wiring LED’s, What went wrong?

Printed From: the12volt.com
Forum Name: Lights, Neon, LEDs, HIDs
Forum Discription: Under Car Lighting, Strobe Lights, Fog Lights, Headlights, HIDs, DRL, Tail Lights, Brake Lights, Dashboard Lights, WigWag, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=81477
Printed Date: May 13, 2025 at 7:46 PM

Topic: wiring LED’s, What went wrong?

Posted By: vinny_d
Subject: wiring LED’s, What went wrong?
Date Posted: August 11, 2006 at 9:48 AM

It was my first time wiring LED's and it ended up being a disaster (kinda figured since i had no clue what i was doing)

heres the scoop

i wired in 4x 3.2v-3.6v 50,000mcd blue LED's 20mA FV, i got the power lead from the fuse box, ran it to the first LED were i jumped from pos to neg to pos to neg kinda thing for all 4 LED's. then at the end i added a 512ohm resistor (think thats my prob..) and then ran it to a switch that ran it to ground. all using 22ish guage wire (think thats could be my prob also). anywho i powered up the car and it worked for 2 seconds when the first LED stopped working and i was concentrating on why it wont work so i gave it a little push thinking the leads were not connected. when i pushed the led all leds shut off, then i looked to my right and saw smoke bellowing fromt the switch where i paniced and unhooked the ground from the battery.

well thats my story.. if anyone can tell me what they think i've done wrong be great i didnt know if the 512ohm resistor was gonna be big or small enough or w/e all i had so i tried. and the 22ish guage wire prob is the problem but if i can get any susggestions that would be just super!

Replies:

Posted By: Big Dog
Date Posted: August 15, 2006 at 11:13 AM

We know the forward voltage Vf is 3.4V, and we wish to pass 20mA (0.020Amps) through the LED.

Using ohms law R=(Batt voltage less LED operating voltage) / current in amps

R = (12 - 3.4) / 0.020

R = 430 ohm resistor for use with one LED.

What about the other three LEDs?

As we have plenty of volts to drop across the resistor we could add another LED in the circuit which would make the circuit more efficient. At the moment, with one LED only, the resistor will get warm depending on power dissapation of that resistor, the reason for heat is that you are converting energy to heat which is wasted power. By using two LED's there are less volts to loose across the resistor, it will still be the same current as passing throught one LED but less heat is being generated.

R = (12 - 3.4 - 3.4) / 0.020

R = 260 ohm resistor for two LEDs in series

posted_image

Repeat the above for the other pair of LEDs.

Most of the time the rating of the resistor doesn't matter as they are usually half watt which is plenty.

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Prepare your future. It wasn't the lack of stones that killed the stone age.

Posted By: bdl666
Date Posted: August 15, 2006 at 12:45 PM

You can use this program to calculate the resistor needed.
And remember that when the engine is running the voltage will go up to 14.4v.

The led's can take a little more than 20mA but don't go over 30mA.

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ieSpell rocks.