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

05/26/05 rev 01/16/07

Long ago, we addressed one of the problems associated with common rail. I advised you to get rid of common rail and tie all power supply negative terminals together, whether "conventional" (primitive) or DCC to prevent decoder damage due to voltage doubling. I am told that one Manufacturer has advised against this. He is right in a way. He wishes to reduce the statistical risk of damage to his DCC system . Can't blame him a bit for that, it's just good marketing.

As I see it, I can easily access and fix his damaged booster for a dollar or two to replace a blown transistor, but replacing a decoder priced from sixteen bucks to a whole lot, for those sound equipped ones, and possibly replacing a melted loco shell is not a desirable alternative. Can we protect both? You bet.

It's Murphy's Law- If it can happen, it will happen and in direct proportion to the number of visitors.

In a mixed DCC and primitive power system, the two are going to get tangled when even the most experienced operator might run his loco across a gap that separates the two systems. You can't fix this with administrative orders or training. You've got to make it goof-proof. In the club environment, the transition from primitive power to DCC may take years and sooner or later Murphy will get you. It took a combination of three events to take one mainline down the first time, and naturally on a public show day, and I vowed it wouldn't happen again.

It's not important but the three coincident events were a wiring error in a module, an operator goof, and a loose heat sink on a booster transistor so it didn't survive the additional stress. At the time, we were using some old MRC power packs on the Primitive analog system.

See- I told you that Murphy would get you :-)

Once upon a time we protected our primitive power supplies from each other with husky series and shunt diodes so that when an operator goofed and ran a gap the train stopped, a sluggish thermal breaker might open, but there was no magic smoke to escape from the power supply. The "magic-smoke" is that stinky stuff you smell when a semiconductor goes Poof ;-) This simple fix went away with DCC because the booster is putting out AC. It is now vulnerable to current being fed back into it from a primitive power supply. When we tied all the Negatives together we eliminated the danger to expensive decoders but increased the risk of damaging a primitive power supply or a booster. The typical booster has overcurrent protection from the positive supply which will protect the sourcing transistors in the output. The manufacturer has rightly decided that this would protect the sinking transistors in a properly installed pure DCC system but if the interfering source is coming from a primitive power supply it might well have the capacity to blow one or both of them.

The solution is simple. Simply limit the primitive power supply current to such a low level that it can't hurt our booster(s)

How? Well the built-in current limiters in the old power packs are usually thermal switches that take a long time to heat up and kick out when overloaded. They were designed to protect that old power pack and usually did the job quite well. We need something faster, and no it doesn't have to be a pricey electronic gadget but rather the 50 year old "vaccuum sealed, low mass, quick response, positive temperature coefficient thermal resistor".

Yep, the old #1156 automobile lamp bulb! Does a fine job and holds the current to about 2.5 Amps and it is bi-directional. 2.5 Amps won't hurt a DCC booster. Some simple solutions need never die, if they remain on the market for their primary use. You'll find many of your experienced fellow modelers still rely on this old trick.

Here is the schematic of the mixed DCC and Primitive system that we now use on the HO semi-modular Pocatello Model Railroad & Historical Society


The bank of twelve DPDT, center off switches are used in lieu of circuit breakers for isolating a fault. If you're using higher current boosters ( I advise against that for HO or N) to serve a larger area then electronic circuit breakers might be advisable.

Regardless of operator errors that might bridge gaps, no harm will befall a decoder, primtive power supply or DCC booster.

Have Fun, Wayne Roderick P.E.

Return to Teton Short Line Home Page