LEDs are diodes with very specific characteristics. The most important values that must be followed are the (maximum) forward voltage [Vf] and the (maximum) forward current [If]. Typically, an LED powered with a voltage equal to Vf, will allow a current equal to If to pass through. The forward voltage of LEDs depends on the material they are made of (thus the color that they emit) and the number of diodes in series they have. For example, a red LED typically operates at 20mA when a voltage of 2.2 volts is applied across its leads.
So, what happens if we have 5 volts to power this 2.2V LED? There are many different LED drivers, but the simplest one is to use a resistor in series. Here is the typical circuit:
The idea of the resistor is to generate a voltage drop across its leads. Byselecting this resistor properly, the voltage across the LED will be as much as required. To select the proper resistor value, we need first to know how much voltage we need to drop on this resistor. Suppose that the power supply has a voltage of 5 volts (Vdd) and the red LED needs 2.2 volts to operate at 20mA. The voltage across the resistor must then be:
VRES = VDD - VLED => VRES = 5 - 2.2 => VRES = 2.8 Volts
Now we can simply use the Ohm's law to calculate the resistor value. The idea is that we want a specific voltage drop (2.8 volts) at a specific current (20mA) (remember to convert the current to Amperes):
R = VRES / If => R = 2.8 / 0.02 => R = 140 Ohms
So, the resistor will be 140 Ohms! When power is provided to the above circuit, the current will climb up to 20mA. At that point, the voltage across the resistor will be 2.8 volts, so the remaining voltage (2.2 Volts) will be applied to the LED.
One more important value that has to be calculated is the power dissipation on the resistor. As you know, resistors come n different rated power values, for example 1/4 watts (250 mWatt), 1/2 watt (500 mWatt), 1 Watt etc. This value indicates how much power can be dissipated on the resistor in the form of heat. We therefore need to know this value for our circuit in order to select a properly rated resistor. The formula to calculate the power is this:
PRES = If2 x R => PRES = 0.022 x 140 => PRES = 0.056 Watts => PRES = 56 mWatts
Therefore, a typical 1/4 or even an 1/8 watts resistor is sufficient to dissipate the 56 mWatts of power.
To make our lives easier...
You may wanna now go to the next page of this article to download the PDF with the LED resistor selector dial and learn how to use it...