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DiscoverCircuits.com -- Hobby Corner
Last Updated on:
Tuesday, November 29, 2011 08:14 PM
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Tester Circuits
LED Circuits White LED Circuits |
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White LED Life Tester Schematic
designed by David Johnson, P.E. |
| There are lots of white
LEDs for sale these days. Many eBay and electronic component surplus sales
claim to sell some very bright devices. Over the years I have purchased some
of them to experiment with. I also have purchased some products, which
contain white LEDs. |
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A major disappointment for me was some LED night
lights I bought from Costco. In just three months of operation, the night
lights I purchased were too dim to be of any use. What I have discovered is
that many of the LEDs start out emitting lots of brilliant white light but quickly
fade in just a few months of continuous operation. I think some
manufacturers from China are using inferior phosphors inside the LED assembly,
which fatigue after only a few hundred hours. The light from the better
manufacturers seem to last much longer. But, how do you know if you have
some good parts or bad parts? When new, one part looks as good as another.
What is needed is a way to measure the degradation. Good parts should show
little or no drop in light output as a function of time. Bad parts might
show a 10% reduction in light after only a few days. |
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| I
think the tester circuit shown below would work. It routes an accurate 30ma of
current through the LED under test, day and night. This 30ma current level is a
bit more than the standard 20ma, which should accelerate the testing process.
The light emitted by the part is monitored by a small PIN photo diode. The photo diode
and the LED under test need to be housed inside a black plastic box. This insures
that light from the environment is not allowed in. Using a digital multimeter,
the current produced by the photo diode can be measured. The photo diode current
is proportional to the light intensity of the LED. By periodically recording the
drop in light output as a function of time, you should be able to flag suppliers of
good and bad parts. Be patient! It may take a few days or even a week or
so to detect a measurable drop in output. |
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Click on Drawing Below to view PDF version of Schematic |
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