Can You Safely Use a Center Tapped Transformer for a Dual Polarity Power Supply?

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Discussion Overview

The discussion revolves around the use of a center tapped transformer for creating a dual polarity power supply, particularly in the context of designing voltage and current sensors for a solar system. Participants explore the implications of measuring the primary voltage while ensuring safe operation and isolation of the transformer.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses concern that measuring the original 120V line could eliminate the negative supply by making the center tap the same potential as the source common.
  • Another participant suggests using a single positive supply with a voltage converter to generate the negative voltage instead of a center tapped transformer.
  • A participant questions the understanding of transformer isolation, suggesting that the transformer should isolate the primary from the secondary, but emphasizes the need for safety when measuring primary voltages.
  • Further clarification is sought regarding the isolation of the transformer and the implications of shorting the primary common to the secondary center tap.
  • One participant reports a practical test where shorting the line common with the center tap worked fine, indicating a potential resolution to the initial concerns.
  • Another participant warns about the dangers of the previous action, highlighting the importance of ensuring that the lead remains connected to neutral to avoid hazardous situations.

Areas of Agreement / Disagreement

Participants express differing views on the safety and functionality of using a center tapped transformer for this application. While some suggest alternative solutions, others raise concerns about safety and isolation, leading to an unresolved discussion regarding the best approach.

Contextual Notes

There are unresolved assumptions about transformer behavior and safety protocols when measuring voltages. The discussion includes practical experimentation that may not align with standard safety practices.

gargeug
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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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just use a single positive supply and then get one of these: http://pdfserv.maxim-ic.com/en/ds/ICL7660-MAX1044.pdf

It will convert a positive voltage to a negative voltage.
 
gargeug said:
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.
 
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 separate 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.
 
disregard this thread. I just ripped apart a transformer and shorted the line common with the center tap, and it works fine.
 
gargeug said:
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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