Continuous standard load (mA) 0.2 seems really low.

Discussion in 'General Electronics Chat' started by RyanD, Feb 11, 2010.

  1. RyanD

    Thread Starter Member

    Jan 14, 2010
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    I'm trying to power a very small circuit so I'm looking into coin batteries but they do not seem to be able to put out that much of a load. I mean, looking at that I would appear that I couldn't even light an LED since they require 20ma. Am I looking at this right, how to circuits use these if they provide such a small amount of continuous load?
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Coin cells will supply much more than 0.2mA; just not for very long!
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    The reason it can't light an LED is voltage, while LEDs require current to light up you must exceed their forward dropping voltage Vf.

    Try 2 or 3 coin cells stacked on the LED, and be prepared for the LED to possibly blow.

    Have you read this?

    LEDs, 555s, Flashers, and Light Chasers
     
  4. RyanD

    Thread Starter Member

    Jan 14, 2010
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    I want it to power a 555 and when the timer runs out(every 2 minutes or so) blink the led. Would something like this work, I tried finding what was required to run the 555 but there are so many numbers in that sheet I got lost. Here is the battery I'm looking at.

    http://datasheet.octopart.com/CR2032-Panasonic-datasheet-48591.pdf
     
  5. Wendy

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    Mar 24, 2008
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  6. RyanD

    Thread Starter Member

    Jan 14, 2010
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    Thanks for the tutorials but those don't help me answer my question if the coin battery could power something like this. In your project you're using two AAA batteries which would easily power it.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    An ordinary 555 has a minimum supply voltage of 4.5V and uses a supply current of 3mA to 6mA when it is doing nothing. With a 4.5V supply its output high voltage is only 2.5V.
    The tiny battery cell is only 3V when it is new with no load and its standard current is only 0.3mA. It cannot power your circuit.
    Your LED needs 3.5V if it is white or blue at 25mA to be fairly bright.

    LED keychain lights use three 1.5V button alkaline cells in series (4.5V). They cannot provide much current so their internal resistance limits the current to about 20mA when the cells are new. They can shine for about 3 minutes continuously but are dimming slowly the entire time.

    I have two chaser projects that use 74HCxxxx high speed Cmos logic ICs to blink LEDs very brightly for months. One is powered from two AA alkaline cells and the red 1.8V LEDs still blink when their total voltage has dropped to only 1.5V. At only 1.5V the red 1.8V LEDs are not bright but the circuit is still operating.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    You are quibbling about batteries, when you ought to be reading, or do you think 3V from coin cells is somehow different than from AAA batteries?

    These project are designed for extra low current for long life, which is exactly what you were asking for. They will work where a conventional 555 circuit won't.
     
    Last edited: Feb 11, 2010
  9. hgmjr

    Moderator

    Jan 28, 2005
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    Be aware that the 555 comes in two flavors. There are bipolar versions and CMOS versions. Since you will be using battery power, you will need to make sure that you purchase the CMOS version.

    If you have any doubt about the correct part number, be sure to have someone here to confirm that you have chosen correctly before you commit your hard earned dollars.


    hgmjr
     
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