# LED Life when overrun

Discussion in 'Electronic Basics' started by ef, Mar 9, 2005.

1. ### efGuest

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

2. ### CWattersGuest

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).

3. ### efGuest

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 !!

4. ### Anthony FremontGuest

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.

5. ### CWattersGuest

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