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LM35 fridge thermostat

Discussion in 'General Electronics Discussion' started by CornelAndrew, Nov 6, 2012.

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  1. CornelAndrew


    Nov 6, 2012

    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.

    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

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    This cannot read temperatures less than zero because there is no pull-down on the output of the LM35. Even with this, the reference voltage cannot go negative.

    See the datasheet.
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