second counting input 5 2 which receives internally in the control device 2 the pwm signals which are fed to the output stage 3 one of the counting inputs 5 1 or 5 2 causes the counter to count upward while the other counting input 5 2 or 5 1 brings about downward counting as a result for example the pulses which arrive at the counting input 5 2 and are therefore transmitted simultaneously to the output stage 3 are counted upward and the pulses which arrive from the feedback circuit are counted downward at the counting input 5 1 if all the pulses which are transmitted to the output stage 3 also pass back to the counting input 5 1 via the feedback circuit 4 the counter reading of the counter 5 is equal to zero and is output at the counter output 5 3 an interrogation circuit 6 is connected to this output 5 3 and signals when the interrogated counter reading has exceeded a specific predefined value the exemplary embodiment in fig2 differs from that in fig1 only in the type of counters two counters 5 and 7 are provided here the first counter 5 is connected by its counting input 5 2 to the feedback circuit 4 while the pwm pulses which are transmitted to the output stage 3 are applied to the second counter 7 internally in the control device 2 both counters 5 and 7 are compared with a comparator 8 which compares the counting contents of the two counters 5 and 7 at regular intervals if the deviation between the counter readings of counters 5 and 7 is above a predefined limiting value the.