What’s the storage life of idled LED light bulbs?
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Previously, we discussed for how long a time an electrolytic capacitor can safely be left idled. Hold that thought.

The bulb held an impressive set of electronic goodies. Aside from 32 LEDs on that circular disk, there was a circuit board that turned out to be a constant-current switch-mode driver.

The first thing I learned was that these things should be e-cycled when they go bad, even though our local e-cycling service does not include these items. The second thing I learned was that there was/is an aluminum electrolytic capacitor in there. That led (no pun intended) to a new realization.

However, the storage life of an idled LED light bulb such as the one I took apart will be constrained by the storage life of that aluminum electrolytic capacitor. These things can’t just be put on a shelf somewhere and then forgotten about for years and years the way a conventional incandescent light bulb can be safely put away.

If my collection of reserve light bulbs included any LED light bulbs like the one I just dissected, there could be a substantial risk of trouble from electrolytic capacitor degradation. My advice would be to put any new LED light bulbs into service fairly soon after purchase. I would even advocate that LED light bulbs should only be sold with a “use by” date on their packaging.

As to why my particular device went dark, I never did figure that out. Nothing appeared to be burned or charred. Unfortunately, the circuitry itself was only partly discoverable because the PWM chip bore no markings.

Early adopters of LED lighting will remember 50,000 hour or even 100,000 hour lifetime ratings printed on the box. But during a recent trip to the hardware store the longest advertised lifetime I found was 25,000 hours. Others claimed only 7,500 or 15,000 hours. And yes, these are brand-name bulbs from Cree and GE.

So, what happened to those 100,000 hour residential LED bulbs? Were the initial estimates just over-optimistic? Was it all marketing hype? Or, did we not know enough about LED aging to predict the true useful life of a bulb?

Any discussion of light bulb lifetime would be incomplete without mention of the Phoebus cartel, an international organization formed in 1924 by the world’s leading light bulb manufacturers to manipulate the bulb market.

The cartel enforced production quotas and bulb lifetimes with a system of monetary fines, backed by the power of GE’s patent portfolio.

Measuring Lifetime of a Bulb

What exactly does the box mean with this 1,000 hour lifetime? This is the bulb’s Average Rated Life (ARL) — it’s the length of time for 50% of an initial sample of bulbs to fail (abbreviated B50). What “failure” means depends on the type of bulb; we’ll explore this in more depth later on. The definition of B50 reveals a common misinterpretation, namely that a bulb will last for its rated lifetime. In reality, only half of them last that long, although this rating doesn’t tell you anything about the distribution of failures around the median lifetime.

Since the LED bulbs contain a number of parts, it’s natural to ask which ones might be responsible for failures.

Interestingly, the LEDs themselves account for only 10% of the failures; driver circuitry, on the other hand, was responsible almost 60% of the time. The remainder of failures were due to housing problems

Locate the Weakest Link: Component Lifetime

The lifetime of a bulb (or power supply) can be no longer than the lifetime of any of its components. Among the components found inside the bulbs, two stand out as life-limiters: the semiconductors and the electrolytic capacitors. Both of these components suffer from a failure rate that is a strong function of temperature.

25,000-hour Cree bulb uses an electrolytic capacitor rated for 130 C as opposed to the 105 C caps in the other two. For similar operating temperatures, this could multiply the expected life of the capacitor by a factor of five. Each of these measures probably contributes to delaying catastrophic failure of the bulb, resulting in the longer rated lifetimes.

Like the soldiers in Douglas MacArthur’s famous line, old LEDs don’t die, they just fade away. We all know what an incandescent lamp failure looks like: one second it’s burning bright; the next, it’s not

As it turns out, lumen depreciation happens to incandescent bulbs, too. By the end of their 1,000 hour life, the output has typically dropped 10-15%, but nobody ever notices. With LEDs, the effect is much worse, and the output continues to fall as the device ages.

Research says that most users won’t notice a gradual 30% drop in light levels; accordingly the industry has defined L70, the time at which the output has dropped to 70% of its initial level, as an endpoint for measuring LED bulb lifetime.

Color Shift Happens But is Unpredictable

Making Sense of It All

I’ve taken a look at some of the technical issues in LED lighting. Of course, there is more to LED bulbs than lifetime — color temperature and color rendering index (CRI) should factor into any purchase decision. There are also a number of larger problems involved, including issues of economics and sustainability.

Certainly moving away from incandescent bulbs to more efficient lighting makes sense, but maybe we never really needed 100,000 hour bulbs in the first place. The lifetime of even 7,500-hour bulbs is long compared to the rapid pace of advance in lighting technology.

The oldest surviving incandescent light, known as the Centennial Bulb (click to see a webcam of the lamp), is a dim carbon-filament bulb that’s been burning nearly continuously since 1901 — over 1 million hours. In its current state, it throws off as much light as a modern 4-watt incandescent.

