Comparator circuit not comparing

Discussion in 'The Projects Forum' started by tom7891, Mar 8, 2014.

  1. tom7891

    Thread Starter New Member

    May 16, 2011
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    Hi, i'm currently building a comparator circuit using a lm324n op amp.
    The problem is, i get an output even when the voltage at the negative terminal is greater than at the positive terminal. I have 6.2v at the positive and 6.5v at the negative. I'm just wondering how accurate these op amps are, because i've simulated the circuit on multisim and it works fine.
    [​IMG]
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    They are fantastically accurate for what you are doing. Check your wiring.
     
  3. #12

    Expert

    Nov 30, 2010
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    I also can't find anything wrong with the diagram.
     
  4. tom7891

    Thread Starter New Member

    May 16, 2011
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    Thanks for the reply. I'll have another check.
     
  5. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    380

    hi tom,
    Recalc the voltage at the junction of the 10K and 2Meg, when supplied with 9 volts.

    E

    EDIT:
    Remember your voltmeter could be loading the -Vin as it will be connected across a 2meg, so it could read lower than expected.
     
  6. ScottWang

    Moderator

    Aug 23, 2012
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    What the P.12(+) that you also connecting to where?
    What's value of R15?
     
  7. crutschow

    Expert

    Mar 14, 2008
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    The data sheet states that the maximum input voltage should be no greater the 1.5V below the supply voltage or 7.5V.
    According to your schematic the (-) input is well above that.
     
    #12 likes this.
  8. #12

    Expert

    Nov 30, 2010
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    Darn, missed again.:mad:
     
  9. tom7891

    Thread Starter New Member

    May 16, 2011
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    0
    Thanks for all the help. It is all wired correctly, i think you're right about the input voltage being too high for the op amp chip. I must have been looking at another parameter when i selected it. I'll have a look for a replacement.
     
  10. crutschow

    Expert

    Mar 14, 2008
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    Why do you need the reference voltage to be so close the the supply voltage?
    If so, then you need a rail-rail input/output type op amp such as an LT1637.
     
  11. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Why can't you simply reduce (e.g. halve) both the + and - inputs to the opamp so that they are within spec?
     
  12. ScottWang

    Moderator

    Aug 23, 2012
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    You can do a testing as :
    R11 - 10K
    R12 - 10K
    R13 - 9.1K
    R15 - 1K + VR10K (1K connecting to +9V, VR10 connecting to R13)
     
  13. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,542
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    Good question. The reference input is 99.5% of Vcc. Is it really the goal of this circuit to detect when the other input is between 8.955V and 9.000V? What is happening in those 45 millivolts?

    ak
     
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