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Old 2nd April 2009, 05:03 PM   #1
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Question Curious LED failure mode

On my electric gate/ intercom, there is red LED to show the gate status.
Lately this LED fails - 3 times so far in a year. But each time, it doesn't die completely, it suddenly goes really dim - yet still works.
It's switched via a dedicated relay on the microprocessor board, powered from 12V dc, and the series resistor is 1k5, so not excessive current, and no obvious reverse bias.

When I replace the LED, the new one work fine instantly, nothing else need repairing.

Is this a known failure mode for LEDs? I've never known them to suddenly go dim, yet still work. The forward voltage drop after failure is rather high at over 2V.
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Old 3rd April 2009, 12:37 PM   #2
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I've seen similar behaviour several times from LEDs that have been driven with too much current.

If you have just a standard 5mm LED in there (or something similar) I'd be willing to bet the 6.7mA you have flowing through it is too much and over time it is cooking its self (especialy if the housing it's in can get hot). So you could try swapping out the curent limit resistor for somethinig in the region of 3k or more and see if this stops your problem.
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Old 3rd April 2009, 04:30 PM   #3
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That's still weird though; the books say typical LED current is 15 to 25mA - continuous, and this one is operating way below that. And it's only in use for maybe 30 seconds of blinking, three or four times a day.
It stays at room temperature - it's just in the plastic intercom wall bracket with no electronics.
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Old 4th April 2009, 12:01 AM   #4
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Uh-huh. I have this with every LED bought from fleabay

My solution that has worked so far is to use a superbright LED and run them at about 600uA current for "standard" brightness.

Haven't had one fail since

Cheers!
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Old 4th April 2009, 12:07 AM   #5
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Ahh.. so they could be a flakey batch. They DID come from a Chinese dealer, so really no idea who made them. I guess the plan now is to demand genuine HP or Kingbright.

Thanks for the input.
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Old 5th April 2009, 10:15 AM   #6
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Maybe they are actually low current LEDs. Try to run one at 5 mA and see how bright it is...

I've seen something similar after a burn in period on some of our products. In one batch a few LEDs had not survived the burn in period (at 70 degrees C) and then the baking of the conformal coating at 90 degrees C.
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