which makes it possible for the transformer to rise again since the current that is actually flowing through the resistor 43 for shutting off the transformer depends predominantly on resistor 17 and since the current for switching on the transformer depends on resistor 12 a hysteresis behavior will result corresponding to the difference of these currents fig7 is a variant of fig1 those elements which correspond to elements of fig1 are provided with the same reference numbers the rectifier input circuit 3 comprises diodes 72 73 74 and 75 anti hum and current limiting is performed by capacitor 70 resistor 71 capacitor 77 capacitor 78 and inductor 76 these elements are generally known to cooperate with rectifier circuits likewise the capacitor 87 and inductor 88 coupled to the dc motor 1 are well known furthermore a capacitor 82 is connected in parallel to the series circuit consisting of the resistor 17 and capacitor 13 the capacitor 82 provides a definite base voltage for the transistor 8 an essential point in comparison with the arrangement of fig1 is that the emitter of the transistor 8 is grounded through a resistor 83 and through two resistors 84 85 which are connected in parallel the series circuit of the resistor 83 and the parallel combination of resistors 84 and 85 lie in parallel with the capacitor 9 it is also important that an arrangement is provided in parallel to the primary coil 5 of the transformer 6 which consists of a diode 79 and of a resistor 80 which is connected in series with the diode 79 the resistor 80 is shunted by means of a zener diode 81 the anode of the zener diode 81 is connected to.