Need help puttin in led's!

[skreetracer87]

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?

 

[Mack]

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.

 

[95GSXBLUR]

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.

 

[skreetracer87]

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. . just someone whose done this before and what they used. thanks

 

[psychlow]

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).

Equation:

R = (B * L/V) - L

Where:
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.