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DCC decoders and our calculators...
Today's mobile decoders have additional functions that can be used to control various lighting effects. Wired decoders have wire leads that are to be connected during installation and depending on the function, may require the addition of a resistor to properly control a lighting effect. Drop-in decoders don't have leads attached, but have solder pads on the decoder to connect wires to for additional function effects. Many (if not most) of the drop-in style have resistors already installed next to the solder pads to simplify hookup.
Our calculators are provided to determine resistance, current and mcd output for LEDs used in static lighting situations. That is, lighting intensities that will be fixed once determined. Many of the DCC decoder functions include dynamic changes that are made to the effect during the normal course of operation (Rule 17 dimming, for example). In these cases, proper resistance selection may not be determined by a simple calculation. This is because decoders vary the output to the light source (LED or incandescent bulb) for a given effect. Many decoder manufactures will recommend resistance values differently for bulbs vs. LEDs because these devices behave differently under dynamic control. We don't recommend using our calculators to determine optimum resistance values in these situations. If this is your need, check your decoder manual for guidance or contact us at firstname.lastname@example.org and we'll try our best to help you.
When choosing resistors, if the calculated value is not a "standard" value, always use the next higher standard value for both resistance (ohms) and current carrying capability (watts).
Being of the firm belief that "one can never have too many tools", we've added several calculators below to assist you in quickly computing the pertinent information needed to properly use our LEDs.
The LED - RESISTOR calculator computes the resistor value (ohms) and size (watts) for the desired LED current , device voltage (Vd), and supply voltage (Vs).
The LED - CURRENT calculator computes the current and resistor wattage for a given resistance, and device/supply voltages. This is a handy tool to recalculate current if you need to stay within a certain wattage specification.
The LED - MCD calculator computes the change in LED current needed to alter LED brightness (mcd output). The LED - RESISTOR calculator can then be used to determine the resistor values necessary for this new LED current. See details below.
IMPORTANT: If multiple LED's are to be connected in series, add their device voltages together for the Vd value.
LED light output brightness levels are measured in "mcd", or millicandelas. Our LEDs are specified to have a given brightness at a specified nominal current (ma, or milliamps) value that will not "overdrive" the LED and shorten its life. All of our LEDs are specify with a nominal (recommended) current value of 20ma (twenty thousandths of an Amp).
If the current that an LED draws is changed (up or down), its brightness (mcd value) will change. The manufacturers that make our LEDs control their processes such that the LEDs are reasonably linear in this mcd/current relationship. For the most part, this is true from about 2 or 3ma all the way up to about 30ma. To understand this better, let's use our 2x3 Super-white LED as an example. The manufacturer specifies this LED to have a brightness output of 320mcd at 20ma. If we reduce the current the LED draws to 10ma, it's brightness will be reduced to around 160mcd (about half). If we go down to 5ma, it will drop to approximately 80mcd (about 1/4 brightness). Conversely, if we were to allow the LED to draw 25ma, it's output will increase to about 400mcd (25% brighter).
A few words about LED life...
Under normal operating conditions (not overly heated during soldering and limited to 20ma of current), our LEDs can be expected to last on average, about 80,000 hours before brightness begins to substantially reduce. Some manufacturers will spec their LEDs at 100,000 hours, but if you look at the fine print, or talk to their engineers, that's when their LED goes completely dark (no output). Besides, 80,000 hours equals a little more than 27 years if you were to operate the device for 8 hours every single day! By that time, we'll probably want to redo our project anyway.
One thing that has a direct impact on an LED's life expectancy is the current it draws. If we operate an LED at reduced current, we will extend it's life even further. At greatly reduced levels, it can last almost indefinitely!
Conversely, if we allow it to see current above the nominal rating (20ma), it's life will be much shorter. Unfortunately, the relationship between LED life and current is not linear like the relationship between current and brightness. Operating an LED at 50% over the rated current level, or 30ma, can reduce it's life by 80%. It'll be really bright, but not for long... If you plan to operate our LEDs at current levels above 30ma, they may behave like flashbulbs.
However (yep, another however), you can subject LEDs to incredibly huge levels of current (175ma) providing it is pulsed current at 1/10 duty cycle, 0.1ms (one ten-thousandth of a second) pulse width. Pulsing LEDs is a discussion for another time. Besides, these levels of brightness (overwhelming brightness) really don't lend themselves comfortably to applications in model railroading.
OK, why even mess with LED brightness?...
Well, because there are situations where it can have a major impact on the visual effect you want to present. Here are some examples:
Play with the numbers...
Once you feel comfortable with using the calculators, they become easy to use to play around with various resistance/current/wattage/mcd values to meet specific criteria you're looking for. It's very quick to re-enter one or two of the values in the calculator entry windows and click to recalculate. Certainly easier than having to reenter everything in your desktop calculator each time or work it out with pencil and paper.
Here's an example of some "tweaking" we did on the Kato Amtrak Superliner with interior and EOT lights:
A little experimenting determined that using our 2x3 Super-white LED (mcd output of 320 at 20ma current), a light output of ~ 240-250 mcd was very adequate to fully light the interior of the car. The car would be obviously lit, but not over-bright so as to look non-prototypical, or "toy-like".
The circuit we designed for this project (this circuit), includes a bridge rectifier. The diodes in a bridge rectifier that "filter" AC or DCC voltages to straight DC voltage (needed for the LEDs) have a slight voltage drop of about 0.6 volts. That is, if we hookup a bridge rectifier, even using plain DC on the input, it's output will be about 0.6 volts lower than the input. This is an inherent characteristic of most silicon signal and rectifier diodes. Our DCC test track uses a Digitrax DCS100 controller powered by an MRC Control Master 20 power supply (pretty standard stuff). Our bridge rectifier connected across the track had an output of 11.4 volts DC so the input had to be right at 12 volts. Using this as the LED source voltage, we began testing a circuit with various limiting resistors. After running through several calculations and a few tests, it was determined that a 510 ohm resistor would do nicely and produce an added benefit.
To run our 2x3 LED at full mcd output we would need a 390 ohm resistor to give 20ma at 11.4 volts. Using the LED Current Calculator and substituting a 510 ohm resistor for the 390 value gives us a 15.3ma result (about 76% of 20ma). Remember, that LED light output is fairly linear as a function of current (ma). 15.3ma is ~76% of 20ma, so 76% of 320 mcd is about 245 mcd. Testing proved this to be very satisfactory output for lighting the car's interior.
Now, here's the added benefit. Had we gone with the 390 ohm resistor for full LED output, a quick calculation determines we would need a resistor capable of handling 156 milliwatts of power. Since this is a non-standard wattage rating, we would need to use a 1/4 watt (0.25) resistor. However, using the 510 ohm resistor, the reduced current in the circuit only requires a 119 milliwatt resistor, so a 1/8 watt (0.125) one will work just fine. Normally, this resistor would run hot, but we've soldered one side of it to one leg of the bridge rectifier and the other side is connected to #30 wire. Both act as heat sinks to help dissipate excess heat.
If maximum LED brightness isn't always necessary, you'll have some room to play with the numbers and may find it advantageous to do so.
© 2008 Ngineering