1. An RJ45 connector comprising: a housing; a plurality of contacts fastened in the housing for mating with a plug; a plurality of terminals for connecting an electrical device; and a magnetic module electrically connecting the contacts and the terminals to form a plurality of signal channels carrying differential signal pairs, the magnetic module having a printed circuit board (PCB) and a transformer mounted thereon, wherein the transformer is connected in one of the signal channels and comprises: an H-shaped magnetic core defining a horizontally longitudinal direction, a horizontally transverse direction and a vertical direction mutually perpendicular to each other, the H-shaped magnetic core having a body portion extending along the horizontally longitudinal direction, a first and a second walls perpendicular to the longitudinal direction and respectively connected to longitudinal opposite ends of the body portion, the first and the second walls each having a plurality of electrode pads disposed along the transverse direction on a bottom side thereof; and a magnetic board covering across the body portion and supported by the first and second walls, a primary coil having a first magnetic wire and a second magnetic wire; and a secondary coil having a third magnetic wire and a fourth magnetic wire; wherein each of the first to fourth magnetic wires has a middle portion winding around the body portion, a first end and an opposite second end connected to the electrode pads, the electrode pads soldered onto the PCB so that the first and the second magnetic wires are serially connected to form an output, an input and a center tap of the primary coil, the third and the fourth magnetic wires are serially connected to form an output, an input and a center tap of the secondary coil; wherein the output and input of the primary coil are respectively electrically connected to two of the contacts through some circuits of the PCB, and the output and input of the secondary coil are respectively electrically connected to two of the terminals through some other circuits of the PCB; wherein each of the first to fourth magnetic wires is made from a copper wire having a diameter of 0.027 to 0.05 millimeters coated with an insulating paint, the body portion having a longitudinal length greater than twice of a product of the diameter and the turn number, the primary coil having a direct-current resistant of no more than 5 ohms between the input and the output of the primary coil and an inductance of 150 micro henries at a frequency of 100 KHz, and the secondary coil having a direct-current resistant of no more than 5 ohms between the input and the output of the secondary coil and an inductance of 150 micro henries at a frequency of 100 KHz.