LM35 fridge thermostat

[CornelAndrew]

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

 

[(*steve*)]

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.