Thermal control with hysteresis

Discussion in 'General Electronics Chat' started by abuyaser, May 7, 2011.

  1. abuyaser

    Thread Starter New Member

    Apr 20, 2011
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    I have built a thermal control circuit but I am trying to add the hysteresis part into it without being successful. I am using the AD22100 temp sensor and the LM393N comparator single power supply.

    Note: the npn transistor is actually inside the comparator

    I need to have it built with hysteresis but can not do it correctly

    Any help is appreciated
     
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Please post a schematic of your circuit.
     
  3. abuyaser

    Thread Starter New Member

    Apr 20, 2011
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    0
    Here is the attached file, it did not upload last time. I did voltage reference for 40 degrees Celsius and want the hysteresis to be between 40 and 35 degrees Celsius.
     
  4. ifixit

    Distinguished Member

    Nov 20, 2008
    639
    108
    Hi abuyaser,

    Q1 collector (LM393 output simulation) should not be inverted with respect to the + input.

    Calculating the value of R5, without changing your circuit configuration, made my head hurt. Since there are two 393’s in a pack, re-design your circuit as shown in the attached schematic. With this setup you can easily change the temperature set points.

    Check my calculations in case I misinterpreted something. The output driving the fan can be changed to whatever you like.

    Regards,
    Ifixit
     
    Last edited: May 8, 2011
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I don't get it. Abuyaser's circuit uses an LT1017, not an LM393. I guess it is because he didn't have a spice model for LM393.
    Ifixit, I'm not sure what your circuit is supposed to do, but negative feedback around a comparator (U1a) will generally cause oscillation. Maybe you have the input pins swapped?

    EDIT: It makes sense if you swap the input pins on U1a.
     
    Last edited: May 8, 2011
  6. ifixit

    Distinguished Member

    Nov 20, 2008
    639
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    Hi Ron-H,

    Yes you are right about the inputs being the wrong way, I made one too many edits before posting:rolleyes:. I'll replace the drawing.

    Thanks,
    Ifixit
     
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