design of temperature regulator

Discussion in 'Homework Help' started by assuc, Dec 14, 2013.

  1. assuc

    Thread Starter Member

    Mar 23, 2012
    77
    1
    Greetings,
    Could anyone help me explaining the following circuit
    lcs project error.jpg

    Actually i have been assigned to build a circuit with pi controller to maintain the temperature set point of 40C of an incubator. the above is proposed to be used as the first stage before pi controller.
    Explanation with mathematical relations would do great.
     
    Last edited by a moderator: Dec 15, 2013
  2. assuc

    Thread Starter Member

    Mar 23, 2012
    77
    1
    I have made the following pi controller design but in this the relay does not switches when temp goes above 40
    Untitled (1).png
    What could be the problem?? Please figure it out asap.
    REFERENCEhttp://www.ecircuitcenter.com/Circui...pid/op_pid.htm
     
    Last edited: Dec 25, 2013
  3. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    Your schematic is too small to see.
     
  4. assuc

    Thread Starter Member

    Mar 23, 2012
    77
    1
    Now it can be viewed
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,509
    2,369
    Must be getting smaller? ;)
    Max.
     
  6. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    How are the opamps powered?
    It can NOT be a single supply, as some of the inputs of the opamps are grounded.

    Bertus
     
  7. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    You don't need all those amplification stages. And what is the purpose of the U2 stage? You need one stage configured as a wheatstone bridge ( as the first stage in your textbook is configured ) and a transistor switch. Maybe one stage of amplification on the front end, but probably not.
     
  8. assuc

    Thread Starter Member

    Mar 23, 2012
    77
    1
    All opamps are poered via a single 12volt variable power supply and all the grounds of opamps are common
     
  9. assuc

    Thread Starter Member

    Mar 23, 2012
    77
    1
    Basically its a PI controller to control the temperature(as in this case 40 C) of the incubator box. U2 is the integrator whose output is summed up with the proportional controller output
     
  10. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    PI controllers don't work with relays. That's you first of many issues.
     
  11. assuc

    Thread Starter Member

    Mar 23, 2012
    77
    1
    so what would you propose to control the ac incandescent bulb which is working on 220v 50Hz
     
  12. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    I answered that in post #7.


    There are many schematics that implement this simple control method. Quickly searching "opamp wheatstone bridge" and "temperature controller" should produce a torrent of information.
     
  13. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    As I told you before: the inputs of the opamps can NOT be put to ground on a single supply.
    The opamps can NOT have any input voltages below the ground value.

    You will need to compare the output of the LM35 with a reference voltage.
    At 40 °C the output voltage of the LM35 will be 0.4 Volts.

    Adding a hysteresis will prevent the relays from scattering.
    On this page of the eBook you will find some info on this:
    http://www.allaboutcircuits.com/vol_3/chpt_8/12.html

    Bertus
     
    Last edited: Dec 26, 2013
  14. assuc

    Thread Starter Member

    Mar 23, 2012
    77
    1
    if that was the only case i wouldn't have posted here, its difficult for me to choose out of tons of schematics so this was the only source to seek infromation!
     
  15. assuc

    Thread Starter Member

    Mar 23, 2012
    77
    1
    The comparator to be placed before 1st stage opamp or where??
     
  16. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
Loading...