LEDs are the preferred method

Bruce prefers to use LEDs with a decoder. Why? Several reasons:

  • Color - they are now available in Golden White and Sunny White colors, which are very close the color shown in the photo to the left. Gone are the days of "blue-white" LEDs!
  • Life - the life span of a full brightness LED is about 5000 hours - that is 2-1/2 years of 8 hours a day, 5 days a week every week! If you are willing to have only 80% brightness - still VERY bright - the life goes up about ten times to about 25 years.
  • Low power - very little power is consumed from your DCC set
  • Cool operation - they use very little power, so they don't get hot!

What sizes are available?

Common sizes are:

  • T-1 or 3 mm - HO or N scale headlights
  • T-1-3/4 or 5 mm - HO and larger headlights
  • SMD (surface mount) frequently 0.06 by 0.03 inches:
    Z headlights and ditch lights and other effects in larger scales

What colors are available?

Common colors are:

  • Golden white - orange lens but they look the most like incandescent - okay if the bulb in inside the loco shining on a lens
  • Sunny white - slightly bluer than the Golden white, but have a "water clear" lens. Bruce owns an Atlas RS-1 that has a similar installation to the one shown on this site with 5 mm Sunny White LEDs replacing the Atlas light pipes - SPECTACULAR!
  • "Bright" white - this is a euphemism for BLUE-WHITE or cool white. They look like fluorescent lights as they come. These can be warmed to you choice with Tamiya Acrylic X-26 Clear Orange paint
  • Colors are available for signaling and other effects: red, green yellow, blue
  • Multi colors are available, too: red/green; red/yellow; red/green/yellow - if you are going to use them with a decoder, you need a common anode version.

How do I connect them?

The answer is very simple, but, if you do it wrong, they won't light or will give you a splendid light show for a few seconds, sometimes taking the driver transistor in the decoder south with them.

Current Limitation

LEDs have a voltage level needed to “fire” them. Below that voltage you get no light. This voltage is related to the way that they generate a particular wavelength of light. A specific color will have a specific voltage, ranging from about 2 volts for red to 4 volts for ultraviolet.

LEDs are current driven devices. THEY MUST HAVE A SERIES RESISTOR!They create light in proportion to the amount of current driven through them. This is NOT a linear relationship. A 10 mA rated device may produce 90% output at 5 mA. The amount of light you desire may come on at only 2 or 3 mA.

Since the output of your decoder is track voltage (about 12 to 15 volts), you need a fairly large value of resistance to absorb the extra voltage.

Calculating Resistance – The total resistance needed in series with the LED can be calculated easily. I have created a table of total resistance (sum of all resistors in series with a LED) vs. current flow for a white LED, assuming setting of 14 volts out of the booster. Higher track voltage or colored LEDs will increase the current slightly – roughly the amount of changing one resistor size smaller.

Resistance

In Ohms

Current
in mA

750

11

1000

8.5

1500

6

2100

4

2700

3

4700

2


As you can see, 1000 or 1500 ohms will get you going with less than 10 mA current. This will probably give enough light for your needs and keep the operating life in the tens of thousands of hours.

Power dissipation will be small enough that 1/8-watt to 1/10-watt resistors will be adequate!

For the theorists in the crowd, here is the formula:

R = E / I = (Track Voltage - LED voltage - Decoder drop) / LED current

If the current value is entered in mA, then the answer is in kilohms. If the current is entered in amps, the answer is in ohms,

Let's calculate the 11 mA number from the above chart, assuming:

  • 14 volt track voltage
  • 4 volt LED firing voltage
  • 1.5 volts Decoder drop: power supply and driver transistor losses

R = (14 - 4 - 1.5) V / 11 mA = 8.5 V / 0.0 11 A = 772 ohms
750 ohms is the closest standard value

While the LED is polarity sensitive, the resistor is not. It can be placed in either direction connected to either lead of the LED. Bruce prefers to put the resistor between the blue lead and the LED, as this provides the greatest margin of safety for the decoder, in the event of a short. The resistor may

If you leave out the resistor, the LED's life is measured in seconds, but the light show is amazing! If the LED fails by shorting out, you will probably toast the transistor in the decoder, too!

Polarity

LEDs are diodes. That means that current only flows in one direction through them. If you hook them up backward, it will probably not damage them, but they will not light.

In conventional LEDs, the longest lead as they come from the factory, or the bent lead as shown in this drawing, goes to the positive voltage supply (usually the blue lead).

