Two months ago I found this train in a shop in Barcelona. I imagine they couldn't sell it during some years, because it was abandoned in the depths of a big drawer and the outer card box was not in a perfect state. That's something usual with Japanese models in Spain, nobody wants to buy them, good for me!!!
But my surprise was to find bulbs instead of led in cab cars, and all my experience until now has been just with led. But finally you will see the only difference is the way you calculate the appropriate resistor, all the rest is similar or easier than led cab cars:
You can see here the detail of the components of the board, all of them in one side of the board. There are two diodes (E225) and one switching diode (C5C).
That's the electrical schema of the board. Sorry for this ugly and poor drawing, but I just had Microsoft Paint installed in the computer from where I'm writing this post:
First of all we have to calculate the right resistor to use for glowing the bulbs in the right way without blowing them, because depending on the central station you are using it can supply more than 12V. You know the formula for that: R = V / A
Let's find first the amperage consumed by the board. With a traditional N scale 12V transformer and connecting the tester in serial, we can see it consumes 50mA:
We measure also the voltage supplied by the decoder in the outputs, in my case is 18V that we have to drop to 12V with the resistor:
So the resistor to use is: R = (18V - 12V) / 0,05A = 120 Ohm
Now it's time to remove all diodes and components from the board, and leave only the bulbs. I also removed the current pick up arms and soldered the red and black wire of the decoder directly to the brass strips. My first idea was to take the decoder's supply also from the board because it has a kind of "current pick up arms", and it's possible to do it if you are just a bit skilled, but it's not my case and I couldn't find enough room for all wires.
That's the board finished. I used a SMD resistor to save space, soldered in the point where bulbs legs are joined.
The final assembly with interior lightning, using two diodes and a resistor, a good solution if your decoder for the cab car has only two outputs.
In this case I used a Doehler&Haass DHF250 decoder. Seems too large, but on the other hand it's only 2 millimeter thickness, so it's really easy to hide under the roof. A cheap and really good decoder.
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