Introduction to the Principle of Refrigerator Protection and Saving Device Designed with 89C2051

This refrigerator protection energy saver has the functions of low protection, overvoltage power control, etc. After trial, it is stable and reliable. This controller can also be used for other home appliances control with a slight modification.

Figure 1 is the electrical schematic of the controller. The voltage comparison circuit composed of the operational amplifier LM324 is used to detect whether the power supply voltage is lower than 180V, higher than 230V and 240V, and send it to the microcontroller for processing.

V4C forms a zero-crossing detection circuit, and the square wave obtained at the {8}th pin of V4C is shown in Figure 2. As shown in Figure 2, the rising and falling edges of the square wave output by the {8}th pin of V4C are the AC zero-crossing points, and the signal is sent to the P3.7 port of the microcontroller for processing.

U7 and others form a thyristor trigger circuit. U7 is a MOS3041, which has zero-crossing detection and related circuits inside. Zero-crossing triggering is used here mainly to reduce harmonic interference. R25 and C5 are thyristor protection circuits. R24 is the thyristor trigger resistor, and its value is determined by the formula R = 220 (root 2) / I (A) Ω, where I (A) is the nominal capacity of the thyristor. It should be noted that if the microcontroller detects the zero point before controlling U7 to trigger the thyristor, U7 cannot detect the zero point because the AC zero point has passed, so it needs to be processed in the program, that is, the trigger waveform shown in Figure 3 is output from P3.7 to trigger normally.

The main procedures to achieve this process are as follows:

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ＬＣＡＬＬ Ｄｅｌａｙ ３ｍｓ
ＪＮＢ Ｐ３．２?Ｓ
ＬＣＡＬＬ Ｄｅｌａｙ ３ｍｓ
ＳＥＴＤ Ｐ３．７
ＬＡＬＬ Ｄｅｌａｙ ３ｍｓ
……
ＣＬＲ Ｐ３．７
ＪＢ Ｐ３．２?Ｓ
ＬＣＡＬＬ Ｄｅｌａｙ ３ｍｓ
ＳＥＴＢ Ｐ３．７
ＬＣＡＬＬ Ｄｅｌａｙ ３ｍｓ
……
ＬＪＭＰ ＭＡＩＮ
……

Why do we use the above method to trigger the thyristor? Because this method can control the power of the motor when it detects that the voltage is higher than 230V or 240V, avoiding the increase of output power and overheating of the motor when the grid voltage rises.

The specific method is: when the power supply voltage is higher than 230V or 240V, a sine wave is discarded after several sine waves. The number of discarded sine waves is determined by the voltage level, but multiple sine waves cannot be discarded continuously, otherwise the motor will run unbalanced. The discarded sine waves are in a certain ratio to the retained sine waves. When it is necessary to discard 2 out of 10 sine waves, the method of discarding 1 out of 5 is adopted. When the voltage is lower than 180V, the thyristor is stopped from being triggered to protect the motor.

The buzzer is used to indicate the status, the digital tube is used to display the status, the 24C04 is used to save the setting data, and the buttons are used to make various settings.
When the power is interrupted for a short time, as long as the program detects that P3.2 has no rising or falling edge for more than 20ms, it means that the power is off, and the program controls the delay of 5 to 7 minutes before triggering the power on.

Line connection: M1 is the refrigerator motor. Cut one of the wires and connect it to points A and B. Only the motor wiring is changed, and the other wiring remains unchanged, which will not affect the operation of the refrigerator.

Since the circuit is controlled by a single-chip microcomputer, this circuit can also be used for refrigerators with failed thermostats. In this case, you only need to modify the program. By modifying the program, it can also be used for electric frying pan temperature control, electric stove temperature control, electric blanket temperature control, holiday pattern lights, air conditioning control, etc.