Trick of White LED. Circuit Clinic 6


Introduction of White LED created a revolution in electronics field to replace the Fluorescent lamps in lighting systems. High power White LEDs are now widely used to make Ceiling lamps, Flood lights, Vehicle lamps, surgical instruments, Projectors etc. When compared to ordinary White LED, the high power LED consumes around 200-300 milli ampere and mounted on heat sink for heat dissipation. Majority of high power White LEDs are flat chips that will be easily destroyed if the voltage and current exceeds. So LED driver circuit is used for the  stable operation of the LED. The theory and working of LED is given in the previous post, so only the relevant facts about White LED is given here.

Unlike ordinary Red, Green, Yellow LEDs, light production is different in White LED.No semiconductor material emits pure white light. White light is a feeling created by the mixing of the primary colors – Red, Green and Blue. The white LED has an Indium gallium nitride blue chip coated with phosphor. This phosphor can emit white light when energized with blue or ultraviolet photons. The blue light emitting chip in the LED is coated with a converter material. When this converter material is activated by blue light, it emits yellow light. The mixture of blue and yellow light is perceived by the eye as white light. The light from the white LED has both peaks in the blue and yellow region but human eye will identify the peaks as white light. That is why, some white LEDs looks more blue and some more yellow. This is due to the difference in the peaks. White LED requires minimum 3 volts for its operation and its forward current can go up to 40 mA. But in high power white LEDs, it is 3.6 to 5 volts and 100 to 350 milli ampere depending on the make.


High power LED mounted on Heat sink



LED Driver

 Led lighting applications  require a constant current source. High power LEDs are usually operated at 350mA current and higher, much as 1A. So LED driver circuit is used to provide constant current to LED for keeping its brightness stable and for  protecting them. LED drivers are expensive circuits . You can try this simple constant current LED driver using Voltage regulator IC LM317. Select R according to the current requirement of LED. Use heat sink for the IC.


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