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Reason for Losing Brightness of the LED Over Time

02/12/2008 7:53 AM

Hi to all!

I have white LED whose size is 10 mm through hole

Which is supplied with 12v dc including 1k resistor in series.

After 1 month approx. its brigntness got lesser with time.

i don't kow the exact result . Does anybody has any clue regarding this behaviour?

Thanks and regards!

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Guru
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#1

Re: Reason for Losing Brightness of the LED Over Time

02/12/2008 8:50 AM

How did you measure the output of the LED?

Was it just an observable difference, which is subjective, or was it a measured difference?

Which ever it was the lifetime given for a LED on its data sheet may be to the 50% light output point, so maybe the LED looses a small amount of light output over the first 1000 hours or so...

If you have some facts and figures we could suggest reasons for the light loss... But all you've given is that its running at about 8 to 10 mA and its 10mm in size?

John.

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Guru

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#7
In reply to #1

Re: Reason for Losing Brightness of the LED Over Time

02/13/2008 8:43 PM

10 mA sounds a little low. I've always used 20 mA as a minimum for LEDs unless the actual specs were known. I have never noticed a degradation over many years in constantly used pcb test fixtures.

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Guru
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#2

Re: Reason for Losing Brightness of the LED Over Time

02/12/2008 9:02 AM

It depends how the white LED was manufactured. If it was with phosphors , It has a short life.

From here http://en.wikipedia.org/wiki/Light-emitting_diode#White_LEDs

"However the ultraviolet light causes photodegradation to the epoxy resin and many other materials used in LED packaging, causing manufacturing challenges and shorter lifetimes."

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Guru
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#3
In reply to #2

Re: Reason for Losing Brightness of the LED Over Time

02/12/2008 9:09 AM

Techno, I thought most white Leds were the phosphorous type?

John.

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#4
In reply to #3

Re: Reason for Losing Brightness of the LED Over Time

02/12/2008 10:57 AM

No, There is only one of at least five generations of white LEDs that use phosphors(read the link).

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#5

Re: Reason for Losing Brightness of the LED Over Time

02/12/2008 11:46 AM

I have had this happen to me in the past. For me the reason turned out to be that the LED was receiving to much voltage and ultimately "burned out" (if that is the right term). But more likely the LED is not receiving enough voltage causing the brightness to be reduced. http://www.mobilehandsetdesignline.com/206100814;jsessionid=4ROC2CKR4QSMQQSNDLQSKHSCJUNN2JVN?printableArticle=true "White LEDs are usually driven with a constant DC current source in order to maintain constant luminosity. In portable applications with single-cell Li-Ion source, the sum of the voltage drop across the white LED and the current source can be lower or higher than the battery voltage. This means that a white LED requires the battery voltage to be sometimes boosted. The easiest way to boost the battery voltage is to use a step-up DC/DC converter. This method significantly optimizes efficiency at the expense of cost and PCB area. An alternative method of boosting the battery voltage is to use a charge pump, also called switched capacitor converter. Let's analyze in more detail the principle of operation of such a device."

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#6

Re: Reason for Losing Brightness of the LED Over Time

02/13/2008 8:41 PM

Hi fireball,

If it really is losing brightness at the same applied current, then it could be ion migration which is what makes transistors lose gain.

S

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#8

Re: Reason for Losing Brightness of the LED Over Time

02/14/2008 4:31 PM

I'm not an expert on the present generations of LED technology, but: if it has only ever been supplied with 12-V DC via a 1k resistor, I can think of the following possibilities as starting points:

a) The pins of the LED are connected the reverse of what they should be (I understand that some types of LED will survive this for quite a while)
b) Poor contact (e.g. dry joint) degrading over time.
c) Environment got warmer
d) Immediate environment was too hot long term
e) The surface got dirty
f) The resistor has changed (does the power supply over-shoot on switch-on - I've seen more than a factor of two with some mediocre designs of supply? and it's not common, but some resistor types won't withstand even 100-mW long-term)
g) Faulty LED or resistor (even the best manufacturers can't find all defects, hence the so-called "bathtub" failure curves)
h) Background lighting levels have increased - the light output of the LED hasn't actually changed at all.
i) Many LED types are inherently somewhat leaky, and this can vary particularly rapidly at the start of life (while still staying within specification). If yours was designed for operation at more than 100-mA, you could see significant variation when operating it at a lower current

Fyz

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#9

Re: Reason for Losing Brightness of the LED Over Time

02/15/2008 12:10 AM

It would help if we had information on the LED... manufacturer... part number...

Having NO information on the part, my first guess is that it has a forward voltage of about 3 volts. This tells me that you have 9 ma of current through it. I would normally expect it to need 20 ma... so anything could happen but this is nothing but speculation on my part. With further information, we could then begin to help you solve your problem.

Bill

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