I am having difficulty finding the right resistor for a 1.5 V, 15MA incandescent lamp, operating on HO scale DCC.
My track has 15.5 V on it and the lamp is 1.5 V. That means I need to absorb 14.0 V at 15 MA. By my calculation, that means I need a 920 ohm resistor.
I tried a 1k ohm resistor and that was too dim. I tried two 440 ohm resistors in series and that seemed to be too bright. This lamp will be mounted inside an HO locomotive so there probably is not room for three resistors in series; i.e., two 440s and a 47.
The question is where can I find such an oddball resistor, hopefully all 920 ohms in one resistor? I tried Radio Shack and Miniatronics on line and they do not appear they carry them.
I guess another question would be are two 440 ohm resistors OK?
I’ve never heard of a 440 ohm resistor. They make 470ohm. The striping would be yellow violet brown. Two 470 ohm resistors in series would be 940 ohms. The actual calculation for 14V/15ma is a 933 ohm 1/4w resistor. However, I think you are making an incorrect assumption in your calculations. 15.5V for track voltage for DCC does not equate to the actual function output voltage on the decoder. The DC component on DCC is a differential voltage between the rails and the decoder has some active circuitry in line to remove the DC component from the DCC signal. The best thing to do is get a voltmeter and see what the actual function output voltage is or experiement with various resistor combinations until you get what you like. I’d recheck your 440 ohm resistors. I don’t think they are 440ohm. I suspect 220ohm (red, red, brown).
The DCC voltage on the track is not the voltage that is applied to bulbs by the decoder’s function outputs. Most decoder functions for headlight and backup light are 12 Volts output.
Try applying Ohms law using 12 Volts as the output voltage and it comes to 10.5 Volts that needs to be dropped. For a 15mA bulb, you would need a 700 Ohm resistor; however, this is not a common value. An 820 Ohm would be the next available value.