Calculating resistor values for LEDS
In order to work out the required value of a resistor, it’s necessary to know the LED’S specifications. With these figures at hand, along with the rating of the power source, we can calculate the desired resistance using Ohm’s Law. All LEDS have a maximum current limit and a maximum voltage. As an example, let’s take an LED with a maximum voltage of 4V and maximum current of 20ma (0.020 amps). We’ll be connecting the LED into a circuit powered by a 12V supply. According to Ohm’s Law, the resistance is calculated thus: OHM’S LAW dictates that: R =( VS – VL) divided by I
Where… R = resistance VS = Supply voltage (12V) VL = LED max. voltage (4V) I = LED max. current in amps (0.020A) In our case: R = 12 – 4 = 8. We then divide 8 by 0.020 to get 400. The total of 400 is the amount of resistance required, expressed in ohms. As resistors come in various standard values, we have to choose the nearest value (always round it upwards), which is 430 ohms.