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Re: Dead LEDs
by Spehro Pefhany
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Quoting Apptech <apptech@...>:
> Getting White LEDs with long lifetimes is easy. Getting them
> at a low cost is another matter.
>
> I've been testing various white LEDs for longevity and been
> getting some rather bad results from Chinese sourced
> products. Reports from others testing LEDs for the same
> purpose are similar. This is not a total surprise as the
> general received wisdom is that this is the case. It is
> however a somewhat surprise as it is not obvious why the
> Chinese products should tend to be so bad. A Chinese
> supplier (name will not be stated) even made some changes to
> try and meet my spec and the results were no better.
>
> By Chinese I mean companies that are Chinese based - NOT
> known internationals who are domiciled elsewhere but may use
> Chinese manufacturing (eg Avago, Cree, Nichia, ...) - such
> companies are demonstrably more liable to get it right.
>
> The universal claim is that white LEDs last 100,000 hours. I
> can assure you that many don't come anywhere close (by 2+
> orders of magnitude in some cases).
>
> My questions are:
>
> - What mechanism makes Chinese LEDs so bad?
Probably some technology they are not using. ;-)
> - Why is this allowed to be? ie why don't they do whatever
> it takes to fix it.
Because it costs more to develop, buy or license the technology than it's
worth to them. In which case you may find (as I have in some cases) that
you're better off to buy the name brand products that have been forced
down in price by the shoddy stuff.
> If anyone feels that my statements are a generalisation and
> that some Chinese white LEDs do have the sort of lifetimes
> one would expect then *PLEASE* do tell me the brands!. I'd
> be extremely happy to be wrong and to be able to source LEDs
> at non-market-leader prices.
>
> For white phosphor LEDs (blue radiator and yellow phosphor
> re-radiator) the degradation mechanism seems to be actual
> LED die output level. A possible mechanism in some cases MAY
> be die over-temperature due to excessive over-rating of die
> current capabilities. Phosphor death does not seem to be an
> issue in what I have seen. (it is in some other cases). Die
> bonding adhesive to the LED structure cup is claimed to be a
> problem in some cases but when this was changed in the LEDs
> I was getting to a Japanese sourced bonding product of good
> parentage it made about zero difference.
There are more than one type of phosphor used in the blue-LED-plus-yellow
phosphor configuration. Here is one commercial supplier:
http://www.mt-berlin.com/frames_cryst/descriptions/led_phosphors.htm
> Interestingly, and not directly related, some name brand
> LEDs give atrocious spectral results at very low currents
> while others are about as good across a wide range of
> currents.
>
>
>
> Russell
Out of curiosity, have you tried blue LEDs from the same suppliers?
I'd be interested if they show a similarly short life.
Blue (and white) LEDs have a relatively high voltage drop compared to
other types such as superbright red, so the power dissipation at the
junction can be twice as high at the same If.
>
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
s...@... Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
> Getting White LEDs with long lifetimes is easy. Getting them
> at a low cost is another matter.
>
> I've been testing various white LEDs for longevity and been
> getting some rather bad results from Chinese sourced
> products. Reports from others testing LEDs for the same
> purpose are similar. This is not a total surprise as the
> general received wisdom is that this is the case. It is
> however a somewhat surprise as it is not obvious why the
> Chinese products should tend to be so bad. A Chinese
> supplier (name will not be stated) even made some changes to
> try and meet my spec and the results were no better.
>
> By Chinese I mean companies that are Chinese based - NOT
> known internationals who are domiciled elsewhere but may use
> Chinese manufacturing (eg Avago, Cree, Nichia, ...) - such
> companies are demonstrably more liable to get it right.
>
> The universal claim is that white LEDs last 100,000 hours. I
> can assure you that many don't come anywhere close (by 2+
> orders of magnitude in some cases).
>
> My questions are:
>
> - What mechanism makes Chinese LEDs so bad?
Probably some technology they are not using. ;-)
> - Why is this allowed to be? ie why don't they do whatever
> it takes to fix it.
Because it costs more to develop, buy or license the technology than it's
worth to them. In which case you may find (as I have in some cases) that
you're better off to buy the name brand products that have been forced
down in price by the shoddy stuff.
> If anyone feels that my statements are a generalisation and
> that some Chinese white LEDs do have the sort of lifetimes
> one would expect then *PLEASE* do tell me the brands!. I'd
> be extremely happy to be wrong and to be able to source LEDs
> at non-market-leader prices.
>
> For white phosphor LEDs (blue radiator and yellow phosphor
> re-radiator) the degradation mechanism seems to be actual
> LED die output level. A possible mechanism in some cases MAY
> be die over-temperature due to excessive over-rating of die
> current capabilities. Phosphor death does not seem to be an
> issue in what I have seen. (it is in some other cases). Die
> bonding adhesive to the LED structure cup is claimed to be a
> problem in some cases but when this was changed in the LEDs
> I was getting to a Japanese sourced bonding product of good
> parentage it made about zero difference.
There are more than one type of phosphor used in the blue-LED-plus-yellow
phosphor configuration. Here is one commercial supplier:
http://www.mt-berlin.com/frames_cryst/descriptions/led_phosphors.htm
> Interestingly, and not directly related, some name brand
> LEDs give atrocious spectral results at very low currents
> while others are about as good across a wide range of
> currents.
>
>
>
> Russell
Out of curiosity, have you tried blue LEDs from the same suppliers?
I'd be interested if they show a similarly short life.
Blue (and white) LEDs have a relatively high voltage drop compared to
other types such as superbright red, so the power dissipation at the
junction can be twice as high at the same If.
>
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>
Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
s...@... Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
--
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
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