UPDATE LED Life Test at 2 Months

[Watson A.Name - \"Watt Su]

I I've had the 4 LEDs operating at 25 mA for all of May and June, and
the first few days of July. This is about 1500 hours so far. The LEDs
are two Nichia NSPW500BS white LEDs, and two white LEDs from Hong Kong
that I got off Ebay. All four are in series so there is no doubt that
they are all running at exactly the same current.

The two Nichias are going strong, with no noticeable signs of dimming.
The two Hong Kong LEDs, sad to say, are so dim that they wouldn't make
good panel lights, so can't even consider them suitable for
illumination. They're really quite dim.

Of the hundred Hong Kong LEDs I bought, all I've used have had this
dimming problem after a few hundredds of hours, so I decided to put the
four on a PS to control the conditions and verify that I'm not imagining
things. I think one problem might be that the Hong Kong LEDs use an
organic phosphor that degrades faster than Nichia's, and might
contaminate the LED chip. But I'm not an expert on this at all, so I'm
open to any other ideas.

I posted an update of the progress of the dimming at the beginning of
last month, and now this one shows that the hong kong LEDs are so dim
that it's probably not worth continuing the test. I may solder two more
unused hong kong LEDs into the circuit, and start the test again, just
for the halibut.

--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@h@e@r@e@@
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[Jim Adney]

On Mon, 5 Jul 2004 18:10:34 -0700 "Watson A.Name - \"Watt Sun, the
Dark Remover\"" <NOSPAM@dslextreme.com> wrote:

I I've had the 4 LEDs operating at 25 mA for all of May and June, and
the first few days of July. This is about 1500 hours so far. The LEDs
are two Nichia NSPW500BS white LEDs, and two white LEDs from Hong Kong
that I got off Ebay. All four are in series so there is no doubt that
they are all running at exactly the same current.

The two Nichias are going strong, with no noticeable signs of dimming.
The two Hong Kong LEDs, sad to say, are so dim that they wouldn't make
good panel lights, so can't even consider them suitable for
illumination. They're really quite dim.

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 jadney@vwtype3.org
Madison, WI 53711 USA
-----------------------------------------------

 

[Tim Wescott]

Jim Adney wrote:

On Mon, 5 Jul 2004 18:10:34 -0700 "Watson A.Name - \"Watt Sun, the
Dark Remover\"" <NOSPAM@dslextreme.com> wrote:


I I've had the 4 LEDs operating at 25 mA for all of May and June, and
the first few days of July. This is about 1500 hours so far. The LEDs
are two Nichia NSPW500BS white LEDs, and two white LEDs from Hong Kong
that I got off Ebay. All four are in series so there is no doubt that
they are all running at exactly the same current.

The two Nichias are going strong, with no noticeable signs of dimming.
The two Hong Kong LEDs, sad to say, are so dim that they wouldn't make
good panel lights, so can't even consider them suitable for
illumination. They're really quite dim.


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.

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Kevin McMurtrie]

In article <10ejul0jvd4ok4c@corp.supernews.com>,
"Watson A.Name - \"Watt Sun, the Dark Remover\""
<NOSPAM@dslextreme.com> wrote:

I I've had the 4 LEDs operating at 25 mA for all of May and June, and
the first few days of July. This is about 1500 hours so far. The LEDs
are two Nichia NSPW500BS white LEDs, and two white LEDs from Hong Kong
that I got off Ebay. All four are in series so there is no doubt that
they are all running at exactly the same current.

The two Nichias are going strong, with no noticeable signs of dimming.
The two Hong Kong LEDs, sad to say, are so dim that they wouldn't make
good panel lights, so can't even consider them suitable for
illumination. They're really quite dim.

Of the hundred Hong Kong LEDs I bought, all I've used have had this
dimming problem after a few hundredds of hours, so I decided to put the
four on a PS to control the conditions and verify that I'm not imagining
things. I think one problem might be that the Hong Kong LEDs use an
organic phosphor that degrades faster than Nichia's, and might
contaminate the LED chip. But I'm not an expert on this at all, so I'm
open to any other ideas.

I posted an update of the progress of the dimming at the beginning of
last month, and now this one shows that the hong kong LEDs are so dim
that it's probably not worth continuing the test. I may solder two more
unused hong kong LEDs into the circuit, and start the test again, just
for the halibut.

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.

 

[Rune Christensen]

Hello

I have found this link on eBay
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=2020&item=5106449490

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" <mcmurtri@dslextreme.com> skrev i en meddelelse
news:mcmurtri-9B28DE.23370306072004@corp-radius.supernews.com...

