The basic theory behind light emitting diodes

 This is a Voltage/Current curve of a typical light emitting diode. The exact values of each spot on the curve are less important, but the curve and what happens in each segment, is more important.


Segment A - Voltage is increasing, but has not yet reached the point where the diode junction is emitting enough light to be visible. If you have a datasheet for the device, this will probably be the first 75%-80% of the rated forward voltage (Vfwd).


Segment B - As the forward voltage enters the knee part of the curve, light may be perceived. Depending on the exact voltage and/or the current, the light may be faint. Once you have entered the curve area, forward current will begin to change as the voltage increases. Depending on the exact device ratings, the device may begin to dissipate some heat. Heat will be low to negligible, increasing as you approach the device rated forward current. Light should be obvious, and increasing as you move deeper into the knee area.


Segment C - The light emitting diode now has encountered a minimum voltage required to rapidly increase current flow, light production, and heat. Too much current, or too much heat generation (without a way to dissipate it safely) can cause catastrophic device failure. This is where the datasheet's Absolute Maximum Ratings should be consulted. Cree Thermal Management documentation suggests that more powerful LED devices (e.g. 250 mw and up) could generate 50%-70% of the total power as heat. High power usage and heat dissipation, can result in a shorter device lifetime.

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