LED (means Light Emitting Diode) is a semiconductor, works like a normal diode, but emits light in forward bias mode. It means LED releases energy in the form of photons when current flows through it. The flow of current is blocked when it is connected in reverse bias mode.
The voltage drop across an LED is about 2.0V to 3.0V in forward bias (when the LED glows), depends on size, colour of the light it emits and manufacturer.
The maximum (or safe) current limit, in milliAmps, for an LED depends on its diameter (size) and approximately equal to its diameter in forward bias. eg: 3mm LED = 3mA, 5mm LED = 5mA and 10mm LED = 10mA etc. The brightness of light emitted depends on the current flowing though it.
To limit current flowing in an LED, connected at higher voltage than its forward bias, a resistor is used in series. The resistor value in series should be such that, the voltage across the LED should be approximately equals to voltage required to glow the LED and should not exceed the safe current limit of the LED.
eg: Assume an LED is to be connected across 12VDC in forward bias (for glowing as indicator). The voltage drop across the LED is about 2.7V and current flow through the LED is 3 mA (i.e. 0.003A).
sol: The voltage should be shared by the LED and resistor. since LED requires 2.7V, the remaining voltage should be dropped across the resistor. i.e., 12V – 2.7V = 9.3V.
Using ohm’s law, R = V / I , required resistance, R = 9.3V/0.003A. therefore, Resistance required = 3100 ohms. since, the resistance value is not a standard one, we may select 3300 ohms (3.3Kohms) resistor.
Once, resistance value is set, now the actual current may be calculated using formula I = V / R, i.e., 9.3 / 3300 = 0.0028A (2.8 mA).
The resistance power requirement may be calculated by multiplying Voltage and Current across the resistor. so, 9.3V x 0.0028A =0.026W (26mW or above).
The LED should NOT be connected in reverse bias above its breakdown voltage. The breakdown voltage depends on model and manufacturing company. In general, the breakdown voltage for an LED may be considered as 5V, but, some LEDs may withstand up to 20V in reverse bias (refer datasheets).
The following coloured lighting may be available on various LEDs: Red, Green, Yellow, Blue, Amber, White, Infrared etc. Some LEDs have the same body colour of light called Diffusion LEDs and some LEDs have transperent (glass like) body, but, emits various colours of light.
Some multiple colour light emitting LEDs are concealed in to a single LED and identified with more than two leads connection.
In general Red-Green LED have three leads, one for red (anode) another for green (anode) and third one is common (cathode) to both. They are available in common anode and common cathode models.
Similarly, Red-Green-Blue LED have four leads, three for three colours and forth one is common.
Some LEDs have two leads, but, built-in chip with three colored (RGB) LEDs, which changes their colour automatically in a fixed time period.
More varieties of LEDs are available now-a-days. i.e., Infra-Red rays emitting (IR) LEDs , UltraViolet rays emitting (UV) LEDs etc., which are explained in Sensors section.
Power LED is similar to normal LED in operation, except its high current carrying capacity. Power LEDs emit more and bright lighting. Power LEDs are available in 0.5W, 1W, 2W etc.
To know its current requirement of a power LED, divide the Power by the Voltage drop across the LED. eg: 2W LED requires 2W/3V = 0.667A (667 mA) approximately.
Some power LEDs are available along with heat sink fitted beneath them
A number of LEDs are connected in series with a resistance as a set on a flexible strip, which may connected directly to a particular voltage (say 12VDC). These sets are again connected in parallel to make a long (infinite) strip, so that all sets may be connected to a single DC power supply.
LEDs are available in SMD (Surface Mount Devices) size also. They are tiny in size, anode (+ve) and cathode (-ve) are marked on the LED.