In article <10ejul0jvd4ok4c@corp.supernews.com>,
"Watson A.Name - \"Watt Sun, the Dark Remover\""
NOSPAM@dslextreme.com> wrote:

I I've had the 4 LEDs operating at 25 mA for all of May and June, and
the first few days of July. This is about 1500 hours so far. The LEDs
are two Nichia NSPW500BS white LEDs, and two white LEDs from Hong Kong
that I got off Ebay. All four are in series so there is no doubt that
they are all running at exactly the same current.

The two Nichias are going strong, with no noticeable signs of dimming.
The two Hong Kong LEDs, sad to say, are so dim that they wouldn't make
good panel lights, so can't even consider them suitable for
illumination. They're really quite dim.

Of the hundred Hong Kong LEDs I bought, all I've used have had this
dimming problem after a few hundredds of hours, so I decided to put the
four on a PS to control the conditions and verify that I'm not imagining
things. I think one problem might be that the Hong Kong LEDs use an
organic phosphor that degrades faster than Nichia's, and might
contaminate the LED chip. But I'm not an expert on this at all, so I'm
open to any other ideas.

I posted an update of the progress of the dimming at the beginning of
last month, and now this one shows that the hong kong LEDs are so dim
that it's probably not worth continuing the test. I may solder two more
unused hong kong LEDs into the circuit, and start the test again, just
for the halibut.


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.

---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.716 / Virus Database: 472 - Release Date: 05-07-2004

 

[Watson A.Name \"Watt Sun]

Tim Wescott wrote:

Jim Adney wrote:

On Mon, 5 Jul 2004 18:10:34 -0700 "Watson A.Name - \"Watt Sun, the
Dark Remover\"" <NOSPAM@dslextreme.com> wrote:

I I've had the 4 LEDs operating at 25 mA for all of May and June, and
the first few days of July. This is about 1500 hours so far. The LEDs
are two Nichia NSPW500BS white LEDs, and two white LEDs from Hong Kong
that I got off Ebay. All four are in series so there is no doubt that
they are all running at exactly the same current.

The two Nichias are going strong, with no noticeable signs of dimming.
The two Hong Kong LEDs, sad to say, are so dim that they wouldn't make
good panel lights, so can't even consider them suitable for
illumination. They're really quite dim.

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.

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).

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


[snip]

 

[Watson A.Name \"Watt Sun]

Kevin McMurtrie wrote:

In article <10ejul0jvd4ok4c@corp.supernews.com>,
"Watson A.Name - \"Watt Sun, the Dark Remover\""
NOSPAM@dslextreme.com> wrote:

I I've had the 4 LEDs operating at 25 mA for all of May and June, and
the first few days of July. This is about 1500 hours so far. The LEDs
are two Nichia NSPW500BS white LEDs, and two white LEDs from Hong Kong
that I got off Ebay. All four are in series so there is no doubt that
they are all running at exactly the same current.

The two Nichias are going strong, with no noticeable signs of dimming.
The two Hong Kong LEDs, sad to say, are so dim that they wouldn't make
good panel lights, so can't even consider them suitable for
illumination. They're really quite dim.

Of the hundred Hong Kong LEDs I bought, all I've used have had this
dimming problem after a few hundredds of hours, so I decided to put the
four on a PS to control the conditions and verify that I'm not imagining
things. I think one problem might be that the Hong Kong LEDs use an
organic phosphor that degrades faster than Nichia's, and might
contaminate the LED chip. But I'm not an expert on this at all, so I'm
open to any other ideas.

I posted an update of the progress of the dimming at the beginning of
last month, and now this one shows that the hong kong LEDs are so dim
that it's probably not worth continuing the test. I may solder two more
unused hong kong LEDs into the circuit, and start the test again, just
for the halibut.

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.

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.

 

[Warren]

Kevin McMurtrie <mcmurtri@dslextreme.com> wrote in message news:<mcmurtri-9B28DE.23370306072004@corp-radius.supernews.com>...

In article <10ejul0jvd4ok4c@corp.supernews.com>,
"Watson A.Name - \"Watt Sun, the Dark Remover\""
NOSPAM@dslextreme.com> wrote:

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
2.628 volts. Drawing 7.92 mils. LED is dimmer with this current draw.
Going to let it go util led won't light. Gave a quick check with fresh
batterys and led still very bright. Warren

I I've had the 4 LEDs operating at 25 mA for all of May and June, and
the first few days of July. This is about 1500 hours so far. The LEDs
are two Nichia NSPW500BS white LEDs, and two white LEDs from Hong Kong
that I got off Ebay. All four are in series so there is no doubt that
they are all running at exactly the same current.

