mesuring small AC currents

Discussion in 'General Electronics Chat' started by _dan_, Apr 6, 2015.

  1. _dan_

    Thread Starter New Member

    Dec 10, 2013
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    Hi, everybody!!!

    I am trying to measure small mains current. My loads are between 6-12 W up to about 600 W. I tried with Hall effect based ICs but they appeared to be quite noisy and not suitable for such small loads. Then decided to go with CT. Now I took CT 500:1, with burden 330 Ohm and op amp (so called precision rectifier) for buffering and rectifying my signal. With this setup I can achieve almost linear output 70mA -> ~70mV and 440mA -> ~500mV - measured with non RMS multimeter.
    My intend is that this output voltage would be measured by MCU.
    Because the shape of the rectified sin, didn't appear to be good? , I placed 4,7uF cap at the output. Is it worth filtering the input of the op amp (the secondary of CT)? Or is it possible to use bridge rectifier instead of op amp precision rectifier (having in mind the voltage drops over each diode)?

    Thanks very much for help!!!!
     
  2. OBW0549

    Well-Known Member

    Mar 2, 2015
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    It would help if you could post a complete schematic diagram of the opamp precision rectifier circuit you are trying to use, then we could help you better. If you are using the "classic" 2-opamp, 2-diode precision full-wave rectifier circuit, filtering the output should be easy; take a look at the diagram on page 6 of this document,

    https://www.site.uottawa.ca/~rhabash/ELG4135L8.pdf

    and note that the capacitor C provides output filtering.
     
  3. _dan_

    Thread Starter New Member

    Dec 10, 2013
    20
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    Thank you for the provided document. As I see on p 6 it is a half wave rectifier and the second op amp is a filter
    I am using 2 op amp, 1 diode schematic. Here is the circ I am trying.
    The graph posted above shows its input - red, and output - blue

    Can I use such filtering over each op amp on my circ?
     
  4. OBW0549

    Well-Known Member

    Mar 2, 2015
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  5. ScottWang

    Moderator

    Aug 23, 2012
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    Why you not just move the C1(4.7uF) to the positive input?
    And does C1(4.7uF) enough to do the filter job?
     
  6. crutschow

    Expert

    Mar 14, 2008
    13,022
    3,235
    You should be able to use a diode bridge directly at the transformer output with the burden resistor at the bridge output since a CT is a current device and the forward drop of the diodes should not significantly affect the voltage across the resistor.
    You can then add an RC filter at the burden resistor output to filter the rectified voltage and give a DC value equal to the average (not RMS) value of the current.

    Note that the current output may not look sinusoidal since many loads look nonlinear, such as rectifier DC supplies, and thus draw currents that are non-sinusoidal.
     
  7. _dan_

    Thread Starter New Member

    Dec 10, 2013
    20
    1
    OBW0549, thank you for the suggested circ, I can try it out!

    ScottWang, thanks for the ideas! If C1 is raised for let's say 22uF and more, then the output rectified sin is more smoothed, but then time constant rises making the circ slower.
    The idea of moving the cap C1 to +5V seems interesting - I must try it :)
     
  8. ScottWang

    Moderator

    Aug 23, 2012
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    I mean that moving the C1 to the (+) positive or non-invert input of the op amp, not the +Vcc.
    But if you want to try what you think then it's ok, anyway it just a test.
     
  9. _dan_

    Thread Starter New Member

    Dec 10, 2013
    20
    1
    I tried sth like that before but with burden before the bridge and another after, with another ct 2500:1 and Schotky diodes. Then there was no need for filtering, since the graph seemed like normal rectified sin wave (just a small cap.). I remember that, due to very low sec voltage at my low primary currents, instead of using 50-100Ohm burdens as specified, I had to place 4,7kOhm before and after the bridge rectifier.
    Is it safe/reasonabole using such high resistance burden?
    What is better to use only a burden after the bridge rec., before it or like me two?
     
  10. _dan_

    Thread Starter New Member

    Dec 10, 2013
    20
    1
    Ok, now I got it :)
     
  11. _dan_

    Thread Starter New Member

    Dec 10, 2013
    20
    1
    Can I omit the load resistor R4 just after the last op amp? I mean can I put that signal to ADC without load directly from the op amp output?
     
  12. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    If your ADC provide the +5V to Vcc then it's OK, otherwsie you may need a voltage divider using two resistors.

    The R4 you used just for the load current not the voltage.
     
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