LED life versus current (at and above rated) in white light bulbs

P E Schoen · 2015-09-28T20:58:25+00:00

Recently (Dec 2014) I bought some 9W LED light bulbs from for about $4 each, and I was pleased with their apparent quality and performance. However, in May, the first one started flickering and then failed. I took it apart and found that some of the individual 1/2W chips had failed. There were two parallel strings of 9 each. I got it to work (somewhat) by jumpering the bad LEDs. I replaced the filed bulb with the second new one. Then, a couple weeks ago, the new lamp started flickering, and also failed. Similar mode of failure. Several chips showed low resistance and did not light, while the others lit using the DMM diode check. So the lifetimes of both lamps were similar, about 2000 hours, and actually less than 1500 since they are used only about 12-18 hours a day. I replaced the bad LED chips with some much smaller 1206 white LEDs, rated about 30mA or 100mW. I figure the current in the LED strings should be about 150mA, so this was a 5x overcurrent (yes, they lit very bright). I kept it on until it started flickering again, after about 5 minutes, and the small LEDs were toast, but also one more of the original LEDs had failed. My thought is that the original LEDs were over-rated and failed because they were driven by too much current. But I could not find any definitive information on the effect of (over)current on lifetime. The sources I found only said that LEDs fail mostly because of excess temperature, and the lifetime is determined by measuring the slight drop in luminous output after continued operation at nominal current after several thousand hours, and then extrapolated to EOL defined as something like 50%-70% of initial output. It seems that there should be some studies on the relationship of current and lifetime, but I could not find anything. It seems like the 5x data point is about 5 minutes or 0.1 hour, and perhaps it is an exponential function with 1x=1,000 hours, 2x=>100, 3x=>10, 4x=>1, 5x=>0.1. Obviously not quite that...

Sep 28, 2015 28 Replies

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dagmargoodboat 10 years ago

The Lumileds graphs used to show 90% lumen maintenance @ 50K hours, IFF you keep 'em cool.

The industry has changed to a different, standardized test regimen since then, and I haven't looked recently to see how that affected their results.

Cheers, James Arthur

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upsidedown 10 years ago

Look carefully what that "test current" is.

IIRC at CREE it is about 30-50 % of Imax.

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Kevin McMurtrie 10 years ago

I've built a few high power (0-5000 lumen) bicycle lights at home so I'm very familiar with this.

LEDs with low resistance had reverse voltage ESD damage from sloppy assembly. The die will slowly burn away until the damaged area consumes more current than provided. An early test for this is to apply a few microamps of forward voltage for an hour. Damaged LEDs will eventually appear to be shorted while good LEDs will stay lit.

LEDs that went open-circuit were likely cracked from thermal flexing. They may have been cheap because they were contaminated or flawed.

I will not see posts from astraweb, theremailer, dizum, or google because they host Usenet flooders.

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Jon Elson 10 years ago

Yes, of course, and that IS what current I am running at, the TEST cuurent, NOT Imax! Imax is SCARY, I think one Amp! It would probably burn holes in the floor, not to mention our poor retinas!

Test current is 350 mA, Imax is 1 A.

Jon

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Jon Elson 10 years ago

Just another data point. The first retrofit I did had 10 LEDs in it, and it has been running for 2.5 years, guesstimated at about 2700 hrs or 27000 unit-hours. The second two units have been running for about 1.5 years, but more hours/day, guesstimated at 3300 hours, or 132000 unit-hours. (Unit- hours referring to hours x number of LED components.)

They seem to be doing QUITE well. I've had 2 bad solder joints on these things, they are leadless SMT, so getting them perfectly soldered to the PC board material is tricky. When they go out, I just go down the line with a DVM in diode test mode to find the bad solder joint, and it is back on line in a couple minutes.

Jon

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upsidedown 10 years ago

Earlier you said

In that case, those are "3 W" LEDs, since the power is usually quoted as Pin = Vf x Imax.

IMHO, a "3 W" LED is usable as 1 W (350 mA) light source and "1 W" as

300 mW (100 mA) light source. Also with such underrating the thermal design becomes much easier keeping Tj well below Tmax.

Sure there are "10 W" LEDs with Imax at 3 A, but light output well below 10000 lm, thus less than 100 lm/W and very short usable life time. Possibly usable as a flashlight, but not much else.

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Archer 10 years ago

Thoothu pithare Govinda !!!

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Jon Elson 10 years ago

Cree is a conservative company, they do NOT recommend running them at 1A. I thought they listed these as 1W, but I can't find that description anywhere in the spec sheet, now.

it's all going the way of the 6 HP vacuum cleaner motors, 5 Hp wall-plug air compressors and the rest of that crazy consumer-rated crap. But, Cree sells mostly to large manufacturers of commercial lighting products, and won't get away with misrepresenting their products.

Jon

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