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LM324 as a comparator

Discussion in 'General Electronics Discussion' started by bpm, Jul 18, 2013.

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  1. bpm

    bpm

    1
    0
    Jul 18, 2013
    Hi all,

    TLDR
    Can an LM324 op-amp be used as a comparator (for <2.5kHz frequencies)?
    What is the simplest way of making sure an LM339 comparator is working?




    I'm trying to build a circuit which needs a comparator. Since the circuit already has an op-amp chip for filtering and gain, I figured I'd use one of the spare op-amps on that chip. So first question, can an LM324 op-amp be used as a comparator (for <2.5kHz frequencies)?

    I ask simply because however I try it, it does not seem to work. So I bought a dedicated comparator chip, an LM339. And this makes even less sense. I attached the 5V supply and ground, and then (just as the most direct test I could think of) put 5V on the + input and ground on the - input. This produced nothing on the output. After much time fiddling, I accidentally knocked the general ground out and at this point I got a potential difference of about 3V from the output pin to ground. This also makes no sense to me, because I though the comparator should deliver 0V or 5V. Plugging the ground back in, this disappeared. I established by trial and error that it didn't matter which pin the chip was grounded by, it always killed the output signal.

    Please help, I think I am missing something very obvious, because what I am trying is really not that complex!

    Thanks.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,496
    2,837
    Jan 21, 2010
    The LM339 has an open collector output. This means you need a pull-up resistor from Vcc to the output. Without this it will flop around like a piece of wet spaghetti. (And that's what you're seeing)

    I don't see why the op-amp wouldn't work, however you need to keep the input signals within the valid range for the device (and you can find that in the datasheet). As I have just reminded myself, that range is quite large.

    How did the LM324 not work? What does it do that you don't expect?
     
  3. john monks

    john monks

    693
    2
    Mar 9, 2012
    If the LM324 is used as an amplifier and another section is used as a comparator you might get some cross-talk.
     
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