Problem with LM339 comparator

Discussion in 'General Electronics Chat' started by samy555, Sep 22, 2015.

  1. samy555

    Thread Starter Active Member

    May 24, 2010
    116
    3
    Hello

    I built this comparator using LM339 but it didn't work

    [​IMG]

    When I add a resistor (R6 = 1K) between VCC and pin #2 (the output pin of the first comparator), it works good

    [​IMG]

    My question is: What is the role played by that resistance?

    thanks
     
  2. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
    1,215
    The LM339 comparators have an "open collector" output. They require a pull up resistor (R6). Google "open collector" for more info.
     
    samy555 likes this.
  3. crutschow

    Expert

    Mar 14, 2008
    13,055
    3,245
    Of course the LM339 data sheet will also show that the output is open collector (but who looks at a data sheet? :rolleyes:).
     
    samy555 likes this.
  4. Bordodynov

    Active Member

    May 20, 2015
    643
    188
    This comparator with an open collector. Additional resistor gives current sourcing. The scheme can be simplified. Make R6 = 10kOhm. Remove the resistor R4. The base of the transistor connect to the output of the comparator.
    LM339.png
     
    samy555 likes this.
  5. wayneh

    Expert

    Sep 9, 2010
    12,154
    3,060
    At least he supplied a schematic, which made the answer immediately obvious. ;)
     
    #12, Sinus23 and shortbus like this.
  6. samy555

    Thread Starter Active Member

    May 24, 2010
    116
    3
    Thank you very much
     
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