RailPro > RailPro Specific Help & Discussion
Choosing LED's
KPack:
Bill - I use two sizes of LED's for nearly all of my installs. I use the "chip" size for head and rear lights, and the "nano" for ditchlights. http://www.modeltrainsoftware.com/smd-chip-leds.html. I use the chip because I can get it to sit flat up against the back of the rear and front lights. The larger round LEDs would be perhaps brighter, but they are rather long.
Get the ones without any resistors on them...just bare wire. You'll want to install your own resistors where you want them. I use SMD resistors, 1K ohm 1/4 watt. I'll see if I find the exact ones I bought from Digikey. I bought a roll of 100 or so....it'll last me forever.
One trick I learned is to use liquid electrical tape to cover the back of the LEDs after they are installed to prevent any light leakage into the shell. Easy to do, and works very well. Removable in case a LED ever burns out, heaven forbid.
-Kevin
William Brillinger:
Thanks Kevin, one key question is still unanswered: Which voltage?
Select Voltage
3 volt DC
5-12 volt DC
7-19 volt AC/DC/DCC(+1.00)
OH, Wait, I maybe you did answer that...
Get the ones without any resistors on them...just bare wire.
Looking at the pictures on the Evan's site, that would be the 3 volt ones.
Thanks Kevin.
Alan:
Bill, voltage is an odd specification when talking LEDs. They don't have an "operating" voltage per se, at least not like you are accustomed to with other devices. LEDs have a voltage drop across them, typically 1.7V - 3.5V. Termed forward voltage drop it is specified in the data sheet as Vf. The voltage applied must exceed the minimum voltage drop of the specific LED for the LED to light. Once the minimum voltage has been applied an LED becomes a current regulated device, not a voltage regulated device. The series resistor sets the current level flowing through the LED which dictates how bright the LED emits.
So, when working with LEDs the variable that is important to you is the current, or more precisely, forward current. The maximum forward current value for a given LED will be specified in the data sheet as If. If you want the LED to be less than full brightness then decrease the forward current by increasing the value of the series resistor. Most LEDs will light with as little as 1mA of current or less. Here is the ultra handy formula you should use when working with LEDs:
R = resistor value
Vs = power supply voltage
Vf = LED forward voltage
I = LED forward current
Or even easier... http://www.ohmslawcalculator.com/led-resistor-calculator
You know a typical LED will light anywhere between 1mA and 20mA. Use the formula to determine the resistor value for 1mA and then the value for 20mA. Those two resistor values now become the min and max for whatever voltage you are using. Choose your brightness level by selecting a value somewhere between min and max. Notice that, other than as an operand in the formula, voltage doesn't matter to the LED. Hence, why voltage is an odd specification for an LED.
When suppliers advertise LEDs for different voltages (Evans) what they are really saying is "This LED has a series resistor of sufficient value to permit 20mA of current to flow at x voltage." The LEDs themselves are not different voltage LEDs because there is no such thing.
KPack:
Yes Bill, the ones labeled as 3V are what you want.
Thanks Alan for the deeper explanation.
-Kevin
PatP:
Bill,
If it helps, I use an iPhone app called "Electronics Toolbox Pro" to figure out resistor size for LEDs (among other things). As Alan described, the voltage drop is color dependent while the current is fairly constant at 20ma with a peak of 30ma to 50ma (the light that burns twice as bright burns half as long), again depending upon color, clarity, etc... In this case, size does not matter, with the smd style being a bare chip and the "bulbs" being the chip embedded in an acrylic shell.
My brother-in-law (a true electronics Guru, he writes the manuals for military radar, radio, sat, etc..) uses an app called "Electrodroid Pro" for his Android. Says it saves the grey cells from overuse.
Either app lets you pick the color and source voltage and will calculate the resistor for you.
This is another of those things that I know just enough to be a nuisance.
PatP
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