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Optoelectronics/Displays

Posted: 12 Jan 2015     Print Version

Keywords:LED  incandescent  schematic  circuit  bulb

I am now disillusioned with LED lamps. LEDs are touted as the future, in part because they are economically the right form of long-term lighting, and there are environmental benefits as a great aside. Well, maybe the environmental argument is true, but the economical one is not.

My wife has converted a substantial amount of our home lighting, as well as our holiday decoration lighting, to LED bulbs. Despite all this investment, I have yet to experience the primary benefit of long life. This made me sit down recently and ask myself why.

As it turns out, the answer is quite simple. The lifetime is not a function of the LED, but rather the total circuit solution.

Figure 1 shows the schematic for an incandescent light bulb. As I once read in a college textbook, the analysis of this circuit is left to the reader.

Contrast the figure 1 schematic with the figure 2 schematic that shows an offline LED lamp schematic minus the LEDs. As I'll show soon, there is no need to analyse the figure 2 circuit operation.

 Figure 1: Schematic for an incandescent light bulb.

 Figure 2: Schematic for an offline LED lamp schematic minus the LEDs.

To estimate the failure probability of an electrical circuit, one can apply the product rule for probabilities. Essentially, the rule is that the overall probability of failure is a product of the individual, uncorrelated failures.

Notice that the figure 1 circuit has one circuit element. Let's assume the probability of failure for a circuit element is 100 ppm. After all, we're discussing a 50-cent light bulb, not a \$30,000 car. Applying the product rule, the ppm failure rate for figure 1 is 0.9999 or 1 out of 10,000 light bulbs.

Next we look at figure 2. There we have approximately 60 circuit elements. Using the same product rule and 100-ppm failure probability, the failure rate of figure 2 is 1 out of 167 lamps. Now this is a big deal, but not yet a catastrophe.

After a quick look around my 2,200-sq.-ft. house, I counted about 50 light bulbs. Once again, applying the product rule, the incandescent lamp failure rate inside my house should be about 1 out of 200. And the LED lamp failure rate should be about 1 out of 4! A 25% failure rate is ridiculous for a product that costs \$10 or more and does nothing more than a product that costs 50 cents.

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