Monday, February 14, 2011

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.  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.

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. 
See schematic below:

Circuit Circuit White LED life tester schematic or circuit

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