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
n Positive and Negative Terminals
n Typical Interior (more on LEDs later in the course)
n Symbol for LEDs
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.