UPDATE LED Life Test at 2 Months

I know I've seen references to the fact that normal LEDs, driven at their rated current, will be dimmer by half after a year. I don't know what the mechanism is, but I've seen this for myself on LED displays that were clearly left on for a long time, displaying the same characters.

I could imagine that the rate of dimming would be related to the drive current and the rated drive current. It sounds like you may be driving yours a bit hard. OTOH, it's quite possible that the "after 1 year" test that I read about, meant a full year of normal use, rather than left on 24/7.

----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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In laser diodes it's aluminum migration. I assume it's the same in LED's but don't know -- and I don't know how the heck white LED's are made, for that matter (gotta catch up).

Its worse at high temperatures, probably exponentially with absolute, but I know when we were trying to buy a solid-state laser for rangefinding it was still mad science stuff -- our mad scientist talked to the laser manufacturer's mad scientist and they decided it would work just peachy keen. Then it got kicked over to their production engineers who said NO! Pissed off people all around.

I've had problems with several white LEDs from Fry's Electronics failing too. They dimmed after a few months of use then they started blinking. It sucks because some are inside a night hiking flashlight to keep it glowing while switched off. It's hand crafted inside a tough acrylic tube like a ship in a bottle. The LEDs can't be removed. I didn't think I'd need to make LEDs replacable.

I'm going back to green LEDs for the next flashlight.

Hello

I have found this link on eBay

He claims that the white leds in the flashlight can last 100,000 hrs. That means 24/7 in over 10 years.

You can get the flashlight for 15$ with free shipping around the world.

Cheers Rune

"Kevin McMurtrie" skrev i en meddelelse news: snipped-for-privacy@corp-radius.supernews.com...

AFAIK, LEDs don't have aluminum in them. At lease not in the mechanical makeup.

What color are they? I bought some cheap blue LEDs, and they were just like that atraight out of the package. After five minutes on the PS, they started to get erratic.

My 2 cents worth>> Have had a "Hong Kong" white LED on for 70 days. Runs off of 2 pen cells (3.12 volts when fresh) no series resistor and draws 26.4 mils. After 70 days battery voltage down to

What's claimed and what's actually going to happen are two different things.

There've been statements here that some LEDs start to lose a lot of birghtness at less than 10k hours. That's not much different than a fluorescent light.

I t hink your measurements are off. Seems to me that a pair of AAA cells couldn't put out that much current for that long. How are you monitoring the current? If you're inserting the meter in series, then that meter is adding series resistance, and it is changing the readings. Best to use a resistance permanently in series and monitor the voltage across that resistance. I've checked white LEDs and found that the current drops off rapidly below 2.8V, to almost nothing.

Today I read an article that claims the die heat contributes to colour change and long term reduction of light output. So I gots to thinking, if you pulse the led at some reasonable rate, would this reduce the die temperature and prolong the life of the led? Maybe you could bung two of you're cheap hong kong leds next to each other, on pulsed at the lowest duty cycle possible to prevent visible flicker, and the other on dc.

The article I read also stated the UV tend to degrade the lenses.

The resistance of the bond wires is always there, and if you put high current pulses thru these wires, they get hot. Just one more heat source to cause degradation. Same also applies to the chip itself.

Well, I haven't noticed any discoloration or change in clarity of the lenses so far. I would say that most of the dimness comes from the blue LED chip fading to nearly nothing.

You could tell if it was the LED chip or the phosphor degrading by shining a blue led into the degraded and un-degraded white LEDs and comparing the glow.

Errr...no. The clear lens is an attenuator of the UV which stimulates the phosphor. The UV LED lens may be a different fomulation. There is certainly little response from a white LED's phosphor to external UV stimulation compared with a straight reflection of the light from an external white LED.

-- Graham W

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Since 'pulsing' the leds would, for a given output, increase the die temp. there would appear to be little mileage in this approach if die temp is the critical criterion.

White LEDs tend to require a higher average current and have a higher voltage drop (for equal average klight output) when pulsed than when operated continuously. Unless the average current is less than a few milliamps, you get more light and less heat with continuous operation than with pulsed operation at a frequency high enough to apear continuous.

By and large, only LEDs that are more efficient at higher instantaneous current benefit from pulsing. This was especialy true of LED digital displays with GaAsP on GaAs substrate, where a segment had an average current of only a few milliamps but the efficiency was maximized at instantaneous currents of 50 milliamps or more, often at least 100 mA. Many people were not aware of this nonlinearity of those LEDs and believed that the benefit of pulsing was due to a quirk of human vision.

Some LEDs, namely at least some InGaAlP ones, have a degradation mode that is a function of temperature and duty cycle. I suspect this is from some sort of diffusion of an ingredient from where it belongs to someplace else and dependent on electric field around the boundary or zone that the diffusion occurs across. Maybe a boundary between different layers (having/lacking whatever diffuses) should be sharp but gets "blurred". LEDs with that chemistry, primarily at higher temperatures and lower average currents, can have a life extension from pulsing. One manufacturer, Agilent, publishes an application brief where they encourage pulsing of their LEDs with that chemistry if the average current is less than 10 mA, and recommends instantaneous currents 10-100 mA (but average current not exceeding 30 mA).

I discuss pulsing LEDs in an attempt to make them brighter (or appear brighter) in a web page of mine,

By and large, if the average current is already close to the maximum rated constinuous current, there is at beast little to gain from pulsing.

- Don Klipstein ( snipped-for-privacy@misty.com)

The phosphor is intended to be stimulated by visible blue light. In a usual white LED, a blue-emitting die (chip), typically with a peak wavelength around 460-470 nm, is used.

- Don Klipstein ( snipped-for-privacy@misty.com)

But you would still be able to compare the (attenuated) level between a new and a 'worn' LED. ....and I thought it was blue, not UV, that most white LEDs used....

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Don, Martin:

Arrrrgh... I thought it was UV. I've re-done the experiment with a visible blue LED and it does seem to externally stimulate the white phosphor at a level (with a new white LED) which should be measurable with a simple photometer. Making a jig and setting the drive current would be critical. [I wish I had access to a Perkin Elmer Spectrophotometer like the one I used to use!]

-- Graham W

These are the lovely bits of information that professional eng. mags seems to skip over so often. I am no designer of LED gear, but the info is interesting to read none the less.