White LED Lifetime
Tests (March 1, 2010)
By: Dave Johnson |
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| A dirty little
secret in the lighting industry is that all white LEDs fade over time. I’ve
been conducting some more experiments on some white LEDs offered on Ebay.
I have purchased many LEDs through eBay over the years but I have yet to find
a quality white LED, made in China. I want to world to know that many of
the so-called white LED bargains on Ebay are not bargains at all. Some
devices fade in intensity very rapidly while others fade more slowly but every
one I have tested would be worthless in a few months to a year. The
buyers of these devices would be especially disappointed with them, if they
were used in applications where prolonged operation took place. I cringe when
I see some of the latest 120vac powered LED spot lights and strip lights being
sold at places like Costco and Lowes. These lights often use large
numbers of the same kind of LEDs I have been testing. |
| The device below is
one example of the many white LEDs sold on eBay. This one came from an
obscure company called Besthongkong. This device is their part number
BUWLC5333W50BB25. They claim that their device has a lifetime of 100,000
hours. In fact, if you define the end of life as when the light output
drops by half, you would have to say that their part is only good for 3,000
hours. And this lifetime was measured at a power level about one-fifth
their specified rating. |
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High Power 20-100mA 5
chips White 5mm LED
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Vf: Typ 3.5V
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Iv: 18000-25000mcd @ 100mA
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Iv: 20lm @ 100mA Max
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Viewing Angle: 50 Degree
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Super Reliable LED, 14 MIL chips.
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| The manufacturer
claims a longer lifetime by using 5 separate LEDs wired in parallel inside the
assembly. I guess their idea is to spread the power out between 5
different parts. The device is rated at 100ma and has an extended metal
heat sink on one of its legs, to aid in removing heat from the part.
Although the part was designed for 100ma, I tested the LED using the constant
30ma current drive circuit shown below. I used a small photodiode,
placed in front of the LED, to measure the light output. I used a
quality digital multimeter to measure the current from the photodiode.
Using this method, the multimeter current reading is directly proportional to
the light intensity. I plotted a curve of the photodiode current reading
as a function of time. Note that this particular LED faded rapidly.
In about four months, the light output dropped to about 1/2 the initial
intensity. In 14 months, the light was a dim 8% of the original reading.
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| Apparently,
Chinese companies don’t care about the quality of their products.
They should be ashamed of themselves. This is all too typical of
products for China. By way of
comparison, I am testing similar devices from Cree, which are made in
the US. I will be conducting more tests on other LEDs too. I
have seen some devices offered on Ebay, which claim to be made by Cree
but I have my doubts. Many of these devices are counterfeit
knockoffs from China, which have the guts to place “Cree” on the device,
to make them appear original. I would bet they will have the same
fading problem that the best Hong Kong part had. I will post my
test findings as I generate them. |
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White LED Output Vs Time |
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White LED Lifetime Test Circuit |
| To speed up the
testing process I have modified my constant current LED driver circuit as
shown below. This circuit can test 4 LEDs at the same time. I have
a separate photodiode monitoring the light output from each device. I
have also mounted a battery operated hour meter to the circuit. This
should make it easier to plot light output Vs time curves for several parts. |
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