The other (shorter) lead goes to the function wire (white, yellow, etc.)

With Surface Mount Devices (SMD LEDs) or if you are not sure, the quick way is to connect a 2700-ohm resistor in series with a 9-volt battery. Probe the LED with this connection. When the LED lights, connect the pad that has the positive side of the 9-volt battery connected to it to the blue decoder lead. This test will apply 3 mA or less to any LED. Enough to light but not enough to burn out the most efficient LEDs during a short period of application.

REMEMBER ONE of the leads needs a resistor in it.

Series

To light several LEDs with one decoder output, you can wire several identical LEDs in series, up to a point. This is an advanced technique for experienced installers.

FIRING VOLTAGE - First you must know the firing voltage of your LEDs. If you don't get that data with the LEDs, you can measure it. Connect the 9-volt battery + resistor or buzzer to the LED and get it to light. WHILE IT IS LIT, connect your multi-meter set on the scale that maximizes out at 5 to 20 volts, and read the voltage. That is your LED firing voltage.

SUPPLY VOLTAGE - You can measure the function voltage out of your decoder (blue + and white -) or you can estimate it. If you follow your set manufacturer's instructions to set track voltage or measure it with something like an RRAmpmeter then you know your DCC track voltage. If you subtract 2 from the DCC track voltage you will have a close estimate of the supply voltage.

LED CURRENT - A good place to start is at 75% of the LED rated maximum current. This usually gives enough light and keeps the LED life in the thousands of hours. If you want to fine tune the light level, adjust the brightness of a single LED to the desired value by varying the series resistance and measure the current that results in the desired brightness.

Now you can connect the LEDs in series with a current limiting resistor as shown above, as long as the sum of the firing voltages doesn't exceed the supply voltage.

A good rule of thumb is to assume that white LEDs will require 3 to 3.5 volts to fire and the supply voltage will be 12 volts. Thus, the longest string of LEDs that you could support would be 3.

total LED voltage = 3 LEDs x 3.5 volts = 10.5 volts 
which is less than 12 volts

In this case, if you wanted 7.5 mA (0.075 amps) of LED current, the current limiting resistor would be:

Supply Voltage - total LED voltage / LED current in amps 
(12 - 10.5) / 0.075 = 
1.5 / .075 = 20 ohms

The power consumed would be so little that virtually any wattage resistor would suffice: 1/8 watt discreet or any SMD resistor.

LED Anatomy

Packaged LEDs include a small die (the actual device) packaged into an epoxy housing with external leads. The longer lead is connected to the anode and should be connected to the positive voltage.

SMD LEDs eliminate the epoxy housing and have the die mounted on a small ceramic holder with connection points on either side.

White LEDs - LEDs emit a small band of frequencies (color) around a center value.  White light is the presence of all colors. How do you do this with an LED? The same way you do with fluorescent lights: you make a fluorescent coating that emits the spectrum you want to see and excite it with blue to ultra-violet light from the LED. Varying the chemistry of the coating changes the color mix.

BiColor LEDs – As you can see in the photos above, the die sits in the cathode cup, so the simplest way to have two colors is to put two dies of different color in the same cup. That makes the cathodes common. Well, since our decoders operate by connecting the cathode to the negative voltage, this version won’t work. There are a few folks who make common anode LEDs. If you want a two-color version, you need to find one of these common anode units, or make your own by connecting the anodes of two SMD LEDs of different colors.

LED Safety

LEDs will probably not hurt you. But how do you keep from hurting them?

If you are working with them on the function outputs of a DCC decoder, static electricity and current limitation (discussed above) are the only issues to seriously consider.

STATIC ELECTRICITY - all semiconductor devices, LEDs are one, are sensitive to static electricity. LEDs are fairly imune. It doesn't hurt to tuch something metal and grounded before you work with LEDs. Also, do not wear a wool sweater!

If you are connecting LEDs to the rails, as with signals or lighting (buildings or cars) , there are other issues to think about.

STATIC ELECTRICITY - since the LED may be subjected to static electricity stresses everytime someone touches the rails, prudence would include use of a capacitor across the LED do absorb the voltage spike. The circuit shown here will suffice.

REVERSE VOLTAGE - LEDs are not designed to be rectifiers. They have limited and usually unspecified reverse voltage ratings. For the greatest safety when connecting them to rails or something where there may be static electricity and alternating current, use a rectifier (bridge is best) with them.

What about using LEDs with 
SoundTraxx' LC series decoders?

