output stage switch 5 b towards ground in this context in turn voltage u mess dropping off at node d 2 is measured optionally both output stage switches 5 a 5 b could also be closed in order to check both valves 9 a 9 b in parallel at the same time during this measuring phase switch 3 remains open in order to determine the coil resistance of coils 2 a 2 b the steps shown in fig2 blocks 10 to 12 are carried out according to block 10 first of all the voltage dropping off over one of coils 2 a 2 b and also over reference resistor 4 is measured if the reference resistor is a resistor having very low tolerances current i supplied by current source 6 is able to be determined very accurately with the aid of the voltage measurement this calculation of current i takes place in block 11 fig2 from the voltage dropping off at node 2 and current value i determined before the resistance of coils 2 a or 2 b may now be very accurately determined in step 12 control signal pwm and pwm 2 of output stage switches 5 a 5 b may consequently.