LED Life when overrun

E

ef

Guest
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
 
"ef" <gef@dh.invalid> wrote in message
news:1gt4xwz.10d69idg9f6bmN%gef@dh.invalid...
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
Click to expand...
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).
 
"ef" <gef@dh.invalid> wrote in message
news:1gt5g65.18aa3iv1bfrq26N%gef@dh.invalid...

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 !!
Click to expand...
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.
 
"ef" <gef@dh.invalid> wrote in message
news:1gt5g65.18aa3iv1bfrq26N%gef@dh.invalid...

So, higher consumption, brighter output, lower life, within limits, any
sugestions as to quantative analysis, no other response needed !!
Click to expand...
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
 
Give these people a try, they know a lot about LED's and lights in
general.
http://www.candlepowerforums.com/
 
"CWatters" <colin.watters@pandoraBOX.be> wrote:


This page has a graph for the "Half life time of Luminous Intensity" vs
Junction temperature....

This reminds me of a project I've wanted to try for some time. Get an
Click to expand...
unencapsulated LED and submerge it in liquid nitrogen, with a pump
create a strong flow across the LED, to prevent bubbles from forming.
The idea is to see how bright I can get it if all the heat is removed.
What I haven't been able to find is LED's, mounted on the frame, bond
wire attached, but no encapsulating plastic..

To reply, please remove one letter from each side of "@"
Spammers are VERMIN. Please kill them all.
 
"Doug Warner" <dwarner22@ccharter.net> wrote in message
news:9q3231h8nq0se2edaj8cl94r7730ec4ifm@4ax.com...
"This reminds me of a project I've wanted to try for some time.

Why?
 
CWatters <colin.watters@pandoraBOX.be> wrote:

"ef" <gef@dh.invalid> wrote in message
news:1gt4xwz.10d69idg9f6bmN%gef@dh.invalid...
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).
Click to expand...
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 !!
 
Doug Warner wrote:

This reminds me of a project I've wanted to try for some time. Get an
unencapsulated LED and submerge it in liquid nitrogen,
Click to expand...
ISTR a bare chip is not a very good light transmitter. Index of
refraction and all that.

In any case some of the plastic can be dissolved.. OTOH a lot of the
heat can be extracted through the metal leads anyway.




Thomas
 
In article <4231e796$0$32634$756600cd@cachenews.cambrium.nl>, Zak wrote:
Doug Warner wrote:

This reminds me of a project I've wanted to try for some time. Get an
unencapsulated LED and submerge it in liquid nitrogen,

ISTR a bare chip is not a very good light transmitter. Index of
refraction and all that.
Click to expand...
Percentage of generated photons escaping the chip as one reduces the
ratio of refractive indices of the chip material to surrounding material.

-----------------

BTW - LED chips do give more light per unit current as temperature
decreases. In my experience this tends to happen more with:

* Longer wavelength types

* Lower voltage drop types

* Types where the voltage drop is low for the wavelength

And less with ones where the efficiency at "ordinary temperature" is so
high as to have less upward mobility.

- Don Klipstein (don@misty.com)
 
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