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single op-amp window comparator

Discussion in 'Electronic Design' started by Lee, May 6, 2005.

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  1. Lee

    Lee Guest

    the usual circuit design of a window comparator uses two
    op-amps...does anyone know about a single op-amp window comparator
    circuit...tnx in advance
     
  2. Pooh Bear

    Pooh Bear Guest

    Yup, you can do it with a couple of extra diodes. It's not as accurate (
    and needs to oversome the Vf of the diodes ) but may suffice. Don't
    have a schematic handy though.


    Graham
     
  3. Ban

    Ban Guest

    with a rectifier in front of the comparator you could do that, but you loose
    the ability to randomly choose the thresholds.
     
  4. Ken Smith

    Ken Smith Guest

    Not really. You loose a lot of accuracy because of the diode drops but
    for moderatly large values of random, you can get any values you want like
    this:

    If you make this circuit and its top to bottom flipped version, the Vx
    voltages are determined by R1 and R2 or Vin depending on Vin.

    !
    /
    \ R1
    /
    \
    !
    Vin -!<--+
    V
    ---
    +-------- Vx
    !
    /
    \ R2
    /
    \
    !


    Further resistor dividers on the Vx and Vx' signals can give you whatever
    dead band you want. As a result, you can set the deadband and the center
    point.
     
  5. Guest


    The inputs to the opamp go where?
    Vx is?


    NT
     
  6. Ken Smith

    Ken Smith Guest

    The top to bottom mirror image goes to the other op-amp input.
     
  7. Jamie

    Jamie Guest

    a simple hack version comes to mind.

    using an OP-AMp from a +&- rail supply to get a
    0 volt output into a mini bridge rectifier to convert
    the signal to always + or - what ever.

    by setting setting one input of the Op-amp to a voltage
    and using the other to monitor the external source voltage,
    one can then adjust the gain using a loop back into the -
    input and adjust the + input for the comparator center point.
    the idea is to produce no voltage or low voltage at the
    output as the reference voltage starts to get near the set
    voltage..
    basicly we are looking at a simple OP-amp circuit that you
    can set the + input using a dual rail supply.

    the output can drive some schmitt
    trigger or mechanical relay etc..

    like i said, just a hack in thinking of course.
     
  8. Lee,
    you should ask for a single resistor solution.
    :)

    Rene
     
  9. Ken Smith

    Ken Smith Guest

    Thats easy:


    T \----------------------
    ---O
    !
    ---------------------------
    Vin ------/\/\/\----VMiddle


    When the voltage is far enough from Vmiddle to be either voltage, it heats
    up the thermstat and it closes.
     
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