I am makin a custom computer desk, and using rgb leds to have custom lighting under it... they will all be the same color, so i only need 3 pins... i need the leds to be able to vary brightess... i an going to use about 300 leds an 20ma each, so thats like 3 amps... thatever i use i would liek to be rated for 5 amps... what i need is something like a transistor, to dim and brighten the leds according to the arduino's commands... what do i use
i an going to use about 300 leds an 20ma each, so thats like 3 amps
How ever hard I try, I can't get that arithmetic to work. (or the grammar, for that matter)
My computer desk is cluttered underneath, advertising the fact by lighting it doesn't exactly fit in my ethos. 
Yep the arithmetic doesn't exactly work for me either. You're going to need big ugly trannies to switch that amount of current. I think MOSFETs are the way to go. I'd suggest the IRF840 (8 amps) although I'll admit its far from my forte.
sorry, its 6 amps duh... anyways, a large transistor will work, right?
i am thinking it might draw a little too much current, so i might just use a small 2n2222, and that can power a larger one... im not sure...
i an going to use about 300 leds an 20ma each, so thats like 3 amps... thatever i use i would liek to be rated for 5 amps...
And don't forget the 300 current limiting resistors.
If you are going to have all the LEDs turn on or off or dim at the same time, you can cut your current requirement by using a higher voltage and wiring the LEDs in strings. If you wire 5 leds in a string (with one properly sized current limiting resistor) then you would need 60 such strings wired in parallel and you would draw a total of 1.2 amps. You will need a switching transistor to control all the strings, and a logic level N-channel MOSFET transistor is the simplest method.
Lots of different ways to wire up 300 leds if you understand ohm's law and why you need current limiting to prevent damage.
Lefty
And don't forget the 300 current limiting resistors.
i didnt... ima do them all in parallel because the leds come with free resistors... and its actually 900, because rgb... i dont really understand the way you are talking about...
You may want to check this out before you commit to any designs: