Oct 26, 2011

In Which Steven Discovers Capacitance

So, weather's colder now, and I've still got issues with the newly-installed Digitrax DCC system.

The issue at hand is that my BD8 block detectors (from The Signaling Solution), which I love and which worked flawlessly with the old Keller OnBoard command control system, were now working oddly.  A train would move into a block, the detector LED goes on, and all seems well.  But when the train left the block, the LEDs were stuck 'on'.

Well, after checking for the usual stuff - ground loops, or other sources to cause current to flow through the one of the side of the feeders being used with the detectors, I did the unmanly thing and asked for help.  In this case, the designer of the BD8s.

I received a very quick and thorough return email, which suggested the problem was the DCC signal in one feeder causing a small current to flow, through capacitance, in the other feeder.  The solution was to put some distance between the feeders, physically, as much as possible.

This was a bit discouraging, as only the blocks with the longest wiring runs were experiencing problems, and those were (inevitably) the hidden and least-accessible sections.  You know, the part that runs thru the bathroom walls, thru the basement stairs, behind the furnace, and so on.  To keep wiring neat in these locations, the wiring had been neatly bundled together and carefully routed.  But the bundling was causing the problems - the alternating DCC signal in one wire was inducing a charge in the other wire.

I did color code everything rather religiously, so I crawled under the layout and started tracing the white and gray wires, which is the Rail B side (for me) and being used for detection.

Very long story short, I was able to pull all those wires out and re-route them (still neatly) along new paths that (mostly) kept them a couple inches or so away from the Rail A sides.  After finishing half of the first block (a hidden track beneath Durango), I ran a test train, but no change.  I continued working, got the entire run separated, tentatively ran another test train... and WaaLaa!  Properly functioning signals again!

After much crawling and re-wiring, I've now completed about 5 of the 10 or so affected blocks, and these changes have fixed all of them.  It seems almost crazy that just moving a wire run a little would makde a difference, but with DCC it certainly does!