Abstract: A transformer comprising: a coil bobbin which includes a pair of core winding portions and a pair of coupling portions for coupling the core winding portions so as to space the core winding portions a predetermined distance from each other and is formed, on its whole outer periphery, with a groove; a winding which is obtained by winding a conductor around the groove of the coil bobbin a predetermined number of times; and a pair of wound cores each of which is obtained by winding an electromagnetic steel plate around each of the core winding portions of the coil bobbin a predetermined number of times; wherein an outer peripheral surface of each of opposite side walls of the groove at the core winding portions is curved so as to have an arcuate cross-sectional shape.

Abstract: A transformer comprising: a coil bobbin which includes a pair of core winding portions and a pair of coupling portions for coupling the core winding portions so as to space the core winding portions a predetermined distance from each other and is formed, on its whole outer periphery, with a groove; a winding which is obtained by winding a conductor around the groove of the coil bobbin a predetermined number of times; and a pair of wound cores each of which is obtained by winding an electromagnetic steel plate around each of the core winding portions of the coil bobbin a predetermined number of times; wherein an outer peripheral surface of each of opposite side walls of the groove at the core winding portions is curved so as to have an arcuate cross-sectional shape.

Abstract: A bobbin case for use in a transformer of the double-insulated type comprises an insulating member of tube-like shape, a partition wall disposed substantially normal to the axis of the insulating member so that the inner space of said insulating member is divided into two recesses for receiving primary and secondary winding assemblies, respectively, from the opposite directions along the axis of said insulating member, and a pair of insulating rims which extend from the outer end surfaces of the insulating member at the side of the recess for receiving the primary winding assembly. The pair of insulating rims are formed such that they are capable of being bent inwardly along the outer end surfaces so as to obtain a creeping distance between said primary winding and a core inserted into center holes in said winding assemblies. By using the bobbin case, the insertion of the primary and secondary winding assemblies can easily and efficiently be carried out.