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Quick Understanding Of LED Bulb Turning Blue
Another LED bulb in my house failed long before I could celebrate my 99th birthday! This was not supposed to happen if we believe the calculations I made in a previous article on this subject and mostly if we believe the 25,000 hours advertised for those lamps… This LED, however, did not suddenly stop glowing like the first one but was progressively turning from white to blue! It started with one blue spot which kept growing as time went. I always thought that the color of a LED is determined by the chemistry of the semiconductor and cannot be changed without changing the diode, so I became very curious. Investigating this phenomenon led me to interesting findings about white LEDs that I thought of sharing in this short article.
That particular LED bulb was the very first one I bought some 4 to 5 years ago to replace the overheating filament bulb in one of my desk lamp. Soon I found out that I could not use it while working with high frequency projects – those small receivers I build just for the fun of it – as the LED was broadcasting a strong interference over a large range of radio frequencies. Therefore it was immediately banned from my workshop and ended up into the spare bulbs box, to be replaced with another, much more “silent” LED bulb; until last year when I used it to replace a burned filament bulb on the ceiling of the balcony. Far enough from my office/workshop the radio interferences did not matter anymore. All was then peaceful in the LED department until the appearance of the blue zone that prompted me to get the ladder and take the culprit to my workshop for further analysis.
I first tried to find some tips on the Internet on how to open this kind of bulb. It looked solid, with a heavy aluminum casted heat sink. One guy on YouTube was opening a similar lamp just by lifting the side of the plastic dome with a small screwdriver. It looked so easy so I tried it! First discovery, it was not plastic but glass, just like a filament lamp… After patching my bleeding finger with a band aid I continued the investigation! Inside the bulb was another, smaller bulb, yellowish/brown color. This one was obviously made of plastic as one side of it was melted with signs of overheating.
It came off easily, as the plastic had become brittle, exposing an array of 6 LEDs. At this stage I powered the lamp again to see if one of the LED had turned blue… Surprise, they were all blue! So the plastic globe was converting the blue light into white light? To confirm I took a blue LED and covered it with the plastic bulb; and yes the blue light became white.
A few extra measurements before continuing my destruction did not bring any further surprise. The power of the lamp confirmed to be around 10 W and the power factor about 0.82… It still generated that strong interference covering most of the radio frequencies!
All the 6 LED were connected in series and each LED had a voltage drop of 2.6 V. It was then easy to test all the LEDs together using a bench power supply.
So the mystery of the white lamp turning blue is explained by the degradation of the yellow “color converter” bulb. The question we could ask is why didn’t the manufacturer use white LEDs instead of going through all this trouble of designing that inside bulb?
The fact is that white LEDs as such do not exist! Of course, you will say, they exist as we can buy them almost anywhere… But they are not “pure LEDs” made with a PN junction only. Each PN junction emits one color only which depends on the chemistry of the semiconductor used (type of semiconductor, doping used etc…). LEDs exist in many colors from infra-red up to ultra-violet but not white. Because white is not a color but the sum of several colors. White doesn’t belong to the color spectrum… Consequently white LEDs must be made by combining colors and there are two methods for doing that:
1) Combining Red, Green and Blue LEDs to produce white color. This method is widely used for TV screens, computers, mobile phones etc… to make white or any color based on those three basic colors.
2) Convert a blue light into white light using a phosphor based material. Phosphor is a substance that displays the property of luminescence. It absorbs some of the blue emission and produces yellow light through fluorescence. The combination of that yellow with remaining blue light appears white to the eye. There are several materials that have phosphor properties and can produce light of different colors.
This second method is the one used for manufacturing high power white LEDs. Today’s typical white LEDs use a phosphor-dissolving resin mixed with silicon or some other resin. The blue LED chip is then coated with this composite. In the case of my LED bulb, the small plastic bulb was probably coated with similar phosphor material, in the same way a fluorescent tube is made. The following picture shows blue and white LEDs matrixes. The yellow part of the white LEDs is the phosphor coating.
This is all great but why blue LEDs? Engineers made the first LEDs in the 1950s and 60s. At the time, scientists developed LEDs that emitted almost everything from infrared light to green light… but they couldn’t quite get the blue. That required chemicals, including special crystals, which they weren’t yet able to make in the lab.
Initial blue LEDs were developed between late 1970th and late 1980th but were not very bright. However, help was on the way: Three scientists, Isamu Akasaki and Hiroshi Amano at the University of Nagoya, and Shuji Nakamura at Nichia Chemicals in Tokushima were working on the problem. Nakamura demonstrated the first high brightness blue LED in 1994. This was followed by improvements by both teams as they studied new gallium nitride alloys using aluminum or indium, as well as more sophisticated diode designs.
The three scientists were awarded the 2014 Nobel Prize in physics for their work. The efficiency and power of the blue LED had open the way for making the high power white LEDs (and also other colors) used today in so many applications. It was the start of the current lighting revolution…
This story is neither complete nor finished. Everyday new materials and new methods are investigated and new and exciting discoveries are made. Organic light-emitting diodes (OLEDs) are now parts of our mobile phones, tablets etc… and also used for printable and flexible displays. Quantum dot LEDs are just around the corner and promise to revolution the LED technology! And this never stops… LED is a fascinating subject and the net is full of information and details about it. I hope that this article will excite the curiosity of some of the readers, encouraging them to find more.
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