How do I work out the value of resistor I need for a L.E.D?

First of all we need to collect the following information:

• The voltage of the supply

• The forward  voltage of the component eg L.E.D

• The current the component uses

We can then apply Ohms law to calculate the value of resistor needed.

For calculating resistance we use the formula R= V ÷ I

lets take the following as an example:

Blue LED = 3.4V Forward voltage and 30mA maximum current, The supply voltage is 12V DC.

Step 1

We first need to work out the difference in the voltage  between the source and the component this is 12V - 3.4V = 8.6V

Step 2

We convert the current rating from mA into A we simply divide by 1000.

We now have R = 8.6 ÷ 0.03

Therefore R = 286 Ohms

However the nearest carbon film value would be 330 Ohms and the nearest metal film resistor would be 300 Ohms

 You can use the calculator to the right to help you work out values yourself or simply use the function below

This free script

provided by
JavaScript Kit

Remember you should find a resistor that is as close to the value displayed it is safer to go higher but do not be tempted to go below the resistance value quoted above