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LED work light-Calculating an LED resistor value

Time: 2009-06-24

        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 = (VS - VL) / I

         VS = supply voltage
         VL = 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 VS = 9V, and you have a red LED (VL = 2V), requiring a current I = 20mA = 0.020A,
R = (9V - 2V) / 0.02A = 350ohm, so choose 390ohm (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 (= VS - VL in this case)
  I = the current through the resistor

So   R = (VS - VL) / I

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