im redoing my dome light LEDs but with 40,000mcd white LEDs (im using 21 LEDs if thats important), what ohm resistor should i use and do you guys know what the formula is to figuring it out so i dont bother any of you gus again in the future

thanks alot again



-Tecao

 

Are you running series or parallel?
The formula is V=IR. Voltage equals current times resistance.
This may be a bit confusing but here goes. First you need to know your LED voltage and current rating. subtract your voltage rating from 12 (or whatever the source DC voltage is). Then plug that voltage into your formula, along with the current rating (NOTE: always place MORE resistance int he circuit to lessen the current)

Example:
Voltage rating:3.6 V
Current rating: 30 mA

12-3.6=8.4V
V=IR -> V/I=R -> 8.4V/0.030A = 280 ohms
I would go with a slightly higher resistor so i wouldnt burn out the LED.

That was all for parallel circuits.

In series you would need to limit current, so 12V/0.030A=400ohms

You almost need to do this in parallel so you dont have 42 components, that would just be a pain in the ***

 

this is how i have the leds.... 40,000mcd 5mm.... i also shaved them to give it a more spread out light

 

SPECIFICATIONS:

Source Material:InGaN !
Emitting Colour:White
LENS Type:Water clear
Color Temperature: 8000K
Luminous Intensity-MCD: Min: 30000mcd Max: 40000 mcd
Reverse Voltage:5.0 V
DC Forward Voltage: Typical: 3.4 V Max: 3.8V
DC Forward Current:25mA
Viewing Angle:20±10 degree
Lead Soldering Temp:260oC for 5 seconds
Intensely Bright


so the formula would be
12-3.4v = 8.6
8.6v/.025mA = 344 ohm??

 

sounds good. so go with the common resistor, which i think is 380ohms

in hindsight, you technically dont need a resistor if you hook up enough LEDs to cover the voltage and current. With 21 im not sure. I am a mechanical engineer so electronics are only a sidenote. see if you can get an electrical guy to double check this stuff, there may be something beyond my scope when you get into this scale. If you send me a diagram of your system i will run it by an electrical guy at work to double check. I need to know how you are connecting all this.

 

..and dont forget to update with some pics with it installed!

 

well its not really for me i already did my dome light but with 20,000mcd the same way metal maxima did his now my brother saw it and was like do mine and he bought 40,000mcd so if this is brighter which im assuming will be im going to redo the max......im doing it in his s-10 ill post pics............. are you sure if you do enough leds you dont need a resistor?? i never heard of that i thought you always needed one............hmmmmmm

 

if you can drop all of the voltage then you can get away without a resistor, im not sure if you can do this in parallel but you should be able to do it in series-parallel

The luminisity doesnt mean anything for the circuit, the electrons dont give a **** if your car is brighter. What you need to compare is the voltage and amperage ratings. Often they are the same. There is a strong dependance on the material used which determines brightness and color.