comparator switching prematurely

Discussion in 'General Electronics Chat' started by elRey, Jul 7, 2009.

  1. elRey

    Thread Starter Active Member

    Feb 23, 2009
    65
    0
    comparator switching prematurely

    I have the circuit below. The comparator works as expected when ref (set by P2) is below ~3.6V. However, when ref is above 3.6V, the comparator still switches when Vin (set by P1) reaches 3.6V.

    What am I doing wrong?

    [​IMG]

    Thanks,
    Rey
     
  2. elRey

    Thread Starter Active Member

    Feb 23, 2009
    65
    0
    more info. DC and the comparator is a LM324 (which I'm now reading is a op-amp).
     
  3. steveb

    Senior Member

    Jul 3, 2008
    2,433
    469
    Why are you powering the OPAMP through a 1k ohm resistor. This just reduces the suppy voltage and allows power load surges to modulate the voltage.

    Anyway, it is likely that you are hitting the maximum allowed common mode voltage range. The specification says that the input range is zero to Vcc-2V, although you may see Vcc-1.5V typically. This would put the voltage max at 4 V to 4.5 V with a 6V supply, but I'm estimating that your 1K resistor is dropping abut 0.7 V which brings you down to the 3.3V to 3.8 V common mode input range according to specs.
     
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