SoundTraxx says that the LC series decoders are not recommended for use with LEDs. Here's why. The design of the decoder has a small amount of current being routed to the lighting circuits during the generation of certain sounds. If incandescent bulbs are used, these pulses of current are not enough to illuminate the bulbs. However, LEDs respond very rapidly and will show a small glow from time to time. There is nothing about this that is incorrect, just a bit less than totally esthetic.

There are two ways to deal with this:

1) Live with it - many folks don't even notice or

2) Fix it - add a resistor and capacitor in parallel with the LED as shown in the drawing below (the magenta colored components). While the drawing below shows the DSD-AT100LC board (the most cryptic as to colors), the basic connection is the same for all LC series decoders, just follow the color code! In this drawing, the component values are a bit hard to read. The resistors are all 1000 (1K) ohms 1/8 watt and the capacitors are 0.1 uF at any voltage larger than 10 volts - Litchfield Station stocks 50 volt versions. Click on the photo to enlarge.

LED Theory

In my Model Railroad Hobbyist article - March 2012, I posted the following:

1. LEDs are current-based devices. The light they generate depends on the current passing through them. They are NOT a linear device. An LED rated for 10 mA may produce 90% of its light at 5 mA. The amount of light you desire may be produced at 2 or 3 mA.

Control the current by placing a resistor in series with EVERY LED! See above on how to select resistor values.

2. LEDs are diodes. That means that current only flows through them in
one direction. If you hook one up backward, it probably won’t be damaged, but it is not going to light up.

3. LEDs don’t have large reverse voltage ratings. Don’t use an LED as a rectifier.

4. Although the light produced by an LED depends on the current, an LED will not start conducting current and producing light until a certain voltage (called the “firing” voltage) is reached. Less voltage than that and you get no light. More voltage and they quickly go over-current and fail.

Each color has its own firing voltage ranging from about 2 volts for red to about 4 volts for ultraviolet. White LEDs usually need about 3 volts but that can vary, based on their design.

5. Don’t connect LEDs in parallel. The firing voltage varies slightly between even identical devices. Placing multiple LEDs in parallel using a single current limiting resistor is a bad idea.

The LED with the lowest firing voltage in the group will hog most of the current. Some LEDs with higher firing voltages may not even fire. Excess current will shorten the life of the lowest voltage LED. When it burns out (assuming it does not short in the process) the next lowest LED takes an even larger amount of current. You get the idea.

6 - LEDs emit a narrow band of wavelengths (color). Because white is a collection of all colors, a single wavelength device can never be white. Instead, the single-wavelength LED die is coated with a fluorescent material. When the LED shines on it, the coating lights up like a fluorescent lamp. The chemical makeup of the coating determines the color emitted. The composition of the coating and the wavelength of the impinging light work together to set the color value.

Since the firing voltage relates to the wavelength, white LEDs may operate anywhere from 2.8 to 4 volts.

Another take

A fellow installer with 27 years in the optoelectronics business, Scott Russell of CR&S Trains, contributed the following insight into LEDs:

The critical parameter to the longevity of an LED is power dissipation as it relates to junction temperature (of the semiconductor junction that is responsible for turning some of the electrons passing through it into photons). Once the junction temperature gets above a safe level, the LED begins to degrade, both in output and wavelength drift. An LED turns about 15% of the input energy into light and 85% into heat. This is a typical value, as it can vary by the efficiency of the material and doping used to produce a particular color.

Obviously, when the LED goes "poof" and you see a little black dot inside, the junction temperature has been greatly exceeded, even if briefly.

The ability of the LED to dissipate heat from the junction is dependent on package design. When LED's (especially IR-LEDs) came in metal packages (that shows my age), the DC forward currents could be as much as 50mA to 100mA. This is why the SMD packages usually have a lower maximum current, since there is nowhere for the heat to go.

Controlling junction temperature is how TV and other remote controls can output energy (usually IR) over great distances. You can drive an LED at 1A or more if it's with a short duty cycle where the LED is on for about 5% of the cycle and off for 95% so the junction can cool. How much pulse current can be handled is also dependent on the gauge of the bond wire used inside the LED to bond the chip to the lead frame (but that's another boring subject, and not relevant here).

That's a long-winded way to get to the point that 1K resistors almost always work, but with more efficient LED's (like the white ones), the extra light generated after 2-5mA may be negligible and the extra current only serves to shorten the LED's life. It may still outlive you, so it may not really matter.

Copyright © Bruce F. Petrarca 2007 - 2017; All Rights Reserved