Low -cost area keeps apart dc bus undervoltage protection circuit of auto -lock relates to undervoltage protection technical field. Including sampling circuit, voltage stabilizing circuit, optical coupling isolation circuit, signal latch circuit and the acousto -optic warning protection circuit that connects gradually, sampling circuit is connected with the dc bus for the voltage acquisition of dc bus. The utility model discloses a dc bus circuit, sampling circuit, voltage stabilizing circuit, the application of optical coupling isolation circuit, signal latch circuit and acousto -optic warning protection circuit has realized the isolation auto -lock protection the when dc bus is under -voltage, simple circuit structure, low cost.