@JiDi posted:I know it's been two years, but I just got one of these loco's, so allow me to kick this horse. I was curious to know why the resistor?
Is it because the voltage off the pin is 5v?
And if so, I assume if I use a 5v rated LED I would not need the resistor.
Thanks in advance for the assistance.
An LED is a diode (Light Emitting Diode). So a diode, once you cross the forward voltage and it begins conducting, it is sometimes called negative resistance. By contrast, an incandescent light bulb, it starts out at low resistance because the filament wire is cold. As soon as you apply power, the resistance increases across the filament as it heats up. At some point, it becomes self regulating. In other words, at 12V the rated voltage of the bulb, the filament increases in resistance to just the right value based on heat to not burn out.
Back to the LED. It's a diode. As soon as you begin giving it voltage above the forward voltage, it conducts and basically is massively conducting with relatively low resistance. It is NOT self regulating. You have to provide external resistance calculated to limit the current through the LED to about 10-15mA. That is why LEDs need a resistor, if you don't put one, they will simply burn out very quickly.
