thx for the answers,
ok, but how to deal with this:
what happens if i have a resistive divider as a load on the secondary side, and want to measure the voltage of a resistance with an opamp (differential amplifier) ,
for the op amp, i have to connect a feedback resistor at the non inverting input to ground,
this makes a current flow from the transformer secondary to the feedback resistor of the op amp, which usualy would not happe, because the secondary side is floating...