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comparator help (super newbie)

Discussion in 'Electronic Basics' started by SklettTheNewb, Apr 6, 2005.

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  1. I'm trying to understand the basic LM239 comparator. I have read
    several sites about them, looked at samples, read datasheet, etc, etc
    and I still am not getting logical results from my simulations.

    Here is the test circuit that I made using CircuitMaker, there are 2
    version where I switched the reference/input legs

    You will see in those images that the output voltages are either 15.8m
    (millivolt?) or 21p (I have no idea what that means)

    The reference and input voltages are 6v and 3v and I have tried
    flipping them, I never get output voltage of +9v (my supply to the

    I would really appreciate a tip what I've done wrong. I'm sure it's
    simple, but 2 hours now and I can't find it.

    Thanks for any help,
  2. Andrew Holme

    Andrew Holme Guest

    Try connecting the other end of R4 to +9V instead of ground.

    Look at the internal schematic on the LM339 datasheet. It has an
    "open-collector" output. It can pull the output pin down to ground, it
    can't pull up.

    Presumably, 21p means pico-volts i.e. 21e-12

  3. Thank you for the post, that actually did give me the result I wanted.
    I'm not completely sure why, but I guess that is my next item to
    investigate ;)

    Thanks again!
  4. John Fields

    John Fields Guest

  5. Got it!
    Circuit is working great now, thank you both for your time, I
    appreciate it.

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