is an output free wheeling diode 42 which rectifies the chopped output voltage of the transistor switch at the output of the switch mode power supply a unipolar storage capacitor 44 is connected to store and smooth the output voltage according to a well known control method the controlled electronic switch 40 or 22 61 in fig4 is operated at a high switching frequency u t compared to the mains frequency of the ac voltage supply at the connections x 1 x 2 by changing the relative switch on duration of the electronic switch 40 or 22 61 it is possible to adjust the output voltage u c at the capacitor 44 or 24 and thus the output voltage of the switch mode power supply u 0 referring again to fig4 we will now describe how the control voltage u t is determined by means of the three stage control device according to the invention the first stage of the control circuit according to the invention is illustrated in fig4 in a box indicated by 50 the second stage is in a box indicated by 60 and the third stage is in a box indicated by 70 the first stage 50 of the control circuit comprises a voltage divider consisting of resistors 51 52 53 a p element that takes the form of an operational amplifier 54 and a blocking diode 56 these components are connected to each other as shown in fig4 the output voltage u 0max can be adjusted via the voltage divider 51 52 53.