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I need help with LED's for a custom tail light project. Answered

One ring of 20 LED's in paralell, and another ring of about 30 in parralell per tail light assy (x4).
Now Its just been a long time since I went to ITT so bear with me.
LED's 2.0 v x6 with a 12vdc power supply will not need any resistors since the voltage is being equally divided correct?
Im trying to wire these to run as tail/stop lights. Ill be using a PWM for the tail lights. But for right now I need to get this math done correctly.
If I keep thing divisible by 12volts (6 LED's) will that solve this?
So I can have 18 lights in the small ring, and 24 in the large one?

Any help here would be great!
Again this is running off of a car electrical system 12vdc.


4 Replies

ydeardorff (author)2010-05-21

Thank you for all your help!
I found this understated webpage that helped me immensely. ledcalc.com
Using, 2vdc LED's at 100mA each I have two arrays per tail light assembly.
The first array is 24 LED's, the second is 18. At a 2 vdc drop per LED I run 6 of them in series with a 22ohm 1.4 watt resistor. So for the larger array thats 4 x 6 = 24, so 4 blocks of 6, and 3 x 6 is 18, so on the smaller array I use 3 blocks of 6.
And your also correct that the voltage supplied via a cars alternator can be as high as 14volts. So that was used as the base voltage to figure out the resistor value.

Now, Im already past proof of concept stage and into final design modifications for these lights. I used a jeweled plastic coaster from the nugget casino over the smaller array to act as a diffusor giving it the look of the GTR tail lights, but with an outer ring of led's to grab the attention of the guy behind you.

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trick8421 (author)2010-04-05

I presume these are red leds. The forward voltage could be anywhere between 2.00 ~2.4 volts. Generally they will be close to each other if they are the same brand and same batch. If you run them parallel and you happen to have a string with a low forward voltage they will tend to drag the current and they may overheat and burn out.. It is safer to have a series resistor in each serial string. Or better yet have a constant current circuit so that even when the engine is reving or idling the output will be constant..

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steveastrouk (author)2010-03-31

I would seriously avoid paralleling LEDs like that. You should always run them in strings with a current limiting resistor. You'll benefit by achieving maximum brightness at all temperatures too.


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jeff-o (author)2010-03-31

Yes, if the voltage stays a nice, even 12V then you shouldn't need a resistor.  But remember, the voltage out of a car battery can be up to 14V, so it might be a good idea to drop just a small resistor in there, or perhaps make each series chain just 5 LEDs plus a resistor.

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