During the use of household appliances, they are often affected by the instability of the mains power, and their service life is reduced. In severe cases, they are easily burned due to voltage surges. The over-voltage automatic power-off described in this example can solve this problem very well.
1. The working principle of the circuit
The circuit principle is shown in Figure 35.
The 220V mains provides a stable 12V working voltage for the switching IC via C1, VD1, and DW1, and VD3, R2, and RP1 form a voltage-dividing sampling circuit. When the mains voltage is normal, DW2 cannot be turned on, the working voltage of pin ⑤ of TWH8778 is lower than 1.6V, the relay J does not pick up, and the mains supplies power to the CZ socket normally through the J-1 normally closed contact; when the mains voltage is high When it is in normal position, DW2 breaks down and conducts, and the potential of pin ⑤ of TWH8778 rises to 1.6V, causing the IC to turn over. Pin ③ outputs high level, the relay pulls in, and the power supply of the appliance is cut off immediately, thus avoiding overvoltage. Hazards from electrical appliances.
2. Component selection
C1 selects 0.47µ/400V electrolytic capacitor, relay J selects 6V DC contactor; RP selects ordinary trimming potentiometer, chip IC can use TWH8778 electronic switch or TWH8752 electronic switch.
3. Production and debugging method
After the device is welded correctly, connect the mains to the input terminal of the voltage regulator, cooperate with the voltage regulator and carefully adjust RP1, so that the relay J is closed when the voltage is 250V, and then connect the circuit to the mains grid to be normal Work.