Tutorial - FJK's "Down and Dirty" guide to Ohm's Law

Ohm's Law - the down and dirty guide to "Doing the Math" yourself.

Ohm's Law comes in two parts, meaning there are two values you need to calculate. The "Ohms" value and the "Watts" value. The formulas for both are as follows:

R <Ohms> = (VfBattery - (VfLED1 + VfLED2)) / I

Where:
Vf Battery = Battery Voltage (usually either 3.7V or 7.4V)
Vf LED1 = Forward Voltage of LED #1
Vf LED2 = Forward Voltage of LED #2 (If you have a second in series)
I = (in Amps) Current you wish to run your LED at (Rebel Reds and Red-Oranges are usually 700 mA, Cree Reds, Red-Oranges, and ALL Blues, Greens and Whites are usually 1A)

The second part of the formula is as follows:

P <Watts> = R * I^2

Where:
R = the resistance value from the first calculation
I = (in Amps) Current you wish to run your LED at (Rebel Reds and Red-Oranges are usually 700 mA, Cree Reds, Red-Oranges, and ALL Blues, Greens and Whites are usually 1A) - This value needs to be squared in your calculation.

In the following example, I calculate a resistor value for a single Rebel Red LED
(Vf= 2.4V) running at 700mA running it off of a 3.7V battery.

R = (3.7 - 2.4) / .7 ==> 1.3/.7 ==> 1.86 ----> Rounded up to 2 Ohms

P = R * I^2 ==> 1.86 * (.7)^2 ==> .91W ---> Rounded up to 1W

Since there are only so many values that are commonly produced, it will be very likely that your will need to round UP to the next resistor size, to get the resistor that you would need (and as I did in the above example).

So, I will need a 2 Ohm, 1W resistor.