Model Railroading > Help with LED building lights

I am in the process of adding lights to the structures on the Texas & Gulf.

So far so good except in most cases the lights are too bright.  I have a transformer
tbat has adjustable outputs from 12 volts down to 3 volts, and I have put resistors on 
all the LED's to take input from the lighting buss from 12 volts down to 3 volts at the lamp.

My thinking, incorrectly it appears, was that at 12 volts on the lighting buss the lights might
be bright but I could always dial it into to the perfect illumination level, however any voltage
above the 3 volt setting does not change the brightness of the LEDs.  At the minimum setting
of 3 volts the lights dim somewhat and that is good enough for me I suppose.

I am used to incandescent bulbs where the brightness varies with voltage.  I want to keep
the resistor to protect the LED's from premature failure.  The room lighting has dimmable LED's
so I can operate in daylight or late evening settings.  Why are these small LED's not dimmable?



Edited 1 time(s). Last edit at 11/11/22 09:09 by WrongWayMurphy.

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LED light output is determined by the current through it.
The voltage has to be above a threshold to cause it to emit light, typically about 3 volts for white and about 2 volts for red.  The package should tell you that value.
The design calculation is to start with a power supply comfortably above the voltage needed and have a resistor in series that will control the current to what you want by "taking" the voltage from the power supply and leaving just enough to fire the LED.
For a bright headlight you might want 10 milliamps, for a low intensity light like a ground light or a security light on a building scene, maybe 0.5 milliamps.  
The calculation to choose the resistor uses Ohm's Law:  Voltage across a resistor equals current times resistance.
For our headlight example, with a supply power of 12 volts (almost universal for DCC lighting functions) we know the LED will need 3 volts so the resistor has to take 9 volts in series from the 12 volt supply.
If we want 10 ma or 0.010 amps at the LED that same current will run through some resistor that yields 9 volts.
9=0.010 x R, which becomes R=9/0.010  and we find R will need to be 900 ohms.
A little bigger R will make a little bit less current and a little bit dimmer LED.  Many modelers use 1000 ohm (or 1K) resistors just because it is easier to find.  My personal choice is 1.2 or 1.5k for headlights.
Now for the security light, running at about 0.5 milliamps:
9=0.0005xR, which gives us R=18,000 or 18k.
I have some ground lights on locomotives that run even less, and they are bright enough that you know there is a "night light" there.

Are you using dimmable LEDs? There is a difference in that the dimmable LEDs have internal circuitry that steps with the voltage. (Like the resistors in the post above mine.) The characteristics you describe with the transformer read like non-dimmable LEDs. A fixed resistor will dim them to various levels, but it's not a smooth curve.

Also, like your room lights, you need a special electronic dimmer. Search for "12v LED dimmer" and you'll find lots of choices that aren't very expensive. They're sort of like those little dimmers now found on lots and lots of Xmas lights.



Edited 1 time(s). Last edit at 11/11/22 10:04 by Lighter.

You can buy dimmers designed for 12 volt LED strips, such as this:  MPJA LED Dimmer.  These use pulse width modulation and allow good control over the LED.  One dimmer could hande many buildings.

Barry Draper



Edited 2 time(s). Last edit at 11/14/22 11:53 by barrydraper.

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I keep it very simple for LEDs:
12v source (normally)

I keep two types of resistors handy: 1K ohm for brighter lights like headlights (or building interiors that really need intensity)
4.3K ohm (easy to find) to make dimmer LEDs that I don't want to be as bright as the above (like exterior porch bulbs).



Edited 1 time(s). Last edit at 11/11/22 20:32 by KA7008.

A completely different tack I took on this problem was to cover the LEDs in side the building with thin white paper.  While this obviously wouldn't work on an exposed porch light it did a nice job diffusing the light and making the color less harsh. 

Nathan Beauheim 
Loveland, CO

I light my structures with Woodland Scenics Just Plug LED lights. The control module comes with dimmer switches.

Posted from iPhone

Or as I did on one section of my layout, I simply mounted a battery holder for 2 AA batteries for a steady and never too high DC power source. If you had lots of LEDs, use 2 D size batteries.