I'm designing a set of voltage and current sensors for use with a solar system. I have developed all of the circuits, but it requires both positive and negative voltage supplies. I want to use the classic design using a center tapped transformer with two regulators. But I'm not sure what will happen when I try to measure the original 120V line that is supplying the transformer. I think that it will eliminate the bottom half of the transformer, since the center tap will be at the same potential as the source common, and thus wipe out my negative supply. Am I reading this situation right? If so, is there a way around this problem?