A coil of a transformer has two side conducting brackets and at least one middle conducting bracket. The middle conducting bracket is elongated or spiral-shaped and has two terminals respectively connected to the side conducting brackets. The side conducting brackets and the at least one middle conducting bracket of some specific type are connected to form different numbers of loops or different types of circuits (series circuits or parallel circuits). Therefore, design and fabrication of the coil is flexible and materials and manufacturing costs are saved. Moreover, the side conducting brackets and the at least one middle conducting bracket require only two or four soldered joints so number of soldered joints required is limited and efficiency of the transformer is improved.