Scale Model Animation 2 – LEDs and Lighting
A LED is not an Incandescent Bulb
After seeing so much interest in light emitting diode (LED) lighting, and so much misunderstanding about LEDs, I think a little more information will go a long way. First, allow me to say a LED is not an Incandescent Bulb – incredibly obvious I hope. Any modeler who has been around for more than a few years probably started using “grain of wheat” bulbs, then “grain of rice” bilbs, then micro-bulbs etc. They all work, they all generate heat, they all can be colored by usually painting the bulb wil an appropriate color. But if you really pay close attention, different bulbs of the same size can be different brightnesses. For example, Miniatronics sold three micro bulbs all approximately the same size, but one was 15ma, another 30ma, and the last 40ma (all 12 Volts or so). They were brighter with the increasing current ratings. Here’s one of the first realities of incandescent bulbs—their brightness is proportional to the power rating of the bulb (Volts X Amps). Higher power = higher brightness = more heat generated. So one of the bigger problems with incandescent bulbs is that they generate more heat with brightness. Those of us who put a grain of wheat bulb in our first plastic loco and discovered one day that the headlight housing has melted and ddeformed learned this lesson. Some of us had to re-learn this lesson as we tried to light up our carefully constructed, and completely enclosed plastic buildings to learn we had rediscovered the secret to our sister’s Easy-Bake Oven from years ago. The fact is that if we wanted a bright light we were going to generate heat. What is not generally known is that the lifetime of the bulb is inversely proportional to the cube of the power running the bulb. So a small increase in operating power (say because we might me using an unregulated transformer or power pack) radically reduced the bulb’s lifetime. Also the converse of that is true—a small reduction in operating power, increased the lifetime of the bulb. This is why in some real signaling systems the bulbs might be rated for 40 Watts at 34 Volts, but would be operated at 32 Volts and 37.6 Watts! This would increase the operating life of the bulb and reduce maintenance costs for bulb replacement. Years ago when I worked for Digital Equipment Corp. (a computer manufacturer at the time), the single largest reason for service calls was for incandescent bulb replacements in front panel displays!
But reducing the power to the bulb also changes the perceived color of the bulb from white/very bright yellow to a more orange/reddish color. The reason why they appeared white in the first place was that your eye became saturated at that particular color—it was always yellow/red! The heat from the bulb would sometimes also discolor the paint on the bulb envelope, or even flake it off completely! By the way incandescent bulbs are also highly susceptible to vibration and temperature shock (cold to hot, hot to cold) further reducing their lifetime.
LEDs to the rescue?
Have I told you a LED is not an Incandescent Bulb? No? Well let me reiterate “a LED is not an Incandescent Bulb.” A light emitting diode is made from layers of semiconducting materials- that is not insulating and not conducting—all solid, and fairly rugged. They generate light (photons) when electrons in the semiconductor combine with “electron holes” within the device. The energy of the emitted photons determine the color of the light, and the same energy is determined by the band gap energy of the materials forming the LED. Remember this for later!
A LED’s brightness is proportional to the current through the LED—and generally for most of the LEDs we use in modeling there is not allot of range! The number and type of LEDs has been growing enormously in the last 10 years, but my comments will be directed to “typical LEDs” that are small, and appropriate for modeling, but will not cover all devices in the universe called “LED” so please drop the commentary about all the “exceptions.” The LEDs of interest in this discussion have a maximum operating current of 20ma (continuous current DC), ALL OF THEM-- by definition.
LED Assortment and “Grain of Wheat Bulb”
If you have what is commonly called a lab grade power supply, you can limit both the maximum voltage and the maximum current that the supply will deliver to its load. If you have access to such a supply and can set the maximum current to .020 Amps (20 milliamps), set the voltage to zero attach a LED to the supply, slowly increase the voltage, and watch the LED. For a typical red LED, you will likely see nothing until perhaps 0.8-0.9 volts (you might barely get any current draw). Increase the voltage a bit more and you will see a steady increase in light until the power supply limits the current at 20ma. Now look at the voltage reading just at the point when your supply hits 20ma—it will read something like 1.8 volts. This is the forward operating voltage for this diode at 20ma (likely its rated max value). A LED can actually tolerate much higher peak currents (many from 8 to 12 times!) but only for very short periods of time—a matter of milliseconds. This is often how LEDs are used as “flashes” in compact cameras. High continuous current in a LED will eventually build up heat internal to the LED and cause a catastrophic failure of the device—and will happen very quickly.
