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Tuesday, June 01, 2021 03:06 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.
(10/19.2008) |
| 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 Circuit Below to view PDF of Schematic |
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eMail David A.
Johnson, P.E. about this circuit |
