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A light emitting diode, or LED, will light once current is applied to the part. A resistor and LED in series can be considered a simple circuit. Why use a resistor with an LED? A resistor can be used for current limitation. This resistor is called the ballast resistor. If the supplied voltage is equal to the LED voltage, a resistor will not be needed.
The following link will show examples of circuits with resistors and LEDs - Resistor for LED.
If you are exceeding the LED voltage, you will need a resistor to resist the additional voltage. When calculating the resistor value, you will need to know the voltage drop across the LED first. An example would be a 12V power supply for a 2V LED. With the voltage drop of 2V, you can determine the additional voltage is 10V above the LED value. If the current in the circuit is 200mA, we can now use Ohm’s Law to find the resistor value. With the additional 10V in the circuit and 200mA, the final result will be a 50 ohm resistor. Please note that the power rating for the resistor will take into effect. Power can be calculated as V^2/R, I^2xR, or VxR. In this case, the absolute minimum power rating for this resistor can be 2W, but should be doubled for reliability at 4W.
The following explains a little more on Ohm’s Law:
Please see this post on Ohm’s law calculations for further information:
One of the things that we need to be aware of is that the intensity of the light emitted eventually depends upon the current flowing n the LED. If an application is sensitive to the minor change in the intensity ( for example photo diodes), pay special attention to the temperature related variation the current with the applied voltage.
When it comes to temperature variation, LEDs also have a large change in luminous intensity and a significant change in forward voltage drop.
Using a current source for driving an LED eliminates the variation you mentioned and compensates for the forward voltage change.
For the luminous intensity change other methods are needed to compensate. It can be done crudely using a temperature sensor, controller and an adjustable current source.
When an LED is used with a photodiode as a sensing system, the photodiode’s variation with temperature also needs compensation. A great way to do that is to use two photodiodes, one that measures the steady state light output from the LED, and the other that measures the changed intensity change caused by the sensing system. By controlling the LED current based on the steady state photodiode, the circuit will compensate for both the LED and photodiodes temperature variations. As a bonus it will also compensate for the significant luminous intensity drop with age of the LED.
Here is the link to the LED Resistor Calculator within the Conversion Calculators Click here
If you want to see the full list of calculators available from the Digi-Key home page Click here