The two Nichias are going strong, with no noticeable signs of dimming.
The two Hong Kong LEDs, sad to say, are so dim that they wouldn't make
good panel lights, so can't even consider them suitable for
illumination. They're really quite dim.

Of the hundred Hong Kong LEDs I bought, all I've used have had this
dimming problem after a few hundredds of hours, so I decided to put the
four on a PS to control the conditions and verify that I'm not imagining
things. I think one problem might be that the Hong Kong LEDs use an
organic phosphor that degrades faster than Nichia's, and might
contaminate the LED chip. But I'm not an expert on this at all, so I'm
open to any other ideas.

I posted an update of the progress of the dimming at the beginning of
last month, and now this one shows that the hong kong LEDs are so dim
that it's probably not worth continuing the test. I may solder two more
unused hong kong LEDs into the circuit, and start the test again, just
for the halibut.


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.

 

[Watson A.Name \"Watt Sun]

Rune Christensen wrote:

Hello

I have found this link on eBay
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=2020&item=5106449490

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

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.

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

Cheers
Rune

[snip]

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.

 

[Watson A.Name \"Watt Sun]

Warren wrote:

Kevin McMurtrie <mcmurtri@dslextreme.com> wrote in message news:<mcmurtri-9B28DE.23370306072004@corp-radius.supernews.com>...

In article <10ejul0jvd4ok4c@corp.supernews.com>,
"Watson A.Name - \"Watt Sun, the Dark Remover\""
NOSPAM@dslextreme.com> wrote:


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
2.628 volts. Drawing 7.92 mils. LED is dimmer with this current draw.
Going to let it go util led won't light. Gave a quick check with fresh
batterys and led still very bright. Warren

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.

 

[The Real Andy]

On Wed, 07 Jul 2004 07:19:50 -0700, "Watson A.Name \"Watt Sun - the
Dark Remover\"" <NOSPAM@dslextreme.com> wrote:

Tim Wescott wrote:

Jim Adney wrote:

On Mon, 5 Jul 2004 18:10:34 -0700 "Watson A.Name - \"Watt Sun, the
Dark Remover\"" <NOSPAM@dslextreme.com> wrote:


I I've had the 4 LEDs operating at 25 mA for all of May and June, and
the first few days of July. This is about 1500 hours so far. The LEDs
are two Nichia NSPW500BS white LEDs, and two white LEDs from Hong Kong
that I got off Ebay. All four are in series so there is no doubt that
they are all running at exactly the same current.

The two Nichias are going strong, with no noticeable signs of dimming.
The two Hong Kong LEDs, sad to say, are so dim that they wouldn't make
good panel lights, so can't even consider them suitable for
illumination. They're really quite dim.


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.

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).

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

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.

 

[Watson A.Name - \"Watt Su]

"The Real Andy"
<.pearson@wayit_dot_com_dot_au_remove_the_obvious_to_reply> wrote in
message newsnv6f0pef668eok0tekrei72hcc15flchi@4ax.com...

On Wed, 07 Jul 2004 07:19:50 -0700, "Watson A.Name \"Watt Sun - the
Dark Remover\"" <NOSPAM@dslextreme.com> wrote:

[snip]

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 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.

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

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.

 

[Mike Harrison]

On Tue, 13 Jul 2004 01:37:16 -0700, "Watson A.Name - \"Watt Sun, the Dark Remover\""
<NOSPAM@dslextreme.com> wrote:

"The Real Andy"
.pearson@wayit_dot_com_dot_au_remove_the_obvious_to_reply> wrote in
message newsnv6f0pef668eok0tekrei72hcc15flchi@4ax.com...
On Wed, 07 Jul 2004 07:19:50 -0700, "Watson A.Name \"Watt Sun - the
Dark Remover\"" <NOSPAM@dslextreme.com> wrote:

[snip]

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 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.

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

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.

 

[Graham W]

Mike Harrison wrote:

On Tue, 13 Jul 2004 01:37:16 -0700, "Watson A.Name - \"Watt Sun, the
Dark Remover\"" <NOSPAM@dslextreme.com> wrote:


"The Real Andy"
.pearson@wayit_dot_com_dot_au_remove_the_obvious_to_reply> wrote in
message newsnv6f0pef668eok0tekrei72hcc15flchi@4ax.com...
On Wed, 07 Jul 2004 07:19:50 -0700, "Watson A.Name \"Watt Sun - the
Dark Remover\"" <NOSPAM@dslextreme.com> wrote:

[snip]

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 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.

