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Principle and application of refrigerator thermostat

Date: 2013-11-29
Viewed: 30
案例名称: Principle and application of refrigerator thermostat

The electronic temperature controller not only has the same temperature characteristics as the pressure type thermostat, but also can easily change the temperature characteristics according to the requirements of the refrigerator manufacturer, eliminating the need for the pressure type thermostat to organize the production of new parts due to the change of temperature characteristics. The program speeds up the new product matching process and reduces the production cost. The electronic thermostat has a semi-automatic defrost function. The defrost heater can be manually activated as needed. When the set temperature is reached, the defrost is automatically stopped.

working principle

1.1 Power supply

As shown in Figure 1, the AC 220V is stepped down by the transformer TR1, and then rectified and filtered to output about 12V DC voltage, which is supplied to the compressor relay RC and the defrosting heating wire relay RH. At the same time, it is regulated by R20, D8 and C7. Approximately 6.8V DC voltage is supplied to the remaining logic control circuits.


1.2 Temperature control

The electronic temperature controller adopts a negative temperature coefficient (NTC) thermistor Rt1, Rt2 as a temperature sensing element, and its resistance value is about 3k8 at normal temperature (25 °C), and the normal working temperature range is about -60 to +100 °C. Between the epoxy resin and the metal casing, it is suitable to reduce the temperature sensitivity. It has the advantages of high sensitivity, small thermal inertia, high temperature resistance, linear resistance in a certain temperature range, and low price. Widely used in temperature control and testing.


The electronic thermostat logic control principle is shown in Figure 2. The opening and closing of the refrigerator compressor is controlled by the temperature of the refrigerating compartment. Rt1 (refrigerator chamber thermistor) is the refrigerating compartment temperature sensor. Rt1 and R19 form a voltage divider. With the change of the temperature of the refrigerator, the voltage of V5 (6) of IC1 (four voltage comparator LM339) changes accordingly. The voltage of pin 4 of IC1 is constant.


The voltage of pin 7 of IC1 is determined by the temperature adjustment potentiometer R4. When the potentiometer R4 is adjusted to the low gear (warm temperature of the thermostat), the equivalent resistance of R3 and R4 is R34=0.49k8. At this time,


When the potentiometer R4 is adjusted to the high level (the temperature controller is cold), at this time,


When the potentiometer R4 is adjusted to the middle position (the midpoint of the thermostat), the resistance of the R4 potentiometer is linearly changed. At this time, V′′(7)=2.3V can be calculated.

It can be seen from the voltage change value of pin 7 of IC1: V′′(7)=0.5(V(7)+V′(7))

Thus, by reasonably selecting the thermistor Rt1, the temperature of the thermostat shutdown can be changed linearly with the position of the potentiometer. When the refrigerator is energized, due to the higher temperature, the resistance of Rt1 is smaller, at this time due to V (6)>V(7), V(6)>V(4)

Therefore, IC1 outputs V(1) = '0' and V(2) = '1'.

At this time, IC2 (four or non-gate circuit CD4001) 4 pin output V02 = '1', the compressor relay RC pulls in, the refrigerator starts to cool. As the temperature decreases, the resistance of Rt1 increases, and V(6) follows. Decrease, when the temperature drops to about 4 ° C,

Rt1=6.7k8, at this time V(6)=4.1V

Since V(4)=4.1V is unchanged, V(6)V(7)

At this time, V(1)=“0”, V(2)=“0”.

Therefore, the output of IC2 V02 = '1' remains unchanged, and the refrigerator continues to cool.

As the temperature is further reduced, the resistance of Rt1 continues to increase. Assuming that the thermostat potentiometer is placed at the midpoint, when the temperature is lowered to about -20 ° C, Rt1 = 19.6 k8, at which time V (6) = 2.3 V.

Therefore V(6)

The output of IC1 is V(1) = '1' and V(2) = '0'.

At this time, the output of IC2 V02 = '0', the compressor relay RC is released, and the refrigerator stops cooling.

After stopping the cooling, the temperature in the refrigerator rises, the resistance of Rt1 decreases, and V(6) increases. At this time, V(6)>V(7)=2.3V (midpoint position), but V(6)

Therefore, the output of IC2 V02 = '0', remains unchanged, the compressor does not work. With the further increase of temperature, when reaching about +4 ° C, Rt1 = 6.7k8, V (6) = 4.1V.

Therefore, when V(6)>V(7), V(6)>V(4), IC1 outputs V(1)=“0”, V(2)=“1”. IC2 output V02 is inverted again. High voltage, compressor relay RC pull-in, refrigerator re-cooling. So repeatedly, the temperature at the refrigerating chamber probe is controlled to fluctuate between +4 and -20 °C.

It can be seen from the above analysis that the temperature control temperature (C/ON, W/ON) of the thermostat remains unchanged at +4 °C, and the shutdown temperature can be changed with the position of the thermostat potentiometer. The temperature characteristics and temperature adjustment The position of the potentiometer is shown in Figure 3. In this example:

C/OFF=-24°C, W/OFF=-16°C

N/OFF=-20°C, C/ON=W/ON=+4°C

Thus, the requirements of the WDF series thermostat constant temperature reset are achieved. By changing the resistance parameters, the temperature characteristics of the opening and stopping can be changed to meet the requirements of different users.

1.3 Semi-automatic defrost

The semi-automatic defrost control circuit consists of the other half of IC1 (four voltage comparator LM339) and its peripheral circuits. Analysis of IC1:


Normally, when the buttons AN101 and AN102 are not pressed: the voltage of 8 pins of IC1 V(8)=V(11)=5.65.6+3&TImes;6.8=4.43V.

IC1's 10-pin voltage V(10)=6.8V.

IC9's 9-pin voltage V(9) changes with the resistance value of the defrosting thermistor Rt2, the temperature rises, the resistance of Rt2 decreases, and V(9) increases; the temperature decreases, and V(9) decreases.

When the temperature of the freezer compartment is high, V(9)>V(8). At this time, even if the defrost button AN101 is pressed, since the output terminal V(14) of IC1 = '1', the output of IC2 V(01) ′′ “0”, the defrost relay RH is not conducting, so no defrost heating is performed at this time.

When the freezer compartment temperature is low, make V(9)V(11), then the output terminal V(13) of IC1 = '0', then the output terminal V01' of the IC2 = '0', remains unchanged, and will not Defrost heating.

At this time, if the AN101 button is pressed, V(10)=0

If the defrost stop button AN102 is pressed during the defrost heating process, then V(8)=V(11)=0.7V

After releasing the button AN102, the IC2 output V01'=“0” remains unchanged, and the cooling state is maintained.

It can be seen from the above analysis that the electronic thermostat realizes the semi-automatic defrost function.

It can be seen from the above analysis that the electronic temperature controller's starting temperature C/ON (W/ON)=4°C remains unchanged, the shutdown temperature varies between -16 and -24 °C, and the defrost reset temperature is about 6.5 °C. It has the requirements of constant temperature reset temperature characteristic and semi-automatic defrosting function. It has the characteristics of high temperature control precision, easy organization and quick design, and can change the corresponding resistance combination to change the startup, shutdown and defrosting temperature easily. Features. The electronic thermostat has been trial-produced in batches and tested by relevant manufacturers, and the market has responded well.

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