case if the voltage eo exceeds the value of v u00d7 n 2 n 1 the current i n2 flows through the winding n2 in a direction to decrease the exciting current in the winding n1 the magnetizing current i n2 is always maintained constant by a circuit arrangement consisting of the transistor tr1 a resistor r4 and a zener diode d1 zener voltage v z as shown in fig1 the relation thereof is expressed by the following equation 2 equ2 v be is the base emitter voltage on the transistor tr1 as has been described above in the present invention the circuit arrangement in which the zener diode d1 is used maintains the demagnetizing current i n2 constant during the period of time t1 so that the exciting current in the main winding n1 is not affected that is the magnetic flux density in the core 2 is demagnetized by the predetermined amount always during the time t1 supposing that the demagnetized amount is designated by u03b4b1 the following equation 3 is established n2 is the number of turns of n2 and u03bc is the magnetic permeability of magnetic material as is apparent from the above description the present invention is constructed such that the voltage eo exceeds the voltage induced by the auxiliary winding n2.