Which LEDS do you use in running board lights and how many do you use in each light???
OK, on my runningboards:
I screwed up! Seriously!
LED's have a "working voltage" of between 1.8 and 3.3 volts depending on color. If you apply vehicle power (12-14v) to a SINGLE LED, it will pop in about a 1/4 second. You can add resistors to each LED to stop the damage, but that means a single resistor for EACH light you have.
You can STRING LEDS together. The negative of the first LED to the postive of the next LED. Everytime you add another LED, it "uses up" another 1.8-3.3v (depending on color). If you do your math right, you can make a string of LEDs that will "use up" all of the available voltage without burning out.
This is where I goofed. I made my strings of LEDs to run at 12v. Forgetting that a vehicle charging system runs at 14v!!! This burns them out. Slowly, but still eats them up.
The amber LED's I used are 5mm bulbs. The holders are the T1-3/4 sized holders that you drill a 1/4" hole for them to fit. Because I goofed my math, I made strings of 6 LEDs instead of strings of 7. Therefore I drilled 18 holes, instead of drilling either 14, or 21.
You can use as many as you like and just hide the extra LEDs up inside the frame, or make a FULL STRING and then make a SHORT STRING and make up the missing bulbs with a single resistor. Which is what I had to do.
TYPICALLY, a resistor in the 400 ohm range is usually safe for most SINGLE LEDs run on a 14v vehicle system. However, you should always double check your OHMS LAW math, and the LED specs when you buy them.
My amber LEDs operate at 1.8-2.2 volts and a MAX current of 30mA (milliamps). With a center voltage of 2v, I could string 7 of these LEDs together to "use up" the 14v and not need ANY resistors.
If I only used ONE LED tho, I need to protect it. ONE LED uses about 2v of power. This means I still have 12v to "get rid of". We "get rid" of excess voltage with resistors. Ohms law says Voltage = Resistance x Current. To solve a missing resistance, use Resistance = Voltage divided by Current. The resistor is the unknown value, but we DO KNOW that we have 12v to "get rid of", and we dont want any more than 30mA of power going thru the entire circuit. (the LED cant handle more than 30mA from its specs)
R= 400 ohms
Lets say you have 5 LEDs. LED spec is 2v and 30mA again. 5 LEDs "use up" 10v. That leaves 4v to "get rid of" with a resistor.
R= 133 ohms.
Always use the NEXT HIGHER RESISTOR if you cant find what your math says you need. Also, always do the math in FULL VALUES. even tho its 30milliamps, do the math in AMPS, which is .030.
Dont worry about the next paragraph. Its for anyone that wants to nitpick definitons:
We arent actually changing the voltage of the circuit, we are LIMITING CURRENT. The entire circuit is still a 14v circuit, we are just using the resistors to prevent more than the 30mA of power from destroying your LEDs. The above math is still correct, even tho Im not technically correct in my usage of the terms. Just making it easier to read for the non-electrical out there.