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LED Life when overrun

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

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  1. ef

    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
     
  2. CWatters

    CWatters Guest

    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. ef

    ef Guest

    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. 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. CWatters

    CWatters Guest

    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
     
  6. GotCoffee

    GotCoffee Guest

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