Many circuits operate LEDs at less than the recommended maximum current, to save power, to permit the use of a standard resistor value, or to reduce brightness. Typically, the forward voltage of an LED is between 1.8 and 3.3 volts. It varies by the color of the LED.
Herein, what resistor do I need with an LED?
Choosing a resistor to work with an LED is fairly simple, but does require some knowledge of the LED and a small amount of maths. Some LEDs such as colour changing LEDs, flashing LEDs and 5V LEDs are designed to run off a 5V supply and therefore don't need a resistor.
Which side of the LED is positive?
Diode and LED Polarity. Diodes only allow current to flow in one direction, and they're always polarized. A diode has two terminals. The positive side is called the anode, and the negative one is called the cathode.
Why is it necessary to use a resistor in series with an LED?
The ballast resistor is used to limit the current through the LED and to prevent that it burns. If the voltage source is equal to the voltage drop of the LED, no resistor is required. The resistance of the ballast resistor is easy to calculate with Ohm's law and Kirchhoff's circuit laws.