News*Home Page > Industry news > LED work light-Calculating an LED resistor value*

An **LED work light** 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_{L} = LED voltage (usually 2V, but 4V for blue and white LEDs)

I = LED current (e.g. 20mA), this must be less than the maximum permitted

If the calculated value is not available choose the nearest standard resistor value of **Led trouble lights** 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