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 colours 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 endfor 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 maximumbrilliance. A 1 kohm resistor in series with a 12 V DC source will caused itto operate at 12 mA. In order to ensure the lifetime of it is preserved, donot exceed the maximum rating of the current. The voltage drop across it istypically 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 froma 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 luminousintensity 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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