LED lighting basics

***need*** completion

>> describe how led works, also how dependent on current not voltage

"Quick and dirty" resistor calculation

Ignore the voltage drop of the LED, and just calculate the current for your circuit. So if you have 12 volts and you want 20 milliamps through your LED,  then just use Ohms law to calculate.

Ohms law is V = I * R (Voltage equals Current in amps (I) times Resistance in ohms)

You can express Ohms law solving for Resistance:  R = V / I

So R = 12 volts divided by  0.02  (20 milliamps is 20 thousandths of an amp)

R =  600 ohms.

This is a fail-safe calculation, and you do not have to know the voltage drop of the LED. You can't use this where the "input" voltage is very low, or you are using a number of LEDs in series. 

Don't ignore wattage!

Many people do not even consider the wattage needed to handle the heat dissipated by the resistor.

Wattage is  Volts times Amps, or Amps squared times Resistance.

Easy way to use the current (because that is your real target) and the resistance (because that is your result)

So, in the example above, a 600 ohm resistor and 0.02 amps:

W = 0.02 * 0.02 * 600 or 0.24 watts. That's about 1/4 watt.

Now wait! do not go out and buy 1/4 watt resistors!!! Running a resistor at maximum wattage makes enough heat to burn you and melt plastic!

So always try to DOUBLE the resistor wattage. In this case, get a 1/23 watt resistor! Trust me!

More accurate way to calculate your resistor:

There is actually some voltage "used" by your LED, so to be more exact, you subtract that voltage (or voltages if LEDs in series) from the "input" voltage. So suppose your LED has a forward voltage drop of 1.7 volts for red, on up to about 2 volts for high brightness, to about 3 for some white ones, to even higher for certain white or blue LEDs.

So, let's say that your LEDs manufacturer specifies 2 volts as the "average forward voltage drop". So your resistor does not need to "control" all 12 volts, because the LED uses 2 of them.. so calculate using Ohms law again, but subtract the voltage drop of the LED from the input voltage.

R = (12 - 2) / 0.02 =  500 ohms...

Suppose you have 2 LEDs in series... then you just add all the voltage drops in:

R = ( 12 - 2 - 2) / 0.02 =  400 ohms...

Led Calculators

Calculate for a single LED: http://led.linear1.org/1led.wiz

Calculate for several LEDs in series: http://led.linear1.org/led.wiz

Cool device to avoid calculating resistors:

The CL2 is a small current regulator in a package the size of a transistor. It always limits the current to 20 milliamps, at input voltages up to 90 volts! This is ideal for many applications, and obviously will work on varying input voltages, which a resistor will not do. 

Click here for the CL2 datasheet