Handy Dandy Little Circuits #31

Light Emitting Diodes

A Light Emitting Diode is commonly called LED . The light emitted by an LED has a narrow wavelenght as compared to an incandescent lamp which contains many wavelenghts . When an LED begins to conduct , the voltage increases gradually while the current increases rapidly . Too much current will overheat the LED and eventually destroy it .
The light emitted by an LED is directly proportional to current through the LED. The light ouput from an overheated LED will soon decrease and the LED may be damaged .

The forward voltage and wavelenght of an LED are directly related , so , it is not always possible to substitute a different LED without changing the voltage and current . Many different materials are used to make various LEDs .
Below is a table showing the general description of wavelenght and voltage operation for some LEDs.

Wavelenght (nm) Voltage Wavelenght (nm) Voltage
565 (green) 2.2 - 3.0 690 (red) 2.2 - 3.0
590 (yellow) 2.2 - 3.0 880 (infrared) 2.2 - 2.5
615 (orange) 1.8 - 2.7 900 (infrared) 1.2 - 1.6
640 (red) 1.6 - 2.0 940 (infrared) 1.3 - 1.7

In all applications , LED's specified operating current limit must be respected for longivity and I have found that operating LEDs at their current limit does not significantly produce more brilliance than at 50% of their current rating and ensure long operating life for most LEDs .
Having said that , I must point out that for some infrared LED types,reducing the operating current may also affect a specified detecting range .

Circuit no-1 below shows the formula and procedure to find the proper resistance value to be used to limit the operating current for all LED types , including infrared .

The series LED connection circuit shown below can be used but I strongly do not recommend it . Regardless of good current calculation and voltage regulation one or more LEDs are bound to fail as an open circuit which can be a real pain trouble shooting to find and replace the bad LEDs . If one or more LEDs fail as a short than other LEDs in the string may well be destroyed by excess current load , an expensive proposition .