DIGITAL COMMAND CONTROL (DCC)

WITH COMMON RAIL?

Wayne Roderick, 3rd Division, PNR, NMRA (life)

07/29/01 rev 01/15/07

The Teton Short line was converted to DCC in 1998 and we like it. In the ensuing years I repeatedly see folks disturbed by the problems or potential problems of converting their railroads with common rail to DCC. YES, you can convert with a minimum of effort and NO, you do not have to replace all those single pole switches with double pole. There is however, a few things you need to know to prevent the infamous voltage doubling that destroys decoders. This paper will address only the problem of voltage doubling.

The voltage doubling problem is not new- it's been with us from the earliest days when we found that any track configuration that could reverse a trains direction created some special wiring problems. The voltage doubling problem can only occur if more than one power pack (booster for DCC) is in use and happens because we manage to get them connected in series so the outputs can sum up. In the old days this might blow out your headlight and you never knew why the lamps had such short life. Decoders cost a bit more than headlights so we need to look closer.

Sadly, some bad information has been put out by well meaning folks and it eventually will lead to decoder destruction, excess rewiring or both. I would hope to serve my fellow DCC enthusiasts by looking at some of the pitfalls and show you how to avoid them. I'll strip this of engineering talk and try to bring it down to a level that we can all understand. Also, keep in mind, that this is an objective theoretical approach. I support no manufacturer (I build my own-VBG) and I have only very limited technical information on their products. If any manufacture has conflict with this, I would welcome a dissertation with their engineers so that we might serve our fellow hobbyists better.

First lets dispel a few concerns so we can concentrate on the meat of the problem.


BOOSTER BASICS:

Simplified booster Here is a booster stripped of all its electronics and frozen in time. The electronics have been replaced with a simple double-pole, double-throw switch. Imagine the switch moving back and forth to turn the DC into the square wave. Some boosters may have automatic circuits that will swap phase (throw the switch the other way) for auto-reverse. I will use 15 volts and three colors in the following diagrams.

Simply study the following five diagrams, and I think you can draw your own conclusions. You might want to print them out.