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Need help puttin in led's!

Posted by skreetracer87, Apr 8, 2005

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  1. skreetracer87

    skreetracer87 Proven Member

    Joined Feb 16, 2005
    Wingate, North Carolina
    Hey im in the middle of instalin my led's and i took my gauge cluster out and soldered one in to try it and it doesnt work. i used a 330 ohm resistor if that tells you anything. what did yall do? oh how do you get the hvac unit out to change the lights out and all? :talon:
  2. Mack

    Mack Proven Member

    Joined May 7, 2003
    Cleveland, Ohio
    LED's are polarity sensitive. If they are surface mount that you are using, they're should be some sort of dot or line indicating the positive side. If it is through-hole, one leg should be longer, and that is the positive.
    Try switching the LED's polarity first off.
  3. 95GSXBLUR

    95GSXBLUR Proven Member

    Joined Sep 28, 2002
    Indanapolis, Indiana
    Light Emitting Diode

    Think of diodes as a one way check valve.. they only want electricity to go in one way and out the other.

    541  0

    1995 Mitsubishi Eclipse GSX
    12.083 @ 126.720 · 2G DSM
  4. skreetracer87

    skreetracer87 Proven Member

    Joined Feb 16, 2005
    Wingate, North Carolina
    I know their have polarity but i wasnt sure about the resistors. maybe i had it backwards on the gauge backing. idk. but im still not sure about the resistors. i know all about led's so i dont need a lesson. LOL. just someone whose done this before and what they used. thanks
  5. psychlow

    psychlow Proven Member

    Joined Jun 30, 2003
    Colorado Springs, Colorado
    Posting this from anotehr thread - it's my original post. If you need help with this, find out the resistance across the diode and I'll help you run this equation.
    I just found this thread. I noticed a lot of you guys have been guessing on what value of resistor to use so you don't burn out the LEDs. There's a simpler way to figure it out if you do a little math and you have a multimeter.

    I'm assuming the circuit is 14.4V (with the alternator running) which is what most cars run - if not, change the value of 14.4 in the following equation (I haven't tested doing it myself just yet - measure with a multimeter).


    R = (B * L/V) - L

    B is the EMF (in volts) across the two leads in the car you're starting with
    R is the resistance (in Ohms) of your resistor.
    L is the resistance (in Ohms) of your LED.
    V is the required EMF (in volts) of your LED.
    EMF = electromagnetic force (voltage).

    Example: I found some red LED's with a 3000mcd rating that I'll probably use from the Shack that run 1.7-2.4V max. They are listed at 20mA max current, so I figure they are about 85 Ohms, but I'd check the resistance of the LEDs with a multimeter and run the equation before I bought the resistors.

    So, for the resistor value, I want:

    R = ((14.4 * 85) / 1.7) - 85
    R = 635 Ohms maximum.
    R = ((14.4 * 85) / 2.4) - 85
    R = 425 Ohms minimum.

    If this doesn't make sense, let me know, I'll elaborate more. If you ever do any electrical work and don't have a multimeter, GET ONE.

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