LEDs

[Tomlo]

I have TON's of LEDs and have no clue what they are rated to run at, Id like to use some in my dreamcast but not sure which ones wouldnt blow?

[Erghize]

Do you have a voltmetre? If so you can use it to check them.

I think, if they're 5mm LED's they'll be either 3.3V or 5V (probably 5V). I remember being told also it's wise to put a resistor on the anode just to be extra sure you don't blow it.

[semicolo]

No, leds have a working voltage, and a limit working current (the led is not garanteed if you go beyond this limit).

You need some simple electronic knowledge to power leds correctly.
Ohm's law is : U = R*I (in a resistant conductor, the voltage across it's pins equals it's resistance multiplied by the current going through it)
See Kirchoff's laws there http://utwired.engr.utexas.edu/rgd1/lesson03.cfm

Say you have a 5v power supply.
You want to power a diode rated at 1,25V and 10mA, we need to use a resistor to limit current (no matter if connected to the anode or the cathode, it'll do the same job).
With Kirchoff's law, we know the resistor will have 3,75V acrosse it's pins.
With Ohm's law : 3,75 = R*0,01
so R= 3,75 / 0,01 => R = 375 ohms.

If you don't know the voltage your diode should be used with you can use a 12V power supply and a potentiometer.
Say you want to have a current of 1mA, you'll need a 12000 ohms potentiometer. Start at 12Kohms and turn the potentiometer until the diode lights and measure the voltage across the diode.

[Tomlo]

I wasnt asking what resistor to slap onto it, I know that much... A potentiometer, never thought of that I could just use a cheap single turn one. I must have a million of them around here like LEDs. I always salvage all I can from broken electronics.