An on-line monitoring method of board card current is presented

and a conveniently retractable coil which comprises two coils is designed as the current sensor. The two coils have the same frame structure and size

but have different number of turns. The coil with less turns is used to measure large current

and the coil with more turns is used to measure weak current. By cooperation of the two coils

accurate measurement of the current can be achieved in large dynamic range. In order to improve the measurement accuracy

an improved digital integration algorithm is proposed. The results show that the error of this system is not more than 0.7% in the current range from 0 A to 1 A

and not more than 0.21% in the current range from 1 A to 100 A.