The primary coil body (10) with its coil, the secondary coil body (11) with its coil and also the high voltage connector body (12) with a connector (13) therein are separate bodies put together by snap connections to provide a pre-assembled unit. A dual connector for the primary coil ends may also be provided as a separate part snap-connected to the primary coil body. The center leg of an E-shaped core part passes through the primary coil body, the latter having a cross extension on which a straight core member fits passing across the ends of the legs of the E part. After perfecting electrical contacts by soldering or welding, this unit is inserted into a single-use mold and is embedded in an insulating casting resin by injection molding. As a result of the snap-type fitting together of the coil bodies and the high voltage connector, as well as the low voltage connector body similarly fastened onto the primary coil body, it is possible to use a one-piece single-use mold for the injection molding operation.