Dual polarity power supply

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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?
 

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Averagesupernova
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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?
The transformer should isolate the primary from the secondary winding so what is the problem? You should however be careful about safety when measuring primary side voltages. I would look into small instrumentation transformers to get a bit of added protection/isolation when measuring the primary side. Leright may have something if you are not concerned about cost and if the current required from the negative supply is not high. However, it has nothing to do with the ability to measure the voltage on the primary side of the power transformer.
 
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I didn't really specify very good now that I read it. The sensors that I am using to measure the original line voltage are going to be powered from this dual polarity supply I am creating. It is essentially a loop, in terms of block diagrams. Maybe I am not understanding transformers thoroughly. When you say it is isolated, can I imagine it as a source in itself, completely seperate from the primary, and thus when I short the primary common to the secondary center tap, there will be no conflict. It will just result in + and - .5 the secondary voltage.
 
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disregard this thread. I just ripped apart a transformer and shorted the line common with the center tap, and it works fine.
 
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Averagesupernova
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disregard this thread. I just ripped apart a transformer and shorted the line common with the center tap, and it works fine.
WHOA! Be careful there. Make sure that the lead you shorted ALWAYS stays connected to the neutral if you do this in your project. If the input wires to the primary are reversed then the common ground/center tap on the output side will be 'hot'. Sounds like a potentially dangerous situation to me and quite likely not legal.
 

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