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LEDs

 

n Purpose: Convert electrical energy to light energy

n Advantages:

 

       ð Energy Efficiency

       ð Variety of uses

 

n Typical Uses:

 

       ð Outdoor Lights

       ð Christmas Lights

       ð Flashlights

       ð Party Lights

       ð Security Lights

       ð Signs and Displays

       ð And, for this course, a component of lab assignments

 

n Positive and Negative Terminals

LED dwg

 

 

 

 

n Typical Interior (more on LEDs later in the course)

n Symbol for LEDs

LED symbol.svg

Cathode is negative, anode is positive

n When wiring an LED you have three options

 

       ð wire each LED single with its own resistor

       ð wire multiple LED's in Series with a single resistor

       ð wire multiple LED's in Parallel with a single resistor

 

n Example Calculations

 

       ð Assumptions

 

               Ø A supply voltage of 5V

               Ø A single bright blue LED

               Ø The LED has a voltage drop of 3.3V
               Ø The LED should run at 20mA

 

       ð Calculations

 

               Ø Subtract the LED voltage drop from the supply voltage: 5 = 3.3 = 1.7 V

               Ø Divide this voltage by the LED current (V = IR): 1.7 V/.020A = 85 ohm

 

               Ø Resistance required = 85 ohm

               Ø Next highest standard resistor is 100 Ohm (within 10%)

       ð Wiring

 

               Ø Find the long leg of the LED which is positive and attach the proper resistor to it.

               Ø Connect the ground to the short negative lead.

 

n Rule of thumb values to use for calculating unknown LED's

 

       ð 20mA for all colors
       ð Blue, White, Green, Pink, Purple, 3.3V voltage drop.
       ð Yellow, Red, Orange, 2.0V voltage drop.