Fridge Thermostat

What to do when the thermostat in your fridge doesn’t work any more? Get it repaired at (too) much expense or just buy a new one? It is relatively simple to make an electronic variation of a thermo-stat yourself, while saving a considerable amount of money at the same time. How-ever, be careful when working with mains voltages. This voltage remains invisible and can sometimes be fatal! 

This design allows for five temperatures to be selected with a rotary switch. By selecting suitable values for the resistors (R1 to R7), the temperatures at the various switch positions can be defined at construction time. With the resistance values shown here, the temperature can be adjusted to 16, 6, 4, 2 and –22 °C. 16° C is an ideal temperature for the storage of wine, while 6, 4, and 2 degrees are interesting for beer connoisseurs and the minus 22°degrees position transforms the fridge into a large freezer. Note for wine connoisseurs: to prevent mould on the labels, it is necessary to place a moisture absorber or bag of silica gel in the fridge. 

Circuit diagram :

Fridge Thermostat Circuit Diagram

The circuit is built around an old work-horse among opamps, the 741. D1 pro-vides a stable reference voltage of 5 V across the entire resistor divider. P1 allows adjustment of the voltage at the node of R1 and R2. To use the above-mentioned temperatures as setpoints this voltage needs to be adjusted to 2.89 V. D2 is a precision temperature sensor, which can be used from –40 to +100 °C. The voltage across this diode varies by 10 mV per Kelvin. In this way D2 keeps an eye on the temperature in the fridge. The reference voltage derived from the voltage divider (selected with S1) is com-pared by IC1 with the voltage across the temperature sensor. Based on this, the 741 switches, via the zero voltage crossing driver (IC2), a triac that provides volt-age to the compressor motor. The zero voltage crossing IC switches only at the zero crossings of the mains voltage, so that interference from the compressor motor is avoided when turning on. 

The power supply for the circuit is pro-vided by a simple bridge rectifier and filtered with two electrolytic capacitors of 220 µF each. 

The design can also be used for countless other uses. You can, for example, make a thermostat for heating by swapping the inputs of the opamp.

Keep in mind the safety requirements when building and mounting the circuit.

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