Switched-mode power supply designed to supply power to a consumer, with an output voltage (U out) regulated to a target value (U s), with - a supply path, which comprises a switching regulator controlled by a driver, a galvanically isolated transformer which is situated downstream from the switching regulator, and an output circuit comprising a rectifier which is situated downstream from the transformer, wherein the consumer can be connected to the output of said output circuit, and - a feedback path via which a signal is supplied to the driver, said signal representing the output voltage (U out) and being transmitted via a light source and a light receiver, wherein, on the basis of said signal, the driver regulates the output voltage (U out) to the target value (U s) via a corresponding control of the switching regulator, - wherein the feedback path comprises a current regulator, which regulates a current flowing via the light source to a current value corresponding to the output voltage (U out), and - wherein the light source is used in the feedback path without a series resistor (Rv) upstream from the light source, characterized in that - the feedback path comprises a voltage regulator, which comprises a downward voltage switching regulator, particularly a synchronous downward voltage switching regulator, and regulates a feed voltage dropping over the light source at an output voltage (U out) in the range of 4.5 V to 45 V to a fixed value, in the range of 3 V to 5 V, below the target value (U s), - wherein the light source comprises a light element, specifically a light emitting diode, particularly an infrared light emitting diode, whose brightness changes depending on a current flowing through it, and - wherein an isolator, specifically a printed circuit board, is arranged between the light source and the light receiver, said isolator being transparent in the range of the wavelength of the light emitted by the light source. - a supply path, which comprises a switching regulator controlled by a driver, a galvanically isolated transformer which is situated downstream from the switching regulator, and an output circuit comprising a rectifier which is situated downstream from the transformer, wherein the consumer can be connected to the output of said output circuit, and - a feedback path via which a signal is supplied to the driver, said signal representing the output voltage (U out) and being transmitted via a light source and a light receiver, wherein, on the basis of said signal, the driver regulates the output voltage (U out) to the target value (U s) via a corresponding control of the switching regulator, - wherein the feedback path comprises a current regulator, which regulates a current flowing via the light source to a current value corresponding to the output voltage (U out), and - wherein the light source is used in the feedback path without a series resistor (Rv) upstream from the light source, characterized in that - the feedback path comprises a voltage regulator, which comprises a downward voltage switching regulator, particularly a synchronous downward voltage switching regulator, and regulates a feed voltage dropping over the light source at an output voltage (U out) in the range of 4.5 V to 45 V to a fixed value, in the range of 3 V to 5 V, below the target value (U s), - wherein the light source comprises a light element, specifically a light emitting diode, particularly an infrared light emitting diode, whose brightness changes depending on a current flowing through it, and - wherein an isolator, specifically a printed circuit board, is arranged between the light source and the light receiver, said isolator being transparent in the range of the wavelength of the light emitted by the light source.