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Power sensor signal conditioing

Discussion in 'General Electronics Chat' started by charbel89, Oct 5, 2008.

  1. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    Woops typo.! Its this one.:eek:

    As you are working down close to zero, the -Vref isnt included, just the +Vref
     
    Last edited: Nov 17, 2008
  2. charbel89

    charbel89 New Member

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    Quick update.

    The new proposed diagram worked. The signal is arriving at the PIC as expected. The AD736 is now correct.

    As I suspected the AD736 output was not correct.

    When clamp not connected the AD736 is zero, however th CA3140 is still showing 5mv on my DMM and 3 on my software..

    I tried to connect Pin 1 and Pin 5 to 0V using a 10K pot however this only cut the 5mv in half on my DMMand down to 1 on my program, I know I can allow for it in my program but it will be nice if I can make sure I get zero as output of the CA3140.

    Many thanks for all your efforts and time.

     
  3. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    Thats good news.:)

    Do you a have a low voltage supply on the project, say about -5Vish.?

    What tolerance are the two 100K resistors that give the Vs/2.?
     
  4. dave

    Dave New Member

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  5. charbel89

    charbel89 New Member

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    They both have a Gold band, which means 0.1 I think.

    With regards to -v I do not think the demo board provides -v.

    Any other way I can achieve this?

     
  6. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    Ideally the CA3140 should be powered from +/- 5V supplies in order to get absolute zero output. Most OPA when working on a single supply will not go to absolute zero. The CA3140 is a good choice for single supplies as will go close to zero output.

    I would use the program to remove this small zero offset, the offset value will remain constant over the full output range.
    The only other alternative is a another small battery, which is a pain.!

    The reason for asking about the 100K's if they were low tolerance, its possible that the junction of the two would not be exactly V/2, which would give a slight dc offset.

    Personally I think the results you are getting are OK.
    Once the project is laid out on a pcb the noise should be reduced.

    I would be interested when you have finished building, to see your test results.
     
  7. charbel89

    charbel89 New Member

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    Thanks Eric,

    I am already allowing for this in my software. At the moment I am deducting 2 from the values I am reading.

    Plugging my laptop charger: is showing a reading of:

    AD736 output......CA3140 output......Software output...reference system
    2mv....................11.3/11.4mv..........(4 raw) .............. 0.045KW

    My watts would be : ((2/10)*240)/1000 = 0.048 KW

    Adding my 2000 watts hair dryer to the load:

    77mv....................230mv...............( 76 raw)............ 1.790 to 1830KW

    My watts would be : ((74/10)*240)/1000 = 1.776 KW

    I think those are pretty good figures especially that they match very closely a reference system I am using.

    Is it possible for me to send the schematics to someone who can build the PCB for me without remortgaging my home to pay for it :D .

    My other alternative is to do it on a veroboard, and I was thinking of adding an LED pushbutton to switch on and off, 2 banana sockets for the clamp and a plug for +5v, Gnd, and output, and place all this in nice case with battery compartment. :D




     
  8. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    It would be a good idea to check the actual mains voltage when you doing these calibration tests.

    As you know Power = V * I or V^2/R

    So if the mains isnt what you think it is, it will throw your results off.

    If you want to measure the Wattage of a 'resistive' load you should have a adc channel for the mains voltage.

    I would lay the board out on veropcb, its a simple pcb.
    I understand there are some freebie strip board layout programs on the web.?
     
  9. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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  10. charbel89

    charbel89 New Member

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  11. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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  12. charbel89

    charbel89 New Member

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    Last edited: Nov 18, 2008
  13. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    Last edited: Nov 18, 2008
  14. charbel89

    charbel89 New Member

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    Yes I am using it now.

    Seems to be missing an important feature ability to drag components around which is a bit strange. I will check the help and tutorials.

    Edit:

    Just found it. It is on the bottom right corner 4 arrows :)


     
    Last edited: Nov 18, 2008
  15. charbel89

    charbel89 New Member

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    Here is the diagram.

    I hope I did not miss anything,
     

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  16. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    I'll look it over.. that was quick!!!! :)

    Always leave room at the board corners for mounting pillars.
     
  17. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    There is missing connection from one end of the 100K's

    also whats a 9V led.?

    Correct and repost.:)
     

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  18. charbel89

    charbel89 New Member

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  19. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    With exception of the missing 0V line for the supply it looks OK.
    Cut the track at the spot marked 'X'

    EDIT: just noticed you have linked the 0V near the centre of the pcb
     

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    Last edited: Nov 18, 2008
  20. charbel89

    charbel89 New Member

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    Please find one that I have connected with lines.

    I think 0v is already there.

    And I am not so sure about the link in the centre.

    Please review.

     

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  21. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    Your 'thick' 0V link in the centre is OK, I did notice it after I had posted, so I posted the edit.

    You have made decent layout in quick time, that layout program is nice and simple to use.

    Consider the LED drain on the 9Vbty.
     

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