I need to replace the mechanical fridge thermostat which is not working. Because i canít 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). Iím using LM35CZ (-55 Celsius to +150 Celsius). What else should i modifiy?

Because iím 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 couldnít 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