branches off between the diode u f and the coil 2 is illustrated schematically and is connected to ground through a further ohmic resistor r dson in such a way that the transistor 3 can bridge the capacitor 1 the transistor 3 is driven by a control unit 4 which is constructed as a microcontroller the control unit 4 is connected on the input side on one hand to the voltage input and on the other hand through a voltage divider to a voltage output the voltage divider includes two resistors r 1 and r 2 the mode of operation of the voltage transformer described above will now be explained below with reference being made to an illustration of a time profile of a coil current in fig2 and to a flowchart in fig3 at a start the control unit switches on the transistor 3 initially for a prescribed time interval t on and therefore the coil 2 forms an rl element with the resistor r dson this leads to an exponential rise in a coil current i l the time interval t on is calculated in this case as a function of a maximum permissible coil current i lmax an inductance l of the coil and the input voltage u in using the following formula t on.