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.
- AC power phasing. To the best of my knowledge, all manufactures use a bridge rectifier to convert the incoming 60 Hertz power to DC. This means that we have no concern for AC power phasing and you could probably use a BIG transformer to power several boosters. Any Mfg challenges on that one?.
- Transients observed by 'scope. Very brief overvoltage transients will occur for many reasons that we users need not be concerned about. Every electronic engineer knows that he must cope with these. His product (decoders) would fail the market if he did not provide for this.
- Why not protective devices on the decoder? There is some protection but it cannot be made small enough to cope with extended overvoltage and resultant high energy that we will be exposed to. That means we must correct it at the source. The lamps and appliances in your home would not last long on double voltage. Yeh- there is a failure mechanism that can cause that too, but good wiring practices make it very rare.
- We will assume that you have connected the command station to the boosters in accordance with the manufactures instruction. This assumption does NOT necessarily include common connections.
BOOSTER BASICS: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.
- RED is 15 volts more positive than GREEN
- MAGENTA is 15 volts more positive than RED
- MAGENTA is 30 volts more positive than GREEN
Simply study the following five diagrams, and I think you can draw your own conclusions. You might want to print them out.
- Click here for a 800x600 pix of simple system using common at the booster output and an external reverse switch.
- Click here for a 800x600 pix of a system using common at the output and with internal auto-reverse. Note the voltage doubling.
- Click here for a 800x600 pix of a system using common at the output and with internal auto-reverse. Note that the external reverse device moved the voltage doubling problem, but did not get rid of it.
- Click here for a 800x600 pix of a system using common at the output of boosters B & C, but not connected to booster A. This will almost guarantee phantom failures that are very difficult to post diagnose.
- Click here for a 800x600 system using common at the negative terminals of all the boosters. In this situation, as pointed out by Reinhard Mueller, in the real world the two commons might cause some problems with overload detection in the booster(s). I would suggest using the exact model booster and identical transformers for all.
There are many other scenarios that we might have presented, but it should be obvious from these, that the common connection for ALL boosters must be prior to any switching device that can create an out of phase condition resulting in a series connection of the boosters. The ultimate safe common connection is the NEGATIVE TERMINAL OF THE BRIDGE RECTIFIERS! Did you notice that the remainder of the boosters (B and C) remain connected to the common rail? GOOD! You see there is not much rewiring needed for your common rail railroad. Now just be careful to not mix COMMON RAIL and BOOSTER COMMON NEGATIVE, and have fun.
On the Teton Short Line, I have many power supplies for special purposes and they ALL have the negative terminal of the bridge rectifier tied to an earth ground. In fact, this ground is available throughout the railroad as a bare copper wire ranging in size from #12 to #4. It is a great referance point for measurement and general troubleshooting.
SUMMARY:My observations are not original and others have expressed much the same in words and pictures over several years. Perhaps this paper will make it easier for you to understand the problem of voltage doubling with common rail and its simple solutions.
My advice: Get your DCC up and running using scenario #5, being careful to keep common rail and booster common negative separate and work toward the future abandonement of the obsolete common rail in favor of a balanced feed system, called Direct Home wiring by some. While the argument for this is beyond the scope of this paper, I did it because I see wonderful things coming down the road for our model railroads and the unbalanced common rail system had to go. Have Fun, Wayne Roderick P.E.
Return to Teton Short Line Home Page