Analysis of the Principle of Overvoltage Protection Circuit for Smart Home Appliances

　　If the grid voltage suddenly rises and exceeds 250V, the voltage at the midpoint of RP will cause VDW to break down and conduct. After VDW conducts, the transistor VT is turned on. After VT conducts, the voltage drop between the collector and the emitter is very small, which is not enough to trigger VS, causing VS to cut off. Therefore, the home appliances in the socket XS are powered off and stop working, thus achieving the purpose of protection. Once the grid voltage drops, VT is cut off again, and the collector potential of VT increases, triggering VS to conduct, and the home appliances are powered on and continue to work.

　　Component Selection

　　R resistor 5.1K1, RP potentiometer 15K choose multi-turn precision potentiometer1, C1 metallized paper dielectric capacitor 0.47uF withstand voltage ≥400V1, C2 electrolytic capacitor 100uF/25V1, VD1, 2 rectifier diode IN40072, VDW voltage regulator diode 12V 2CW121, VT crystal transistor 3DA87C, 3DG12, etc.1, VS bidirectional thyristor 6-10A withstand voltage ≥600V1, CZ power socket 10A 250V1

　　The debugging of the device is very simple. When the grid voltage is 220V, adjust RP so that VDW does not break down. When the voltage rises to 250V, VT is saturated and turned on. It is more convenient to use a voltage regulating transformer to simulate the changes of the mains during debugging.