LED Life when overrun

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 !!

higher

any

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.

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.

Give these people a try, they know a lot about LED's and lights in general.

This reminds me of a project I've wanted to try for some time. Get an 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..=20

To reply, please remove one letter from each side of "@" Spammers are VERMIN. Please kill them all.

Why?

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

Percentage of generated photons escaping the chip as one reduces the ratio of refractive indices of the chip material to surrounding material.

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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 ( snipped-for-privacy@misty.com)