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Hope this helps some of you wanting to learn more about wiring leds. its different than bulbs, they must be polarized and they must have resistors in most cases. the specs for each color led are different, and you need to know these values to calculate the proper resistor. it will also vary depending on how many leds you connect together. they can be wired in series, however there is a limit to this, usually only 3-4 leds is possible. so if you want more, you need to make many series and then hook each one up in parallel. this is so they retain brightness. use this link to calculate the resistor and then goto radioshack and buy it. if they dont have the exact one you need, always choose the next highest resistor. Also remember you can add resistors together! if you need 300ohm resistance and they dont have it, you can buy a 200ohm and 100ohm and hook them together.

LED Series Resistor Calculator

 
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I bought two 3.7V 20mA blue LEDs from radioshack. The calculator states I need a 360 Ohms resistor but then I wire it up to a 12V it is less brighter than a 100 Ohms resistor. Does (14.4V - 12V= 2.4V) make that much of a difference?

 
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yes it does. hook it to the car battery with the car running and it will be noticably brighter. when i hooked up an extra led to push total required voltage to 18v, they hardly lit up at all.

 
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do i really need a 360Ohms resistor for a 3.7V 20mA LED coming from a parking light? It just seems to me like an overkill... haha

 
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Using the proper resistor value for LEDs is important to get the voltage in the right range. Some LEDs can handle slightly higher voltage and will light up more brightly but you run the risk of the LED burning out prematurely.

Personally, I like metal film ressistors over the carbon film as they have a closer tolerance and better heat disapation. If you have trouble finding ressistores, visit mcminone.com. They carry a huge assortment. Shipping cost are a dealbreaker unles you order several ressistors at once. I think it cost me the same to ship 1 ressistor as it did for 30 of them.

 
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um, thats a resistor you would need if your power source is only 11V! any led you plan on using with the motor on and alternator making power should be resistored for at least 14.4V minimum. that would mean you need a 510 ohm. if it were me, i would resistor for 15v because alot of times the voltage climbs to 15-16 in winter. for 15v power, you would use a 560 ohm.

 
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How did you get 560 ohms Steve? I used the calc and it gave me 390 ohms.

2 LEDs, 3.7V, 20mA, 15 total V

this is so hard to understand!! ahhhhhhh

 
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in your first post you said A led not TWO lol, and went on to say for a single parking light, which i assume is only one bulb. thats different so the 390 would be correct for two leds wired in series

 
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this thread is a trip for me. hahah thanks!

 
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OHHH pictures!

 
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Nice post man ..... inform the unknown