Here is an idea I have been following in the Yahoo SoundTraxx group. If you use a CL2 constant current device, the current will stay at 20ma when fed by anywhere from 5 volts to 90 volts. No resistor calculation needed. It is a LED driver. It can be used by 1.5 volt lamps also. Good for lamps that operate at 20ma. Some model railroad controllers operate above 12 to 14 volts and occasionally, someone has a burnt out 1.5 volt lamp because of that. Lamps are sensitive to voltage changes even with a resistor and the brightness can vary.The CL2 is sold by Mouser Electronics. It is quite small.----------------------------------------------------------------------------------------------------Another option I found in a different DCC forum.The LM317 can be used as a current regulator using one resistor. Look at National's application note. They are available in the sameTO92 package as well as SMT. I have used them with Miniatronics 1.5Vbulbs at just under 40ma as head and tail lights in trolleys. Theyoperate for 12hr days during shows and have been in service over twoyears. With the SMT wrapped in Kapton I slip them in the boiler between theweight and shell in brass locos, but they require good solderingskills and don't drop them on/in the carpet.
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I have not ried these options but they look very good.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
A regulated power supply/source is always better than a simple series resistance type circuit.
Interesting product(and cheap at 44 cents). BTW, that is Mouser Electronics .....
http://www.mouser.com/Search/ProductDetail.aspx?qs=Plqv86A3fKMm41oemcboMA%3d%3d
Jim
Modeling BNSF and Milwaukee Road in SW Wisconsin