Remember I spoke of the different LED materials determining LED emission color? Well the different materials also change the forward operating voltage mentioned above. So… while red LEDs might have a 1.8-2.1 volt drop, green LEDs can have a 2-3.5 volt drop, blue and white LEDs can show a 3-4 volt drop! This is important and confusing because if you try to put two LEDs of different colors together (in parallel) if could be difficult or even impossible to light both together. One will come to full brightness at 20ma and at the lower operating voltage before the other. The attempt to light both could in fact destroy one of them! Picking up an arbitrary LED of a particular color (say from a surplus seller) without knowing its electrical characteristics, doesn’t give you enough insight in how to pair it with another LED, or even light it to its maximum brightness alone without some measurement or experimentation.
But this does NOT mean you need to know everything about a LED to use it! If you are trying to light a LED from the typical hobby power 12 volt supply. First invest in a small number of resistors in the range from 470 Ohm to 3300 (3.3K) Ohm (470, 560, 680, 1000, 1200, 1500, 2200, 2700, 3300) would be a good set. For any given LED you choose to light, start with the highest value in your set, attach it to either end (No it does NOT matter one bit) of your LED and power it up (via decoder, power pack, etc.). It may not even light! But drop the value to the next lower one in your set until you get a pleasing result in your model. Once the “correct” value is determined you can use that value (and replace the one missing from your reference set). For a single led operated at max 20ma from a 12 volt supply a 1/4W resistor will do fine. Most modelers don’t like to operate LEDs at maximum brightness as they are too bright! So higher resistance will be used—try it and pick the brightness you like. I like headlights and markers to be really bright and building lighting to be much less.
LEDs are Not Hot Stuff!
The really great thing about these LEDs is that they run cool, and will not melt the plastic enclosures in models. There are new, high-power, multiple-LED assemblies that run so bright and consume much more power that they require heat sinking. These haven’t found much use yet in small scale models. The other really great thing about LEDs is that you can get them really small. In the LED assortment picture above you can see two small rectangular specs above the row of LEDs (all devices are sized proportionately). These are “surface mounted device” (SMD) LEDs. Their small size allows their placement in unbelievably small scale models like marker lights, hand lanterns, pole lights, desk lamps, etc. You can learn more about how I handle them in the February 2012 MRH ( http://issuu.com/mr-hobbyist/docs/mrh12-02-feb2012-ol?viewMode=presentation&mode=embed). White LEDs as small as .040 by .001 inches exist and are so bright in their resin package that they appear to emit light from all sides and appear as if they were a scale light bulb! Such small LEDs can be wired and protected by encapsulating them in white glue, ACC glue, Micro Krystal Klear, epoxy, etc.
Color in the Eye of the LED
LED color can sometimes be problematic, but it really should not. I stopped buying anything but white LEDs because you can get almost any color you want by purchasing white LEDs and then coloring them with Tamiya clear paint or permanent markers—both work well. You might ask to get a yellow tint with a “white” LED should I use yellow Tamiya paint? Answer: no! You need orange! (Strange but true) Many “white” LEDs emit a blue tint to them (sometimes called “pure white). Beware that the labels (pure white, white, sunny white, warm white, bright white, etc.) vendors provide to describe their “white” LEDs are open to their interpretation. Try using one before you commit to a large purchase. You can easily mix the Tamiya clear colors to get your own shades, but always try it in conjunction with the LEDs you will actually use in your modeling. Correspondingly, if you do decide to use a colored LED, remember that the color differences will mean that you will likely use different resistors with each color. In the article I cited above, you’ll also find mention of using 20 ma current regulators for lighting LEDs. I try to use these more and more with LED lighting, since I can guarantee maximum brightness (especially for building lighting) independent of supply voltage variations. You might think that supply variations will not matter? Think again—in both DC and DCC layouts of much size you can easily find 1-2 volt drops in track voltages for many practical reasons. Measure your own layout sometime and see.
By the way have I told you a LED is not an Incandescent Bulb! 
More to come.
Have fun!
Best Regards,
Geoff Bunza
