Electronics question about LED's

i tried to put LED's in my case the other day, and it didnt work, i'm pretty sure its the resistor im using thats f*king it up, so , a) what resistor should i use if i have 2 4V led's in - parallel, - series, and what resistor should i use if i have 3 4V led's in parallel/series ^^

 

We need more info bro... like the voltage rating of the LEDs you're using and the supply voltage, then we can give you a resistor size to use

 

There's no such thing as a 4V LED...

 

You can buy max forward voltage LED's, which come in 4-4.5 and 5v settings... I know this because I have 2 4v ones and 2 4.5 volt ones. If you check out the guide in this forum then it should give you the answers you are after. For that you will need to know the supply voltage, the max forward voltage per resistor and the max amps also.

 

Most high intensity White, Blue or Green LED's have an absolute maximum Vf of 4.0V, for UV LED's it is 4.2V.

 

Oh ****, lmao, I didn't read that correctly...

but yeah, we need to know the correct voltage of the LEDs and the supply voltage you are using

 

And once you've got the LED information, you can go to Linear's LED Wizard to get all the resistor & circuit info you asked about!

You'll need source voltage (from your power supply), diode forward voltage, and diode forward current, as well as # of LEDs in your circuit. The Wizard will provide you with a bunch of suggestions, w/ resistor information.

If you don't have exact LED info, I've used this site's info on my LEDs - which I didn't buy from them - without problems so far.

It's also OK, I think, to fudge the resistor values a bit. One LED string I built needed a 150ohm resistor; I stuck a 180ohm in because I didn't have any 150s, and everything lights up just fine. No smoking ruins yet, anyway!