the output signals 13a and 13b of the control circuit 4 the associated fig3 and 4 represent the best embodiment and indicate additional details compared with fig1 fig3 illustrates a power supply unit whilst in fig4 the closed loop speed control stages and the power output stages are shown as well as intermediate driver stages and a monitoring circuit in fig3 the output terminals l1 for 310 volts dc and l2 for 12 volts dc are shown next to the input terminals for the ac network 6 whilst l3 illustrates the ground terminal in addition there are a number of test points tp such as are useful for checking the circuit the system voltage is initially protected from short circuits by means of a fuse element 305 an lc unit consisting of the inductance 304 and the capacitors 306 307 ensures while interacting with a varistor 308 the limiting of overvoltage pulses and the suppression of electromagnetic compatibility faults the system alternating current voltage is then rectified with the rectifier circuit 2 and fed via a series resistor 32 ntc resistor to the line 10 or l1 the associated ground line is marked with the letters gnd and leads to the output terminal l3 the dc output voltage on the line 10 is smoothed by the capacitor 311 in parallel with which a discharge resistor 312 is connected the system voltage which can be tapped off at the resistor 304 is additionally connected via series resistors 314 315 to the input of the auxiliary circuit part 1 from here a further capacitor 318 is connected to ground potential the auxiliary circuit part is also connected directly to ground potential by one or more lines the smoothing.