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Optoelectronics/Displays  

Inside an A19 LED bulb

Posted: 08 Aug 2011     Print Version  Bookmark and Share

Keywords:A19  LED  bulb  CFL 

Standard A19 format light bulbs, used in most lamps and luminaires these days, are now available in LED versions. Some of these are dimmable, some not, and some only with specific dimmers. They all advertise 25,000 to 50,000 hours' expected lifetime, based on three to four hours' daily usage. If you use them appropriately and sparingly, you might expect your light bulbs to outlive you.

But why are the bulbs so expensive? Do they provide real value for the price? And why are some bulbs twice the price of the others?

In our scientific quest for answers, MuAnalysis Inc. tore apart five A19 LED bulbs: a Philips 60-W equivalent, at 12.5 W and 800 lumens, and 40-w equivalents from Feit, GE, Pharox and Sylvania. Our examinations of the five bulbs raised still more questions (figure 1).

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Figure 1: Five A19 LED bulbs: a Philips 60-W equivalent, at 12.5 W and 800 lumens, and 40-w equivalents from Feit, GE, Pharox and Sylvania.

Each of the bulbs comes in a specially designed package, unlike tungsten filament and CFL bulbs, which ship in nondescript shrink wrap. The fancy packaging adds to the overall cost.

These bulbs clearly are not yet positioned as commodity items; they are expensive and are expected to last. But the price of electronic gadgets has dropped so much of late that longevity is no longer the main concern. So why is a common light bulb more expensive to buy than a cheap digital camera?

Looks count in a category as simple as light bulbs, and each of the bulbs we examined has a unique appearance. For example, the GE bulb has a ceramic neck and fins and a glass bulb, and is more costly than those using plastic and metal.

All of the bulbs have a small printed-circuit board contained within the neck, relying heavily on large electrolytic capacitors and transformers (figure 2). The reliability factor of LEDs has increased tremendously. But how long will electrolytic capacitors perform under such hot operating conditions?

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Figure 2: All of the bulbs have a small pc board within the neck, relying heavily on large electrolytic capacitors and transformers.

Bulb comparisons
The dimmable bulbs incorporate silicon-based Infineon or ST power transistors. We did not find silicon carbide technology in any of the lamps.

Each lamp uses a different LED driver IC. Pharox uses four ICs, 40 resistors and 16 capacitors, plus transformers and many diodes. All this to dim a 40-W-equivalent light bulb—but only, in some cases, if you purchase a new dimmer, since your old dimmer might not be compatible.

Philips uses a remote phosphor, relying on the bulb envelope material to do the wavelength conversion. Lutetium oxide is used as a fluorescent medium in more than one lamp. We have looked at many LEDs over the last few years and have never seen lutetium used before. More conventional phosphors, such as barium and cerium-doped YAG (yttrium aluminium garnet), are also seen, along with a cocktail of rare earths such as europium and strontium.

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