LEDs, Voltage, and Amps

[SteveJ34]

I was intrigued by the recent discussion about lighting plexi with LEDs, did a couple of searches and found some cool lighted mouse pads, case mods, and other similarly lit projects.

So, I decided to toy with a couple of ideas, picked up a sheet of .22 plexi, some super bright LEDs, and a couple of different resistor sizes based on some voltage readings on the back of the package.

Here lies my question(s).....

If the LEDs I have recite 20mA, 3V, I would assume:

1. If I power with a 3V Lithium Cell I need to add no resisitance.

2. If I power with a 5V DC from power supply for example, what resistor size should I use to step down the voltage?

I'm trying to understand the mathmatical formulas so if I am building a project with 1,2, or more LEDs wired in parallel, what do I need to do so I don't reduce their life and/or burn them out altogether.

I'm sure there are members that are more electrically inclined than I and look forward to your replies.

[Bgnome]

stolen from oscar's site:

Calculating the Resistor Values

The value of the resistors used for limiting current to the LED is determined by the style of LED’s used. In my example, I set up the driver board to operate Red super brights @ 20mA. To determine the resistor value you need, first subtract the LED forward voltage value from the supply voltage value. Then you divide that number by the LED operating current to give you the limiting resistor value in Ohms. This is how mine looks:
Supply voltage = 5V
LED forward voltage = 1.9V
LED current = .020A   
(5V – 1.9V) / .020A = 155 Ohms   
The closest common value I found for the resistor array was 150 Ohms, so that’s what I used. Here are some typical values of different colors of super bright LED’s:   
  S-B Red: 1.7 ~ 1.9V
  S-B Green/Yellow: 2.0 ~ 2.2V
  S-B White: 3.5 ~ 3.7V
  S-B Blue: 3.5 ~ 3.8V   

A possible enhancement to this driver circuit would be to have a couple of banks of different value resistors, and by moving jumpers the board could support different types of LED’s. Hmmm…