Abstract: A transformer includes an iron core having a shaft around which a plurality of coil modules is mounted. Each coil module includes spiral, upper and lower coil sections each having an outer loop and a plurality of inner loops received in and coplanar with the outer loop. The inner loops of the spiral upper coil section and the inner loops of the spiral lower coil section include a common innermost loop through which the shaft extends. Upper and lower ends of each coil module are respectively interconnected to the outer loops of the upper and lower coil sections. The spiral upper section is superimposed on the spiral lower section to form a two-layer structure. The lower end of an upper one of two adjacent coil modules interconnected to the upper end of a lower one of the two adjacent coil modules.