A current transformer supplying a power for an electronic controller comprises two independent core magnetic circuits, wherein a first core magnetic circuit is a closed loop formed by connecting a U-shaped core and a linear core, a primary conductor extends through the closed loop, and a secondary winding for power supply is wound on the linear core; a second core magnetic circuit having an opening shape is disposed in parallel to the linear core of the first core magnetic circuit, and the open end of the second core magnetic circuit is coupled to the first core magnetic circuit through air gaps. The area of the cross section of the linear core is less than that of the cross section of the U-shaped core, so that the linear core can be magnetically saturated earlier than the U-shaped core. The centerline length of the U-shaped core is 1.5 to 4 times of that of the linear core. The current transformer of the present invention can not only normally start and work in case that a primary current is far lower than a rated current In, but also achieve the purpose of inhibiting rapid increase of an output current of the secondary windings and smoothing the output current in case that the primary current is far more than the rated current In.