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Quad comparator not comparingPlease Help

  1. Jan 24, 2010 #1

    Im having problems with my circuit, I am using a quad comparator IC (LM339N) but for some reason the output its generating does not seem to change with either changing inverting or non inverting inputs.

    I mean, the output, remains same even when the non inverting input (+) is less than, inverting one.

    the reference voltage, is inverting one, which I have grounded(also pin12 grounded with it), and with supplying VCC to +ve one(non inverting) , and to Pin 3 of the LM339.

    Please Help !

    Faraz Nizamani
  2. jcsd
  3. Jan 24, 2010 #2
    Here is a link to a datasheet for this device: http://www.national.com/ds/LM/LM139.pdf [Broken]

    Now here are a few things to check:

    This device is open-collector, so that it needs a pull-up resistor at the output: have you fitted one?

    The common-mode input voltage range has limits 0V to V+ -1.5V: are you respecting that? Possibly not if your reference voltage is 0V; it may be preferable to have a positive reference voltage.

    Check all the other connections, and if the IC is in a socket be sure that all pins have seated correctly.

    Finally, if all else fails perhaps you have a defective IC. Try another one.
    Last edited by a moderator: May 4, 2017
  4. Jan 24, 2010 #3


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    I believe the LM339 has a open collector output. Try putting a 3kohm pull-up resistor on it. Take a look at the datasheet:"www.national.com/ds/LM/LM339.pdf"[/URL]

    Oops. Adjuster beat me to it.
    Last edited by a moderator: Apr 24, 2017
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