A transformer includes a divided primary winding in an isolating transformer power supply circuit and a secondary winding between the parts of the primary winding, a magnetic core having an air gap, and a bobbin surrounding the core with the individual windings applied to it, the transformer has a row of terminal posts to which the windings are connected is designed so that the primary winding is divided into at least three partial windings. The, the secondary winding that is under the highest load for the longest period of time is divided into at least two partial windings, the partial windings of this load secondary winding are enclosed by two partial windings of the minimum of three partial windings of the primary winding on the bobbin, and optionally one or more additional secondary windings are arranged outside the winding structure of the partial windings of the primary winding and load secondary winding.