noob led question

[Brad]

i don't know if this is the right place to as this, but i have been wanting to learn how to wire an led. i tried once connecting it to a molex connector in my pc with a couple wires, but it only blinked once then wouldn't go again. i have a feeling i fucked up lol. anyways any help appreciated. links to a tutorial or something would be nice.

[NFG]

LEDs are simple.  Use the proper voltage, and watch the polarity.  That's it.  You need a resistor to drop the voltage to the amount your LED requires (and they're mostly 3.3V but not always) and for that you can use Ohm's Law:  V=IR

V = Voltage drop, how many volts you want to cut from the supply
I = the current draw of your LED, in Amps
R = the resistance value of your resistor.

So:

Say you have a 3.3V LED that draws 3 milliAmps, and you're using a five Volt source of power.  That's .003 Amps, at 3.3 Volts, and you need to lose 1.7 Volts.

R = V/I

V / I = 1.7 / .03 = 56.66...

So you need a resistor somewhere in the neighborhood of 56 Ohms.

And on your LED the long leg is Positive, the short is negative.  If you're removing an LED you'll prolly find the legs are the same length 'cause they've been cut.  Use your multimeter to determine which end of its location is GROUND, the other is power.  Or look at the LED - there's two metal bits in there, a kind of diving board over top of a cup.  Compare it to the LED with existing long legs, and you'll know which way the old one goes.

[ORTA]

Hey Lawrence that's one hell of an info. I've been looking for my old notes about the P-I-R-E formula wheel but sadly I didn't find them, can you post it here? because I don't remember it anymore    

[NFG]

I'm not realy sure what a P-I-R-E formula wheel is.  V=IR is something I was taught when I was asking about LEDs, it's handy info that everyone should know.

For the record I had forgotten parts of it and I used google.  Google works kids, use it!

[phreak97]

most high intensity leds work at 20-25mA, but theyll draw more if you give them the chance, that's why the resistor is there.. some leds tend to be suicidal:P
for a 20-25mA 3.5v led running off a 5v source (the red wire in a pc molex connector) use an 85 ohm resistor.

[vileitm!]

On the other hand, the Vf of a standard indicator LED (red,green,orange,yellow) is about 2V at a current of 10mA. LEDs are current controlled devices, you should know what current a given LED likes to run at before using it. If Vf is unknown: assume 2V, select resistor accordingly, measure the Vf (with a multimeter) and adjust the resistor to suit.

[Bostich]

Orta,

Here you go: http://www.the12volt.com/ohm/ohmslaw.asp

[ORTA]

Hey bostich thanks a lot!!