rz connected to the secondary side the dependence of resistance rs the load resistance rz is however not definitive therefore the measuring resistance rm is selected so that the effect of resistance rs on time constant u03c4 virtually disappears for example selecting an rm with a resistance that is at least 10 times as great as the resistance of rs the rise and decay time constant u03c4 of measuring voltage um is then almost exclusively dependent on the inductance ls of the transformer as follows equ2 the variation of the measuring voltage um in the rising branch can be described with equ3 the variation of the measuring voltage um in the decaying branch can be described with equ4 by first supplying the rise and decay time constant u03c4 can be determined as follows the measuring voltage um to a threshold decision circuit se see fig2 as shown in fig3 this threshold decision circuit has a lower threshold uu and an upper threshold uo for measuring voltage um as soon as the measuring voltage um exceeds lower threshold uu a counter in a timer unit ze is started when the measuring voltage reaches the upper threshold uo the counter is stopped the counter status is then proportional to the rise time constant u03c4 if the time constant u03c4 is determined while the measuring voltage is decaying the time between dropping below the upper threshold uo and reaching the lower threshold uu is determined the upper and lower voltage thresholds can be described as follows equ5 where u03b4t is the time of transit measured by timer unit ze of the measuring voltage between the two voltage thresholds uu and uo see fig3 from equation 5 the.