Instantaneous voltage reduction protecting device

The invention aims to be used in countries/regions where voltage change of commercial AC power is large and to reduce the repetition frequency of charging and discharging of an electrolytic capacitor so as to make the electrolytic capacitor to have a long service lifetime and a small capacitance. When 90-80% of fixed AC voltage is input, only a second semiconductor switch is communicated, and load is applied to the AC voltage which is increased by 10% by an auto-transformer. When the input AC voltage is 110% of fixed voltage, only a third semiconductor switch is communicated, and load is applied to the AC voltage which is decreased by 10% by the auto-transformer. If the input AC voltage is decreased to a value, less than 80% of the fixed voltage, all the switches will be disconnected, and the inverter can conduct DC/AC inversion, and alternative AC power can be supplied to the load within a short time.