Life curves?

JohnR66

Flashlight Enthusiast
Joined
Aug 1, 2007
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Location
SW Ohio
I'm looking for information regarding the life of LED 5mm, 3mm and power types at various outputs. The power LEDs would have to be estimated since a real time test could take years to complete. The small epoxy LEDs should have data given their relatively short lifetime.

For example, If a 5mm LED lasts 5000 hours at 50% lumen loss at 20ma, what would this be at 15ma? I think the curve would be exponential rather than linear, right?
 
There doesn't seem to be much data on what happens when you drive LEDs under their rated current. I'm reasonably sure that the lifetime curve would be exponential, but any real-world tests, even with 5mm LEDs, would probably take years. For example, extrapolating data at higher currents to lower currents tells me a 5mm white LED which lasts for 6,000 hours at 20 mA would last for about 10,000 hours at 10 mA, and based on the equations lifetime seems to asymptomtically approach ~15,000 hours at very low currents. However, other data I've seen and common sense suggest it might be much longer than that. For example, I've heard that running 5mm whites at 5 mA on a board with fat traces for heatsinking can result in lifetimes of hundreds of thousands of hours. I'd say offhand that the data at high currents can't be extrapolated to low currents. I know I don't personally don't run white LEDs at >10 mA in any device where I need long lifetime.

Low current data for power LEDs would be even harder to come by. As it is they last long enough at rated current to make gathering real world data time-consuming. For example, I have a test Q3J Luxeon running at 350 mA for over 41,000 hours and I haven't measured a decrease in brightness over the last 3 years (didn't have a light meter before then). Based on this, I'd say underdriving power LEDs to 100 or 150 mA could very well result in lifetimes exceeding a century.
 
Here is a link I found some time ago that may interest you.

http://www.molalla.net/~leeper/5mmdeg.htm

Thanks! very interesting information here along with the link back to the old CPF thread with the cross sectioned LED photos.

The 5mm LEDs sure fade fast if over driven even a little.

I'd say 5-15ma for long term life and 20-25ma for flashlight use since a flashlight is used only intermittently. Some manufacturers of LED lighting need to take a hint as some of the LED in the store displays have faded in only a couple months!
 
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