The utility model relates to a current transformer technical field that is correlated with is a controlling means for adjusting current transformer transformation ratio of taking a percentage more, and it includes that coupling draw -out power supply and transformation ratio switch control circuit, and coupling draw -out power supply can the unit including getting, rectification filter circuit and voltage conversion circuit, get can the unit the output be connected with rectification filter circuit's input, rectification filter circuit's output is connected with voltage conversion circuit's input. The utility model discloses a coupling draw -out power supply not only provides the power but also can real -time?supervision be surveyed the electric current size for the transformation ratio switches control circuit, further switches control circuit adjustment current transformer transformation ratio through the transformation ratio, adopts the emitter follower who has the positive feedback than switching control circuit, and the break -make through the control triode changes resistive?voltage?divider and compares, constitutes the positive feedback, makes voltage comparater upset back status pole for stable.