true most freight cars have plastic wheels, but many have metal wheels, and all it takes is one wheel to bridge an insulating gap and cause a short.

The AR wiring standards will be:

1. Each standard AR section gets its own power bus. Terminals won't be needed, instead feeders will be connected directly to the bus.


2. AR devices will reside in the power cabinet, making them easy to track with the other power devices and easy to debug problems.


3. The AR bus wires will be colored blue and yellow to distinguish them from the other layout wires. It doesn't matter which rail gets which color, so by convention I make the rail that is closest to the nearest wall for most of the AR section yellow. No reason, just because.


4. The wire for each AR bus is 16 gauge stranded. 14 gauge would be overkill because a) the length of the AR buses top out at 20' and b) the amperage draw will be much smaller than for the power district buses. 16 gauge stranded wire is available reasonably cheaply at Home Depot in multiple colors. The blue/yellow bus wires will be twisted to keep them together. They are labeled with "AR" and the name of each section, such as "AR-Upper Tier".


5. AR feeder wires are 22 gauge like the other feeder wires, are black, and like the other feeders are soldered to every other rail joint. Each feeder is directly connected to the appropriate AR bus wire. This is done by stripping a 1/2" off the bus wire insulation and the end of the feeder wire insulation, wrapping the exposed feeder wire around the bus wire, soldering, covering with electrical tape and then scotch tape, as electrical tape doesn't stick well. The electrical tape isn't really needed except that it might prevent oddball shorts in strange situations.

The photo below shows part of an AR section with the associated wiring:



The track nearest the camera is the return track for the lower tier. Below the track you can see the twisted blue/yellow AR bus wires, and at the left you can see an attached label with the name "AR - Lower Tier". On the right you can see two black feeder wires connecting the rails to the bus. You'll note that there is electrical tape around the connections, and scotch tape on top of that.

Part of the AR wiring standards remain undecided. First, I'm not sure what AR device I'll use. I have one Digitrax AR-1 on the layout now, which provides a simple AR function for one AR section, and it works like a champ. There are a few complaints about this on the internet forums, but mine has worked perfectly out of the box without need for adjustment. It may be that most of those who have trouble are using larger scales, or older locomotives. N scale may help in that I'm using lower voltage (the Digitrax DCS100 command station has an N scale setting) than HO, or that N scale locomotives draw less current.

On the other hand, I might have just been lucky with my one AR-1. But, I am currently leaning toward sticking with the AR-1 until/unless troubles arise. However, I hear good things about DCC Specialties' PSX, which replaced the similar Tony's Trains products, so they may be worth the small extra outlay in costs.

One product I recommend *not* using for auto reversing is Digitrax' PM42. I bought this because it was advertised as a low cost way to have 4 AR sections. That is technically true -- if you have a separate 12-18V AC power supply lying around doing nothing. And if you have a DT300 or DT400 command throttle to program it. And on top of that you'll need to do a nest of soldered wiring for the inputs and outputs. But the final kicker was that after I'd spent a couple of hours soldering everything and getting it all in place I found out that one of the 4 sections was faulty. ARGH. Yes, Digitrax support is excellent and they would quickly replace it, but that would have meant unsoldering everything, filing a support ticket, printing it out, and going to the post office to mail it. I've decided to accept the bad section and let the other 3 get used for power management.

The other pending decision is whether to connect the AR sections directly to the power booster or route it though a power district, and if so which one (maybe put all ARs in one power district). Logically you'd probably want ARs under a power district so that they don't short the whole layout. Except that there are frequent discussions on the forums about conflicts between an AR device and a PM circuit breaker. Fortunately, as all these devices will be in the same cabinet I can defer the decision, and once made it can be changed easily.

Finally, AR sections, like power districts and "live" switch frogs, require insulated gaps. You can either cut a gap in a rail and optionally fill it in or you can use an insulated rail joiner. I prefer the latter, and as a standard use the Peco insulated joiner even for Atlas track. It's smaller, hence less obtrusive, but it does a much better job of holding the rail in place. It can be disguised well as part of track scenicing. A situation may come up where the rail joiner is not practical, such as where the joint is on a tight curve, but until then insulated joiners are the standard.