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White LED Lifetime Tests  (March 1, 2010)
By: Dave Johnson
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.

High Power 20-100mA 5 chips White 5mm LED
  • Vf: Typ 3.5V
  • Iv: 18000-25000mcd @ 100mA
  • Iv: 20lm @ 100mA Max
  • Viewing Angle: 50 Degree
  • Super Reliable LED, 14 MIL chips.

 

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.
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.

 

White LED Output Vs Time

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.

Please send comments to me


October 2009      Issue 2

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