LED Life when overrun
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Posted by ef on March 9, 2005, 5:39 am
Please log in for more thread options Any pointers to LED life when over-run, ie superbright white led usually run at 20ma, what life can be expected at say 40ma, 80ma ?? Cause of over-run, higher voltage than 3.3v | |||||||||||||||||||||||||
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Posted by CWatters on March 9, 2005, 12:09 pm
Please log in for more thread options LEDs aren't normally connected directly to a voltage source so your expression "Cause of over-run, higher voltage than 3.3v" is a little odd. To connect an LED to a battery with voltage Vbat use a resister in series.. R= (Vbat-Vled)/I example Vbat = 12V Vled = 3.3V I=20mA then R = (12-3.3)/0.02 = 435 (or the nearest standard value to this). | |||||||||||||||||||||||||
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Posted by ef on March 9, 2005, 12:16 pm
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> > Any pointers to LED life when over-run, ie superbright white led usually
> > run at 20ma, what life can be expected at say 40ma, 80ma ?? > > > > Cause of over-run, higher voltage than 3.3v >
> LEDs aren't normally connected directly to a voltage source so your > expression "Cause of over-run, higher voltage than 3.3v" is a little odd. > > To connect an LED to a battery with voltage Vbat use a resister in series.. > > R= (Vbat-Vled)/I > > example > > Vbat = 12V > Vled = 3.3V > I=20mA > > then > > R = (12-3.3)/0.02 > = 435 (or the nearest standard value to this). I know, I know, but I prefer the higher light output, when run at higher voltage, and am prepared to sacrifice most of 100,000 hours life to achieve this. I do not need the leds to function after I'm dead !! So, higher consumption, brighter output, lower life, within limits, any sugestions as to quantative analysis, no other response needed !! | |||||||||||||||||||||||||
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Posted by Anthony Fremont on March 9, 2005, 1:03 pm
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> I know, I know, but I prefer the higher light output, when run at
higher
> voltage, and am prepared to sacrifice most of 100,000 hours life to
any
> achieve this. > > I do not need the leds to function after I'm dead !! > > So, higher consumption, brighter output, lower life, within limits, > sugestions as to quantative analysis, no other response needed !!
Basically (much like an incandescent bulb) slightly greater perceived brightness will result in a dramatically shortened life. If you insist upon exceeding the specs, you will be "rewarded" with a mediocre increases in brightness followed by dimming of the bulb (deteriorating loss of efficiency) and eventual bulb failure. I suggest you do your own experiments. Start by doubling current over the max spec and then work your way down. You won't have to wait anywhere near 100,000 hours for quantitative results. ;-) Understand that LED's are not voltage operated devices, they are current operated. You should never apply a voltage without some kind of resistance to limit the current. | |||||||||||||||||||||||||
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Posted by CWatters on March 9, 2005, 7:16 pm
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> So, higher consumption, brighter output, lower life, within limits, any
> sugestions as to quantative analysis, no other response needed !! This page has a graph for the "Half life time of Luminous Intensity" vs Junction temperature.... Quote: Regarding the luminous intensity (IV) which is the main characteristic of LEDs, the half-life (time when the luminous intensity has been reduced to 50% of the initial value) obtained from the accelerated operating life test, is estimated as shown in Figure 3-2. http://www.marktechopto.com/engineering/visible.cfm | |||||||||||||||||||||||||
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> run at 20ma, what life can be expected at say 40ma, 80ma ??
>
> Cause of over-run, higher voltage than 3.3v