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How long will this LED last with this battery?

Discussion in 'Power Electronics' started by samakbass, May 10, 2012.

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  1. samakbass

    samakbass

    1
    0
    May 10, 2012
    I have an Led with the following specifications:
    3-3.2 V
    Size (mm) : 5mm
    Lens Color : Water Clear
    Reverse Current (uA) : <=30
    Life Rating : 100,000 Hours
    Viewing Angle : 140 Degrees
    Absolute Maximum Ratings (Ta=25°C)
    Max Power Dissipation : 80mw
    Max Continuous Forward Current : 24mA
    Max Peak Forward Current : 75mA
    Reverse Voltage : 5~6V

    IF i use one LR41 battery connected to it, how long will it last? Do I really need to use a resistor for this setup?


    If I use one LR44 battery, how long will it last?

    If I use one CR2032 battery, how long will it last?



    How can I solve these problems?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,499
    2,839
    Jan 21, 2010
    First find data on these batteries indicating how much energy they store and how their voltage drops with time and load.

    To determine if a resistor is required, find out what their internal resistance is when fresh. Use this and their voltage to determine the LED current. If this does not exceed the maximum rating of the LED, you're OK.

    An exact answer will be tricky to arrive at since the LED current will fall off exponentially, never reaching zero. Consequently the LED will remain alight (theoretically) for ever. However, for practical purposes, you might decide on a certain minimum brightness equating to "off".

    Are you making "throwies?" If so, you're probably not too concerned with the life of the LED, as long as it exceeds the life of the battery. In this case you can probably overdrive the LED significantly as the damage you cause to it may be inconsequential (as much as I hate to say that).

    Naturally the battery must have a voltage > Vf of the LED. If not you need to connect several in series until they do.

    The actual current a LED draws is cases like this will vary somewhat from LED to LED and from battery to battery, and almost certainly with temperature.

    You can determine the answer by actually connecting a LED to a battery and timing it. I believe the batteries you're suggesting have fairly high internal resistance, so the current will be limited.

    Just don't try using the same approach with a power source having a lower internal resistance.
     
  3. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    LR41 and LR44 are 1.5V batteries. They will not light a 3V to 3.2V LED at all.

    CR2032 is 3V and could theoretically light it for about 9 hours, but the maximum continuous current it is specified to supply is 3ma, so it is really not an approriate battery for driving this LED except for short flashes and even then at only 15ma. The internal resistance would most likely limit it to less than that.

    Bob
     
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