I have a set of K-line 15" streamlined passenger cars. For some reason, the lights in the observation car are significantly brighter than the other cars. The observation car also has a drumhead light and two marker lights at the rear. I would like to dim the observation car lights if I can, but I’m not sure how. Putting a diode in series with the board dims the main lights pretty well, but then the marker lights don’t light. (they are fed in parallel across a bi-directional two-diode circuit, so they only get 1.5V). Any ideas on why the observation car lighting is brighter, or how to dim it without losing the parallel marker lights? I would have thought the observation car lighting would be dimmer, since there are more bulbs and a 1.5V drop in the circuit.
Can you give some more details? What’s on “the board”? How do they get a 1.5-volt drop in either direction with only two diodes? Are the main lights in parallel, series, or what?
Hi Bob - I tried to post an ASCII diagram, but for some reason all white space is truncated when I preview the post. There are 4 diodes total, two in each direction on parallel branches. The marker lights are in parallel to the two diode branches, so I guess in either direction there is a differential of 1.5V across the diode branches. That whole thing is in series with the main cabin lights, which are all in series with each other. The arrangement is described in the patent; check out 6231421 at free.patentfetcher.com if you’re interested.
The easiest is to paint over a few of the bulbs. To dim the lights in a set of my cars I used a paint, blue, that is used for painting designs on glasses and is translucent. I found it at Michael’s. If you totally want to blank a bulb use black. While you have the cars open don’t forget to add passangers.
Try putting two diodes in parallel (opposit polarities) in series with the car lights. That will give you an additional 1.5 volt drop in voltage. If one set helps but it’s not enough add another.
It seems to me that the marker lights should dim in about the same proportion as the main lights when you put a diode in series with the entire circuit. The only explanation that comes to mind is that they are in fact just dimming, but are dim enough to start with that they appear dark.
If this is the correct explanation, you could try the overall series diode again, but rearrange the marker-light circuit so that three diodes in series rather than two supply the marker lights. The 50-percent increase in the peak marker-light voltage would be almost exactly compensated for by the 30-percent drop from the diode in series with the entire circuit (1.5/sqrt(2)). You would not need any diodes in antiparallel, since the overall series diode guarantees that there will never be any reverse voltage. This way, the main lights should dim about as you have already observed; but the marker lights will keep their full brightness.
I have often suggested diodes in anti-parallel for AC voltage dropping; but they are problematic for dimming incandescent lamps in a train. The relative voltage change on the lamps is always greater than the relative voltage change on the track; and lamp efficacy is extraordinarily sensitive to voltage. Light output varies as the 3.5 power of voltage. In any case, two diodes in anti-parallel will get you only about half a volt of drop in the RMS voltage. (The RMS voltage drop, which is what the lamp responds to, is somewhat less than the forward drop of the diodes.) I usually suggest using bridge-rectifier modules, with the + and - terminals wired together, as a convenient substitute for four discrete diodes in this application; but even that will get you only about a volt per bridge rectifier.