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

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 http://www.gcw.org.uk/ PGM-FI page updated, Graphics Tutorial
WIMBORNE http://www.wessex-astro-society.freeserve.co.uk/ Wessex
Dorset UK Astro Society's Web pages, Info, Meeting Dates, Sites & Maps
Change 'news' to 'sewn' in my Reply address to avoid my spam filter.

 

[R.Lewis]

"The Real Andy" <.pearson@wayit_dot_com_dot_au_remove_the_obvious_to_reply>
wrote in message newsnv6f0pef668eok0tekrei72hcc15flchi@4ax.com...

On Wed, 07 Jul 2004 07:19:50 -0700, "Watson A.Name \"Watt Sun - the
Dark Remover\"" <NOSPAM@dslextreme.com> wrote:

Tim Wescott wrote:

Jim Adney wrote:

On Mon, 5 Jul 2004 18:10:34 -0700 "Watson A.Name - \"Watt Sun, the
Dark Remover\"" <NOSPAM@dslextreme.com> wrote:


I I've had the 4 LEDs operating at 25 mA for all of May and June, and
the first few days of July. This is about 1500 hours so far. The
LEDs
are two Nichia NSPW500BS white LEDs, and two white LEDs from Hong
Kong
that I got off Ebay. All four are in series so there is no doubt
that
they are all running at exactly the same current.

The two Nichias are going strong, with no noticeable signs of
dimming.
The two Hong Kong LEDs, sad to say, are so dim that they wouldn't
make
good panel lights, so can't even consider them suitable for
illumination. They're really quite dim.


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.

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).

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


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.

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.

 

[Mike Harrison]

On Tue, 13 Jul 2004 18:34:36 +0100, "Graham W" <graham@his.com.puter.INVALID> wrote:

Mike Harrison wrote:
On Tue, 13 Jul 2004 01:37:16 -0700, "Watson A.Name - \"Watt Sun, the
Dark Remover\"" <NOSPAM@dslextreme.com> wrote:


"The Real Andy"
.pearson@wayit_dot_com_dot_au_remove_the_obvious_to_reply> wrote in
message newsnv6f0pef668eok0tekrei72hcc15flchi@4ax.com...
On Wed, 07 Jul 2004 07:19:50 -0700, "Watson A.Name \"Watt Sun - the
Dark Remover\"" <NOSPAM@dslextreme.com> wrote:

[snip]

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 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.

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

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.

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....

 

[Don Klipstein]

In article <onv6f0pef668eok0tekrei72hcc15flchi@4ax.com>, The Real Andy wrote:

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.

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, http://www.misty.com/~don/ledp.html

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 (don@misty.com)

 

[Don Klipstein]

In <40f429a4$0$92625$ed2619ec@ptn-nntp-reader02.plus.net>, Graham W wrote:

Mike Harrison wrote:
On Tue, 13 Jul 2004 01:37:16 -0700, "Watson A.Name - \"Watt Sun, the
Dark Remover\"" <NOSPAM@dslextreme.com> wrote:


"The Real Andy"
.pearson@wayit_dot_com_dot_au_remove_the_obvious_to_reply> wrote in
message newsnv6f0pef668eok0tekrei72hcc15flchi@4ax.com...
On Wed, 07 Jul 2004 07:19:50 -0700, "Watson A.Name \"Watt Sun - the
Dark Remover\"" <NOSPAM@dslextreme.com> wrote:

[snip]

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 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.

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

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.

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 (don@misty.com)

 

[Graham W]

"Don Klipstein" <don@manx.misty.com> wrote in message
news:slrncf8r3f.l7n.don@manx.misty.com...

In <40f429a4$0$92625$ed2619ec@ptn-nntp-reader02.plus.net>, Graham W
wrote:
Mike Harrison wrote:
...
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.

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, 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 http://www.gcw.org.uk/ PGM-FI page updated, Graphics Tutorial
WIMBORNE http://www.wessex-astro-society.freeserve.co.uk/ Wessex
Dorset UK Astro Society's Web pages, Info, Meeting Dates, Sites & Maps
Change 'news' to 'sewn' in my Reply address to avoid my spam filter.

 

[The Real Andy]

On Tue, 13 Jul 2004 23:10:15 +0000 (UTC), don@manx.misty.com (Don
Klipstein) wrote:

In article <onv6f0pef668eok0tekrei72hcc15flchi@4ax.com>, The Real Andy wrote:

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.

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, http://www.misty.com/~don/ledp.html

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 (don@misty.com)

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.