The important values needed of a resistor can be calculated by using these equations. To find the resistance value using Ohm’s Law again, subtract the total LED forward voltage from the chosen power supply voltage. This example has 6 LEDs in series so a user would need to multiply the LED’s nominal forward voltage of 3V by 6 to get the total forward voltage of 18V. Subtracting 18V from the 24V supply leaves 6V to be dropped across the ballast resistor. Next, divide that 6V by the LED forward current of 350mA to give the needed resistor value of 17.1 Ohms. Using the next equation shown, a user can find the needed power rating of the resistor by multiplying the 6V with the 350mA forward current. This gives a power rating of 2.1W.