Find Hysteresis

Discussion in 'General Electronics Chat' started by dileepchacko, Aug 28, 2009.

  1. dileepchacko

    Thread Starter Active Member

    May 13, 2008
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    Can anybody find out the Hysteresis of the attached circuits. Consider the vltage source will ingreese from 0 to 270V.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,638
    2,343
    Hello,

    Here is the circuit you posted in the PDF:

    [​IMG]

    I assume the output of the comparator will go to 0 Volts when low and to + 5 Volts (=V2) when high.
    The upper switch limit will be when the output is high:
    The resistor R5 will be parallel to R2.
    The voltage at the input will be 5 * R1 / (R2||R5+R1)
    The lower switch limit will be when the output is low:
    The resistor R5 will be parallel to R1.
    The voltage at the input will be 5 * (R1||R5) / (R2 + (R1||R5))

    The hysteresys will be the difference between these values.

    Greetings,
    Bertus
     
    Last edited: Aug 28, 2009
  3. bertus

    Administrator

    Apr 5, 2008
    15,638
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    Hello,

    The calculations I gave will not work.
    The comparator has open drain output, so a resistor to the powersupply is needid.
    See datasheet.
    This resistor will add to the R5 when the high switch voltage is calculated.

    Greetings,
    Bertus
     
  4. dileepchacko

    Thread Starter Active Member

    May 13, 2008
    102
    1
    Thanks Bertus...
     
  5. hgmjr

    Moderator

    Jan 28, 2005
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    V_{\small{(-H)}}=\Large {{\frac{\frac{5}{20}}{{\frac{1}{20}}+{\frac{1}{1}}+{\frac{1}{68}}}}}\, \normal {=\,0.234807\,Volts}

    V_{\small{(+H)}}=\Large \frac{{\frac{5}{20}}+{\frac{5}{(68+R_{pu})}}} {{\frac{1}{20}}+{\frac{1}{1}}+{\frac{1}{(68+R_{pu})}}}

    Here are the two Millman's Theorem equations.

    Once you choose a value for Rpu, you can plug it into the second equation and solve for the positive hysteresis setpoint.

    hgmjr
     
    Last edited: Aug 28, 2009
  6. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    To restate what has been said a different way, think in terms of voltage dividers. When the two inputs are at the same voltage, that is when it will switch.

    Each leg with 2 resistors will divide the voltages to fixed values, which sets the hysteresis.
     
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