LM35 fridge thermostat

Discussion in 'The Projects Forum' started by CornelAndrew, Nov 6, 2012.

  1. CornelAndrew

    Thread Starter New Member

    Nov 6, 2012
    2
    0
    Hello,

    I need to replace the mechanical fridge thermostat which is not working. Because i can’t find a similar model, i decided to build this schematic.

    http://www.escol.com.my/Projects/Project-03(Thermostat-1)/Proj-03.html

    It is working well, but only for positive temperatures. I need the schematic to operate from -5 Celsius to +4 Celsius (to use VR1 to se the temperature inside the fridge in the range of -5 Celsius +4 Celsius).

    The schematic is using LM35DZ (0 Celsius to 100 Celsius). I’m using LM35CZ (-55 Celsius to +150 Celsius). What else should i modifiy?

    Because i’m using a 5,2v stabilized power supply, i operated the following modifications:

    1.ZD1, R6 are out. R5 is 550 ohm.
    2.VR1 is 5K instead of 2,2K (i couldn’t find a 2,2K pot)

    I measured the voltage in some points. At 22 Celsius:

    TP1> 1,42v
    Pin2 of LM35 > 1,48v
    Pot hot wire > 1,42v

    At -2 Celsius
    TP1> 1,90v
    Pin2 of LM35 > 1,27v
    Pot hot wire > 1,90v

    Datasheet
    http://www.ti.com/lit/ds/symlink/lm35.pdf
     
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    I recall they needed a pull down resistor (to a more negative potential) to swing the output negative. That would be from pin 2 on your LM35 to the negative 12V line in the schematic. Accuracy will be poor because your negative "reference" is just two diodes. Note: all of the signal levels are with respect to "ground" in the circuit which is NOT the negative 12V line.
     
    Last edited: Nov 6, 2012
  3. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    bountyhunter is right.
    In the datasheet this is given to measure negative temperatures:

    [​IMG]

    A negative voltage is a needed to measure negative temperatures.

    Bertus
     
  4. CornelAndrew

    Thread Starter New Member

    Nov 6, 2012
    2
    0
    Thank you. So if i'm using a 5,2v supply, what should be the value of R1? I don't know how to calculate ...
     
  5. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    Unless you use a LM34DZ, which will go well below 0°C.
     
    Last edited: Nov 6, 2012
  6. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    The LM34 is calibrated in degrees fahrenheit.
    The LM35 is calibrated in degrees celcius.

    If you want to use the LM34 the setpoints need to be recalculated.
    (fahrenheit to celcius conversion).

    Bertus
     
  7. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,998
    745
    R1 would be 1.2V/ 50uA =24K nearest value is 22K.

    The LM35 gives out 10mV per 'C so at say 5C it will give out 50mV, once it goes below zero C, it become resistive and so you need to provide a resistor to pull it down,
     
    Last edited: Nov 6, 2012
  8. crutschow

    Expert

    Mar 14, 2008
    13,024
    3,236
    The accuracy does not depend upon the accuracy of the negative voltage. The negative voltage is just to bias the low impedance output stage so it can generate a negative output voltage.
     
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