Page 10 - NMRA Roundhouse March-April 2020

Installing DCC in a
Rapido Steam Generator Car
Mick Moignard
The completed car in operation heating the Empire Builder consist - photo by Brian Clark
MODELLING ARTICLE
S ome time ago Brian Clark asked generator both operated at 4.5v, which that current through a simple voltage
meant that feeding either of them direct- ly from a DCC decoder would not work. Decoder outputs for both motor and functions are always around 12v, with motor speed and effects all managed using Pulse Width Modulation (PWM). PWM works by switching the output
on and off very rapidly, varying the ratio of on and off times, so that the average voltage varies as the ratio of on to off.
But the voltage when the output is on is still 12v or so, which means that a 4.5v motor and heating element won’t last long, especially at full output! Some sort of voltage dropping circuitry would be needed. I had also noted that the entire car consumed around 0.4 amps when operating, of which only 0.02 amps were the lights, meaning that the smoke gener- ation circuitry really requires a 0.5 amp feed to give some safety margin. Feeding
The decoder tucked behind one of the light supports on the car’s chassis. Photo Mick Moignard
me to take a look at his Rapido Steam Generator car which he had long wanted to be able to
control it properly on his DCC layout via DCC commands rather than have to open the roof hatch to switch it on and off when parking it.
He said that he would like to control the lights and the smoke from DCC function buttons, or even be able to consist the generator car with a loco and use the consist functions. He told me that he had an email conversation about it with Rapido who had confessed that DCC operation wasn’t something they de- signed into the car.They had made him some rather half hearted suggestions, which Brian hadn’t followed up, because he felt they might not work.
When I got to look at it, and at their suggestions, I rapidly concluded that he was right; they would not have worked at all. I started to explore the problem and found that the circuit board in the device takes track power from pickups on both trucks. On the circuit board, along with the usual diodes, resistors and a regulator are two manual switch- es. One enables a fan and the smoke generator, the other, the lights on the car ends, which are both lit together when on DCC track power and directional on DC track.
Poking about the circuit board with a multimeter while operating it showed that the fan motor and the smoke
dropping resistor would make that resis- tor heat up, and heat up quite a quickly, too. After all, 0.5amps at 12v is 6 watts so quite a sizeable resistor and heat sink would be needed. But there isn’t that much space in the car, and it does have a plastic body, and we all know that plastic bodies and heat don’t go well togeth-
er. Add to that the fact that 0.4amps required to run the smoke generation is beyond the capability of most decoder function outputs. But Brian really did want to be able to operate the smoke from a function than by twiddling the throttle. After all, on the prototype, steam is either on or off. I puzzled with these findings for while as to how to solve them. I also noted that the fan
ran in the same direction regardless of what the DC or DCC polarity was on the rails made sense, really, as the fan blows air into the smoke generator.This air then exits, with smoke, via a piece
of drinking straw yes, drinking straw! through a vent in the car floor to simu- late the leaks of steam under and around these steam generator cars.
Two ideas, realisations, came together
to solve the issue. First, was recollection that some TCS decoders offer motor control from a function button. Motor outputs from decoders more often don’t not exceed 1 amp, so usng a decoder in such a way would easily deal with the 0.4 amp requirement of the smoke circuitry.
The second, and key, realization was that rather than mess about with adding volt-
ROUNDHOUSE - March/April 2020
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