LEDs Fundamentals and Applications


Introduction to LEDs (Light Emitting Diodes)

The primary component in optoelectronics are the LEDs. Light emitting diode is a diode with PN junction of crystal material that produces luminescence around the junction when forward bias current is applied. The junctions of this light emitting diode are made from Gallium Arsenide(GaAs), Gallium Phosphide(GaP)or a combination of both(GaAsP).



The available colors are red, white, yellow, green and blue.Some are housed in plastic affixed to the base header of a transistor package. Others are contained in plastic packages that have a dome shaped head at the light emitting end. Two wires protrude from the opposite end for applying forward bias to the device.

These days, surface mount types are commonly used.

One of the typical example of its application is as shown below.



The forward bias current of a typical LED ranges between 10 and 20 mA for maximum brilliance. A 1 kohm resistor in series with a 12 V DC source will caused it to operate at 12 mA. In order to ensure the lifetime of it is preserved, do not exceed the maximum rating of the current. The voltage drop across it is typically 1.8 to 2.0 V DC.



Definition and Terminology 

Incident Flux Density

This is defined as the amount of radiation per unit area expressed as lumens/cm2 or watts/cm2. This is the measurement of the amount of flux received by a detector measuring its output.



Emitted Flux Density

This is defined as radiation per unit area and is used to describe light relected from a surface. This measure of reflectance determines the total radiant luminous emittance.



Source Intensity

This is the flux density that will appear at a distant surface and is expressed as lumens/steradian or watts/steradian.



Luminance

This is a measure of photometric brightness and is obtained by dividing the luminous intensity at a given point by projected area of the source at the same point.

A typical specifications of a simple LED is as shown below.


Absolute Maximum Ratings at 25° Celcius

Reverse Voltage = 5V

Forward Current = 20mA to 30mA

Forward Current Peak at 1/10 Duty Cycle, 0.1ms pulse width = 100mA to 150mA

Power Dissipation = 100mW to 150mW

Operating Temperature = -40° Celcius to +85° Celcius

Storage Temperature = -40° Celcius to +85° Celcius 

Operating Characteristics

Typical Forward Voltage at 20mA = 1.7V to 2.0V

Maximum Forward voltage at 20mA = 2.5V

Reverse current when 5V reverse voltage is applied = 10 uA

Peak emission wavelength at 20mA = 627nm to 700nm (value depends on the colout of the LED)

Spectral line half width at 20mA = 20nm to 45nm

Capacitance at 0V forward voltage and (f=1MHz) = 35pF to 45pF



Manual Soldering of LEDs

Soldering tool wattage should be less than 30 Watt.

Soldering temperature should be less than 300 °Celcius.

Soldering time should be less than 30 seconds. 


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