I'm having a little trouble with my wattmeter design

Discussion in 'The Projects Forum' started by cadamhill, Oct 10, 2015.

  1. cadamhill

    Thread Starter New Member

    Oct 10, 2015
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    I want to take a signal ( 1 - 50 amps AC input) and run it through a current controlled voltage source and then through attenuation and then a precision rectifier. Do I need to have a level shifter incorporated, if I plan to convert this signal to 1-5 V DC before putting it into the ADC. My intention is to monitor current from a the utility line and convert it to a digital signal represented by 1-5V DC. Can you please provide me with some assistance with the below circuit? Here is what I have started. I think I have a good start but here is my lack of understanding. My development board (which contains the ADC) will only accept a 3.3V signal. I assume that is the signal that powers the development board and not the input. I have a lot of uncertainties and ask for help getting me on the right path. Thank you in advance.

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  2. cadamhill

    Thread Starter New Member

    Oct 10, 2015
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    I'm using an LA55p current transducer for my current sensing.
     
  3. cadamhill

    Thread Starter New Member

    Oct 10, 2015
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  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Read this contemporary thread.

    Do you know what is the internal resistance (between the loop inputs) of the indicator?
    Do you know if the indicator grounds one of the loop inputs?
     
  5. cadamhill

    Thread Starter New Member

    Oct 10, 2015
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    No, I do not. Here is my version of this circuit. Something is not adding up. I need to play around more.

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  6. cadamhill

    Thread Starter New Member

    Oct 10, 2015
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    So I have my CCVS circuit complete and it appears to work correctly. The only problem I have is how to I tweak this to pick up input currents below 14.15 Amps.? More clearly, there will be times when the input current will be below 14.15 amps and that is below 1V. Can the design be modified to range down to 0-5V? Would I adjust the mA to 0-20mA or leave it at 4 to 20mA?

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  7. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Read this app note. Particularly the stuff around Fig 6. If you reverse the order of the two indicators; put the indicator above your shunt resistor that goes to your opamp, you can return the negative of the loop power to one end of the shunt resistor, and avoid having to use a differential amplifier to convert the drop across shunt resistor into a ground-reference signal.

    Are you sure that your current transducer doesn't already convert the AC current it is sensing to a filtered DC current proportional to the magnitude of the AC current?
     
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