This fully automatic refrigerator protector has simple circuit, complete functions, reliable and sensitive action, and is a useful accessory in the use of refrigerators.
working principle
The circuit principle is shown in Figure 1 (click here to download the schematic). When the voltage of the 220V mains grid is too low, which may damage the refrigerator motor, the voltage divider value of R1 and RP1 decreases, turning on VD5, turning on the integrated circuit IC, and then turning off VD7, followed by the bidirectional thyristor VS Turn off, thereby cutting off the power to the refrigerator. Adjust RP1 to set the undervoltage protection value.
When the grid voltage is too high, it may cause the refrigerator motor to overheat and burn. This is the increase in the voltage divider value of R2 and RP2, which turns on the transistor VT, and then VD6 turns on, and then IC1 turns on, turning off VD7, and the thyristor VS turns off. Adjust RP2 to set the overvoltage protection value.
R7 and C2 constitute the power-off delay start circuit. When the power is turned on immediately after the power is turned off, the voltage on both ends of C2 cannot be abruptly changed, so that IC1 ② pin protection is turned on at a low level, thereby turning off VD7 and VS, which protects the refrigerator compressor. The delay time is determined by the values ​​of R7 and C2. According to the figure, the delay time is about 6 minutes.
See the table below for component selection and component list.
| Numbering | name | model | Quantity |
| R1, R2, R3 | resistance | 10K 1 / 8W | 3 |
| R4, R5 | resistance | 270Ω 1 / 8W | 2 |
| R6 | resistance | 100Ω 1 / 8W | 1 |
| R7 | resistance | 4M 1 / 8W | 1 |
| C1, C2 | Electrolytic capacitor | 470u / 25V | 2 |
| C3 | Polyester capacitor | 0.1u / 600V | 1 |
| RP1, RP2 | Trimming resistor | 1K | 2 |
| VD1ï¼VD7 | rectifier diode | IN4001 | 7 |
| VT | Crystal transistor | Switch type such as 3DK2C, etc. | 1 |
| VS | Bidirectional thyristor | 8ï¼10A ≥600V | 1 |
| IC | High-power switch IC | TWH8751 | 1 |
| T | Power Transformers | Secondary voltage 15V power 5W | 1 |
The production and debugging of this protector are very simple, so it will not be described in detail. When adjusting RP1 and RP2, the AC voltage regulator can be used to simulate the change of the mains voltage.
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