I was contemplating my own mortality recently, when I realized 2015 passed by in a blur. Ruminating about the loss of yet one more year, I looked at my LED-laden lamp and thought, “Sure, you might last 50,000 hours, while my expiration date is virtually unknown.” But, was this true?

Happy to take the thought process from my own shelf life to that of an LED, I began to wonder if that 50,000, 35,000, or 25,000 hours of use from an LED could possibly be accurate. For one thing, I don’t mark down the date of purchase for my LEDs. Do you?

The article discussed tests on five samples of 46 types of bulbs by European partners and consumer watchdogs. The test involved turning bulbs on for 165 minutes and off for 15 minutes until failure. The results of the test:

Five types stopped working before 6,000 hours
Five with claims of at least 25,000 hours stopped before 10,000 hours
66 out of 230 samples failed before 10,000 hours even though claims were for at least 15,000 hours of use

Not a fan of the Daily Mail? What might you believe? The U.S. Department of Energy’s website discusses tests for a variety of bulb types. For instance, it states that, “CFLs are tested according to LM-65, published by the Illuminating Engineering Society of North America (IESNA). A statistically valid sample of lamps is tested at an ambient temperature of 25° C using an operating cycle of 3 hours on and 20 minutes off. The point at which half the lamps in the sample have failed is the rated average life for that lamp. For 10,000 hour lamps, this process takes about 15 months.”

In that vein, how long do you think a bulb claiming 50,000 hours would need to be tested? Try 5.7 years. What it comes down to is that there seem to be no real sure-fire ways to test the long-term performance/viability of the LED.

Those cost and performance figures seem to be drawn out of the thinnest of air.

At least 10 years after they were first applied for general illumination, the subject of how to assess the reliability of LED lighting products is still an on-going subject of debate, and sometimes, confusion. Below you’ll learn about some metrics and processes for reliability designing LED lighting systems.

The lifetime of incumbent lighting technologies like incandescent or fluorescent lamps is usually characterized by average rated life, which is the time in hours at which half a test population of continuously running lamps fails (i.e., light is no longer emitted). For incandescent lamps, average rated life is about 1,000 hours, for compact fluorescent lamps, about 12,000 hours, and for linear fluorescents, about 25,000 hours. In contrast, LEDs, operating under appropriate environmental conditions can emit light for 100,000 hours or longer, making an average rated life measurement impossible.

Instead, LED lighting lifetime is characterized using the L70 metric which is the time in hours at which the light output measured in lumens (L) drops to 70% of initial. The shortcoming of L70 from a reliability engineering perspective is that the test methods, published by the recognized authoritative body for lighting, the Illuminating Engineering Society, are based on lumen output measurements for the LED packages only and don’t take other system components such as the driver, thermal management design, and even optics into account. Incumbent lighting manufacturers have not focused heavily on overall system reliability because the lamp is nearly always the first component to fail. With the advent of LED lighting, manufacturers and end-users have come to understand that other parts of the lighting system can be the limiting factor in overall life as the table below illustrates.

Reliability, in its formal sense, is the probability that a part or system will perform as intended for a specified time period under specified operating conditions. Product reliability assessment is nearly always based on accelerated life test statistical modeling and projections. For LED lighting systems, both lumen depreciation and failures of other components contribute to overall reliability

In the context of field operation, robustness is an unquantified indication of the ability of a product or system to operate as intended in the specified environment by confirming that design and production processes yield products that can withstand higher levels of stress than would be expected in normal operation. The electronics industry has for decades employed robustness testing protocols as part of their verification programs. The concept has only recently been embraced by the greater LED lighting community, as the realization has taken hold that LED-based lighting products are not much different from other types of electronics, except, of course, that they produce light.

Robustness testing typically involves subjecting production samples to a variety of overstress conditions, making it more part of an overall QA program as opposed to reliability testing that uses design prototypes. And unlike reliability testing where the goal is to test to failure to learn about failure modes, the expectation for robustness tests is that all products will operate correctly at the conclusion of the test.

Robustness tests for LED lighting are similar to tests for other types of electronic equipment published by JEDEC, IEC, and other organizations, but tailored to the special characteristics of LED products.

LED lamps have a longer lifespan than conventional filament, halogen and CFL lamps.

LED lamp rated at 25,000 hours
As you can see a LED lamp will last over 22 years (at 3 hours per day) against a filament lamp of less than a year and a CFL of just over 9 years.

All lamps including LED have a number of switching cycles. This is the minimum number of times the bulb can be switched ON and OFF before failure in normal use, lamps may go on to last longer. For a typical value of 12,500 the lamp can be switched ON and OFF once a day for a minimum of 34 years!