A center-tapped transformer includes tubular first and second spools, a primary winding unit, and first and second secondary winding units. The first spool has a first surrounding wall, a first pin-mounting wall, and a first pin set mounted on the first pin-mounting wall. The second spool has a second surrounding wall, a second pin-mounting wall, and a second pin set mounted on the second pin-mounting wall. The second surrounding wall has an inner diameter larger than an outer diameter of the first surrounding wall. The primary winding unit is wound on one of the first and second surrounding walls. The first and second secondary winding units are wound on the other one of the first and second surrounding walls.