# Calculating LEDs resistor value

LEDs must have a resistor connected in series to limit the current flowing through them, otherwise they will burn out almost instantly.

The resistor value, **R** is given by:

R = (V_{S} - V_{L}) / I |

** V** = the supply voltage

_{S}

**V**= the LED voltage (usually 2V, but may also be 4V for blue and white LEDs)

_{L}**I**= the LED maximumcurrent (e.g. 10mA = 0.01A, or 20mA = 0.02A)

Make sure the LED current you choose is less than the maximum permitted and then

**convert**

the current to amps (A)so the calculation will give the resistor value in ohms. To convert

the current to amps (A)

mA to A divide the current in mA by 1000 because 1mA = 0.001A.

If the calculated value is not available choose the nearest standard resistor value which is

**greater**, so that the current will be a little less than you chose. In fact you may wish to choose

a greater resistor value to reduce the current (to increase battery life for example) but this will

make the LED less bright.

#### For example

Assuming the supply voltage is **V _{S} = 9V** , and you have a red LED (

**V**), requiring a current

_{L}= 2V**I = 20mA = 0.020A**,

**R**

**= (9V - 2V) / 0.02A = 350**, so choose a 390 resistor (the nearest

standard value which is greater).