Want to make an accurate electronic thermostat for your refrigerator? The circuit of a solid state thermostat described in this article will surprise you with its “cool” performance.

Introduction


The unit once built and integrated with any relevant appliance will instantly start exhibiting an improved control of the system saving electricity and also increasing the life of the appliance.

Conventional refrigerator thermostats are expensive and not very accurate. Moreover these are prone to wear and tear and therefore not permanent. A simple and much efficient electronic refrigerator thermostat device is discussed here.

A thermostat as we all know is a device which is able to sense a particular set temperature level and trip or switch an external load. Such devices can be electromechanical types or more sophisticated electronic types.

Thermostats typically are associated with air conditioning, refrigeration and water heating appliances. For such applications the device becomes a critical part of the system without which the appliance may reach and start operating under extreme conditions and ultimately get damaged.

Adjusting the control switch provided in the above appliances ensures that the thermostat cuts off power to the appliance once the temperature crosses the desired limit and switches back as soon as the temperature returns to the lower threshold.

Thus the temperature inside refrigerators or a room temperature through an Air conditioner is maintained to favorable ranges.

The circuit idea of a refrigerator thermostat presented here can be used externally over a refrigerator or any similar appliance to control its operation.

Controlling their operation can be done by attaching the sensing element of the thermostat to the external heat dissipating grid normally situated behind most cooling devices that use Freon.

The design is more flexible and wide ranged compared to the built-in thermostats and is able to exhibit better efficiency. The circuit can easily replace the conventional low tech designs and moreover it’s much cheaper compared to them.

Let’s understand how the circuit functions:

Simulation and Working

fridge thermosatt using IC 741 and PTC thermistor


The diagram alongside shows a simple circuit built around the IC 741, which is basically configured as a voltage comparator.A transformer less power supply is incorporated here to make the circuit compact and solid-state.

A bridge configuration comprising R3, R2, P1 and the NTC R1 at the input forms the main sensing elements of the circuit.

The inverting input of the IC is clamped at half the supply voltage using a voltage divider network of R3 and R4.

This eliminates the need of providing a dual supply to the IC and the circuit is able to produce optimum results even through single pole voltage supply.

The reference voltage to the non-inverting input of the IC is fixed through the preset P1 with respect to the NTC (Negative Temperature Coefficient.)

In case the temperature under check tends to drift above the desired levels, the NTC resistance drops and the potential at non-inverting input of the IC crosses the set reference.

This instantly toggles the output of the IC, which in turn switches the output stage comprising transistor, triac network, switching off the load (heating or the cooling system) until the temperature reaches the lower threshold.

The feedback resistor R5 to some extent helps to induce hysteresis into the circuit, an important parameter without which the circuit may keep flip-flopping quite rapidly in response to the sudden temperature changes.

Once the assembly is completed, setting up the circuit is very simple and is done with the following points:

REMEMBER THE ENTIRE CIRCUIT IS AT AC MAINS POTENTIAL, SO EXTREME CAUTION IS ADVISED WHILE GOING THROUGH THE TESTING AND THE SETTING PROCEDURES. USE OF A WOODEN PLANK OR ANY OTHER INSULATING MATERIAL UNDER YOUR FEET IS STRICTLY RECOMMENDED; ALSO USE ELECTRICAL TOOLS WHICH ARE THOROUGHLY INSULATED NEAR AND AROUND THE GRIPPING AREA.

How to Setup this Electronic Refrigerator Thermostat Circuit


You will need a sample heat source accurately adjusted to the desired cut-off threshold level of the thermostat circuit.

Switch on the circuit and introduce and attach the above heat source with the NTC.

Now adjust the preset so that the output just toggles (the output LED comes on.)
Remove the heat source away from the NTC, depending upon the hysteresis of the circuit the output should switch off within few seconds.

Repeat the procedure many times to confirm its correct functioning.

This concludes the setting up of this refrigerator thermostat and is ready to be integrated with any refrigerator or similar gadget for an accurate and permanent regulation of its operation.

Parts List


R1=10k NTC,
R2=Preset 10K
R3,R4=10K
R5=100K
R6=510E
R7=1K
R8=1M
R9=56 OHM/1watt
C1=105/400V
C2=100uF/25V
D2=1N4007
Z1 = 12V, 1 watt zener diode

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