V_S      = Supply Voltage

V_L      = LED Voltage (usually 2V, but sometimes, might be 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 which is greater than your calculated value, 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 (or, to increase battery life for example) but this will make the LED less bright.

For example:

Suppose the Supply Voltage (V_S) = 9V, and you have a Red LED (V_L = 2V), requiring a Current (I) = 20mA = 0.020A, then, the Resistor will be:

R = (9V - 2V) / 0.02A = 350 Ohms,

So, choose 390 Ohms (the nearest standard value which is greater).

TIP:

If you want to work out the LED Resistor formula using Ohm's law, the Ohm's law says that the resistance of the Resistor will be:

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

Really, the result will be the same that we had before. For more information on the calculations, please, see the Ohm's Law over any website.

In the next topic, we have a table with recommended Resistor value for different Power Supplies and colors of LEDs and an automatic “LED-Calculator” to get an accurate LED Resistor value.

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