View Full Version : LED Resistor Calculator

02-02-2006, 10:37 AM
I was thinking about some LED mods and came across this site (http://www.metku.net/index.html?sect=view&n=1&path=mods/ledcalc/index_eng) that calculates the resistor size you need for whatever LED setup you want to run.

Very cool.

02-18-2006, 10:08 AM
I picked up some LEDs to put 5 LEDs in parallel for the front panel of the case. Based on their calculations I needed to include a 100ohm resistor if I use a 12v source (actually only needs 87ohms but the 100ohm resistor is the closest). Long story short (unlike me :D) the lights work without the resistor. So now I am wondering what's the point of the resistor?

Now after a bit of thinking, I believe the resistor is there to drop the voltage to allow the voltage across the LEDs to be within thier operating range. Without the resistor the LEDs are running at higher than spec voltage and would shorten thier life.

Anyone wire up LEDs before and can shed some light on this?


02-18-2006, 11:42 AM
I haven't wired up LED's before, but I do have electronics background in the US Navy. The resistor is used in that case just as you have thought, to drop the voltage applied to the LED's.
Also, you may find it easier to wire the LED's in series.

02-18-2006, 12:39 PM
Thanks huncherman. There is a reason why I am using parallel rather than series. I it couldn't as many LED's as I wanted in one chain (5-5mm). Parallel allow more resistors in a single chain.

Another possible reason (no sure if true though) is if an LED conks out the others in the chain might as well whereas in parallel if one fails the others should still run (assuming it doesn't exceed the operating voltage).

BTW - I was trying the LED's on the 5v rail and not on 12v :bonk: and when I tried one LED on its own on the 12v line it went poof! Oops - I got 10 to play with and really only need 5.