The traditional controllable saturation reactor (CSR) consists of single toroidal core, DC (direct current) controlled loop (including DC controlled winding and DC biasing source) and AC (alternating current) excitation loop (including excitation winding and AC source). A detection winding and secondary winding are added up to the CSR configuration and form a hybrid DC comparator. Research shows the terminal voltage of the detection winding is asymmetric waveform when the secondary winding of the comparator is open and the CSR core is stimulated both by AC and DC biased sources. Both theory analysis and experiment verify the feasibility of the differential RMS (root-mean-square) between positive and negative half waves of the terminal voltage from the detection winding fitted for the feedback variance to balance DC biasing magnetic potential and form a self-balancing comparator. The zero-flux technique that the primary ampere-turn is equal to the secondary is the function base for the comparator. The operation details of the comparator including the control characteristics both of open-loop and close loop, the satiability judgment criterion, static error property and test range are introduced. The experimental results testify to the truth of the principle of the proposed DC comparator.