The circuit is shown in the following figure, and BT33 constitutes a delay circuit. When the power supply is just turned on, the relay J 1 does not work, and the contact J 1-2 is disconnected. Five minutes later, BT33 was turned on due to the rising charging voltage of capacitor C 1, relay J 1 was attracted, contact J 1-2 was closed, and refrigerator was turned on by J 1. When the voltage is normal, D2 and BG2 are turned on, while D3, D4 and BG3 are turned off, so relay J2 does not work. When overvoltage occurs, the voltage regulator D4 will be broken down and turned on, and BG3 will be turned on at the same time. The relay J2 will be attracted, the contact J2- 1 will be disconnected, and the relay J 1 will be released when it loses power to protect the refrigerator. When the power supply voltage is lower than the allowable value of the refrigerator (180V), D2 and BG2 change from on to off, and the collector voltage of BG2 rises, which makes D3 and BG3 on, and the relay J2 is turned on, which also turns off the refrigerator.