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Thread: can anyone calculate this 4 me?

  1. #1
    daz
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    can anyone calculate this 4 me?

    You can give me the equation, but that won't help because my math skills are just about virtually non existent. So i really need an answer to the question rather than the tools to arrive at it. So please understand why i'm asking for the answer to this. Doesn't have to be close, just a ballpark estimate. Using a 2032 3V button battery, about how long would a 20 ma LED stay lit ? Second question, what do low current LEDs draw and how long would that last? Assuming low current LED are what, 2 mA?

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    2032 batteries are rated at about 200mAH, that is if you discharge them at a constant rate of 1mA they will last 200 hours (if you are lucky) because 1mA x 200 Hours = 200mAH. If you discharge at 20mA they should last 10 hours because 20mA x 10 Hours = 200mAH but they won't because they are only designed for low current. I think you'd be better of with the low current LED running at 1mA (if it's bright enough). Try a 1k series resistor to run the LED at 1mA. The LED Vf is about 2V and the battery is 3V so you need to drop 1V at 1mA which is 1k.
    Last edited by Dave H; 03-21-2018 at 09:12 PM.

  3. #3
    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by daz View Post
    You can give me the equation, but that won't help because my math skills are just about virtually non existent. So i really need an answer to the question rather than the tools to arrive at it. So please understand why i'm asking for the answer to this. Doesn't have to be close, just a ballpark estimate. Using a 2032 3V button battery, about how long would a 20 ma LED stay lit ? Second question, what do low current LEDs draw and how long would that last? Assuming low current LED are what, 2 mA?
    We can't tell from the 2032 datasheet as the cell is specified for very low current or intermittent (pulse) operation. Under those conditions the capacity is at best 200mAH i.e. it will last 200 hours if you draw 1mA. Now you want 20mA so the life will at very best 200/20 =10 hours. Now, since this is high current and continuous you'll get a lot less than that. My advice it to hook one up and time it. Then you'll know for sure in just a few hours.
    Experience is something you get, just after you really needed it.

  4. #4
    Better Tone thru Mathematics bob p's Avatar
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    It would help to know what you're trying to accomplish. IME a 2032 doesn't last all that long for supporting a flashing LED. I've used battery powered flashing LEDs in the past as signal/marker lights and in those applications we used a 9V battery to get better service life.
    "Stand back, I'm holding a calculator." - chinrest

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  5. #5
    daz
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    It's just a LED to indicate the status of a latching switch used to switch a amp function that itself will not support a LED. But theres no room for a larger battery, hence the button battery. But 200 hours is a lot. 1/2 that would be fine. So i'll try a resistor ala Dave's suggestion and meter it to see what it draws. Thanks all.

  6. #6
    daz
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    Worked as you said....LED with 1k draws a hair over 1 ma.

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