© John Hewes 2010, The Electronics Club

An LED must have a resistor connected in series to limit the current through the LED, otherwise it will burn out almost instantly.

The resistor value, R is given by:

R = (V_{S} – V_{L}) / I |

**V _{S}** = supply voltage

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

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

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

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

To convert 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

If the supply voltage V_{S} = 9V, and you have a red LED (V_{L} = 2V), requiring a current I = 20mA = 0.020A,

R = (9V – 2V) / 0.02A = 350, so choose 390 (the nearest standard value which is greater).

#### Working out the LED resistor formula using Ohm’s law

Ohm’s law says that the resistance of the resistor, R = V/I, where:

V = voltage across the resistor (= V_{S} – V_{L} in this case)

I = the current through the resistor

So R = (V_{S} – V_{L}) / I

For more information on the calculations please see the Ohm’s Law page.