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How long will this LED last with this battery?

Discussion in 'General Electronics Chat' started by samakbass, May 9, 2012.

  1. samakbass

    samakbass New Member

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    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. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    The LR41 and LR44 batteries are 1.5 volt and will not light the LED at all.

    The C2032 is 3 volt and will light the LED. How much is unknown.

    We get many different types of C2032 batteries and the mAH ratings varies greatly depending on who makes them.

    Normally a resistor or some limiting device is a must but in the case of the C2032 the internal resistance of the battery is so high that I did once use the part with out a resistor. (not proud of that)

    The C2032 will not work well if the current is above 2mA! Probably the LED will pull the battery voltage down until you only get 1 to 2mA.

    A 3.2V LED and a 3.0V LED will act differently on a 3V battery!

    I don't know your LED but a 3.2V LED will pull 20mA, at 3.1V it will use maybe 2mA and at 3.0 volts maybe 200uA. Because the LED is off much below 3 volts the battery really does not get drained.

    On the low end a 3.0V LED has a similar effect but 0.2 lower.
     
  3. Ratchit

    Ratchit Well-Known Member

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    samakbass,

    Just look at the specifications. It says that the LED will last 100,000 hours. That is over 11 years. That assumes that you do not exceed the max current of the LED. Whichever battery you use is irrelevant.

    Ratch
     
  4. dave

    Dave New Member

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  5. ronv

    ronv Well-Known Member Most Helpful Member

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    The LR44 battery is only 1.5 volts and 120 ma hours so it won't light your led.
    The CR2032 is a 3 volt 220 ma hour battery so it might or might not light your led.
    These batteries are made to supply very low current so are not very good to light your led if you want it very bright for very long.
    How long do you want it to last? Does it have to be real small?

    I've got to type faster... :)
     
    Last edited: May 9, 2012
  6. samakbass

    samakbass New Member

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    I want the battery power to last for a few hours, maybe 4 hours. It does not have to be very small.
    What if I connected 2 or 3 LR44 batteries to use with it?
     
    Last edited: May 9, 2012
  7. ronv

    ronv Well-Known Member Most Helpful Member

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    If size is not important you could just use a 9 volt battery and a 330 ohm resistor.
    You should get around 30 hours.
    Easy to hook up to.;)
     
  8. ChrisP58

    ChrisP58 Well-Known Member

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    The brightness of an LED is dependent on the current through it. So a few questions need to be asked first.

    - How bright do you need the LED to be?
    - How much current does it take to get that much light from it?
    - Do you need the brightness to be constant over the full runtime?

    A resistor is used in LED circuits to control the current. But the current will only be constant if the applied voltage is fairly constant. If the voltage varies, the current, and therefor the brightness, will also vary.

    It is hard to know how much battery capacity to put into your system without knowing more details of what it needs to do.
     
  9. Mosaic

    Mosaic Well-Known Member

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  10. samakbass

    samakbass New Member

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    Those are Good points. When you say brightness, what measurement can brightness be measured in for LEDs? Do you mean in terms of mcd? The LED I used in the example have 20,000 mcd. I would like them to be running at anywhere from 15,000 to 20,000 mcd.
     
  11. audioguru

    audioguru Well-Known Member Most Helpful Member

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    20,000mcd LEDs that I have seen have a very narrow beam that makes them appear bright when shining directly at you but invisible when shining a little off angle. Almost like a laser beam. Is that what you want?

    Then why does the spec say 140 degrees viewing angle?
     
  12. colin55

    colin55 Well-Known Member

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    You can buy keyhole torches for $2.00 from junk shops. They use a 3v cell and a white LED connected directly to the battery. Mine lasted for 2 years.
     
  13. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    It was on or 2 years? or It was off for 2 years.

    Your LED needs 20mA to be bright. Small batteries will not output 20mA. If your battery could output 20mA and it had a 200mAH rating then 10 hours.
     
  14. colin55

    colin55 Well-Known Member

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    Not true:
     
  15. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    How is that "not true"?? Think about it colin. Do the math. If your LED was pulling 20mA from a battery that could source enough current, and had a capacity of 200mAH, theoretically it would last 10 hours. There is nothing wrong with that statement.
     
  16. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The problem is that a 200mAh battery is tiny and has its mAh rated at an extremely low current.
    Look at the datasheet for a CR2032 lithium coin cell battery. It is rated at 240mAh when its current is only 0.19mA.
    I don't know how many seconds it can light a 20mA LED which is almost a dead short to it.

    But the 120mAh and 160mAh tiny Li-Po batteries for my electric RC airplanes provide A FEW AMPS so the airplanes can fly straight up like a rocket for some manoeuvres and last about 10 minutes for each charge.
     
  17. DerStrom8

    DerStrom8 Super Moderator Most Helpful Member

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    Right, I understand that. I'm not talking about a real battery. I'm talking about the hypothetical one ronsimpson mentioned--one that could actually source enough current. If it could source 20mA and was rated for 200mAh, then it would last ten hours. I'm only verifying what he said. Not how real batteries work.
     
  18. duffy

    duffy New Member

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    I've done this before, too, after using smaller and smaller ballast resistors until finally realizing the internal resistance of the coin cell was so high it would barely hit 20ma with no external resistor and left it out.

    And I wish I hadn't. Not because there was any error to it, but because, for years afterwards, the guys in the studio who saw me make that stupid light were hooking up LEDs with no resistors up to every kind of battery and power supply and burning them out - then telling the boss they saw me do it that way.

    %#&@*!!!
     
  19. picbits

    picbits Well-Known Member

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    I made a prototype for a customer once which ran a white LED off 3 x SR41 cells. Space was really tight but I put a tantalum across the cells to provide a decent peak current and used a 10F200 to PWM the LED at an optimum current vs brightness ratio.

    I tested the unit for an hour on / few hours off. The total run time of a set of cells was over 30 hours (design spec was 10 hours +)
     
  20. Mosaic

    Mosaic Well-Known Member

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    pb: I am interested in your circuit. I have a 12f675 running an rgb led now and I'd like to use sr44 type batts to do it. Any advice on the run times I can achieve?

    I am seeing max drain @ around 24 mA with RGB on. Single colors are around 12mA. 12675 sleep is around 1/3 mA. I am using a Li ion AAA 3.7V cell atm. with dropping resistors based on the Vf of the RGB (3.2, 3.2,2.0)
     
    Last edited: Jun 9, 2012
  21. Mr RB

    Mr RB Well-Known Member

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    "Chinese Honesty"?

    The new oxymoron.
     

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