Lee said:
I am looking fot the math formula to figure out the voltage drop i need
and the associated resistor. such as the led uses 3 volts i have 12 i
need a resistor that will drop 9 volts. also how to figure out the size
resistor (watts).
thanks
Lee
First, determine the amount of current you wish to have flow through
the LED. You will need to get this information from the datasheet.
Once you have this piece of information, you can apply ohms law to
determine the needed value of resistor. Take the supply voltage minus
the voltage drop of the LED at the desired current and divide by the
desired current. This will give you the needed resitance. Typically,
I use 10ma for a general LED and using that value as an example with
your numbers: 12V - 3V = 9V / 10ma = 900 ohms. You can then use this
figure to pick the a standard resistor that is near this value. Once
you have the resistor picked, double check what current values this
will produce and verify that it is within acceptible range.