The idea was not quite appreciated by Mr Gustavo and he asked me to design a refrigerator thermostat circuit which could sense the cold temperatures inside the fridge, rather than the hot temperatures at the rear of the fridge.
So with some effort I could discover the present CIRCUIT DIAGRAM of a refrigerator temperature controller, let's learn the idea with the following points:
The concept is not very new, neither unique, it's the usual comparator concept that has been incorporated here.
The IC 741 has been rigged in its standard comparator mode and also as a non inverting amplifier circuit.
The NTC thermistor becomes the main sensing component and is specifically responsible for sensing cold temperatures.
NTC means negative temperature coefficient, meaning the resistance of the thermistor will rise as the temperature around it falls.
It must be noted that the NTC must be rated as per the given specs otherwise the system will not functions as intended.
The preset P1 is used for setting the tripping point of the IC.
When the temperature inside the fridge falls below the threshold level, the thermistor resistance becomes high enough and reduces the voltage at the inverting pin below the non-inverting pin voltage level.
This instantly makes the output of the IC go high, activating the relay and switching OFF the fridge compressor.
P1 must be set such that the opamp output becomes high at around zero degree Celsius.
A little hysteresis introduced by the circuit comes as a boon or rather a blessing in disguise, because due to this the circuit does not switch rapidly at the threshold levels rather responds only after the temperature has risen to about a couple degrees above the tripping level.
For example suppose if the tripping level is set at zero degrees, the IC will trip the relay at this point and the fridge compressor will be also switched OFF, the temperature inside the fridge now starts rising, but the IC does not switch back immediately but retains its position until the temperature has risen at least upto 3 degrees Celsius above zero.