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Current Transformer Interfacing

Karan123
Intellectual 510 points
Other Parts Discussed in Thread: LM311, LM324, LM741, LM358

Dear Support,

I want to measure AC Current using different CTs from  50/5 to 2000/5 in a single Unit. Means want connect any CT between the range given .

1) I have checked this link states  the  "This AC voltage is rectified by the first op amp, and then amplified by the second op amp."

First Stage :

R1 = Burden Resistance

R2 should be chosen at least 10 times greater than R1

R2 = R3 for Unity Gain .

Second Stage :

R5/R4 + 1.   Keep gains to 100

 

Guide me equations and value for other component ? Any AppNote ?

2) In this Figure,  Vo= - Vin(R2  /  R1)

 

Which Method is good suitable for my application?  I have to fed to 10-Bit ADC .

 

Any other suggestions are welcome...

Thanks

-

Kr

  • 0
    TI__Mastermind 25805 points
    Hello Kr,

    Can you please provide us with more information about the voltage waveforms on the secondary side of the current transformer for both CTs (e.g. min/max voltage and frequency)? Also, what power supplies are available for the amplifier?
  • 0 in reply to Pete%20Semig
    Intellectual 510 points
    Hello,


    Thanks for update ...

    1) My Line Voltage and frequency in 220V AC 50 HZ.

    2) Please Check datasheet of CTs as Below.

    www.rishabh.co.in/.../107

    3) Power Supply for amplifier 0 to +5V DC .

    I do prefer to use for LM324 , LM311 , LM358 , LM741 due to availability in Local market.


    Thanks
    Kr
  • 0 in reply to Karan123
    TI__Mastermind 25805 points
    Hello Kr,

    Thanks for the additional information. I assumed you were working with a line voltage of either 120V or 220V, but need to understand the voltage waveform at the amplifier. For this I believe you will need to select the burden resistor.

    Concerning the amplifier, if its input is bipolar you will require a negative supply voltage. Furthermore you will likely need to add a reference voltage...this is typically done by using a difference amplifier or instrumentation amplifier. You could build a difference amplifier out of one of the aforementioned op amps, but mismatch of the 4 external resistors will affect the accuracy of your solution.

    I hope this helps.
  • 0 in reply to Pete%20Semig
    Intellectual 510 points

    Hello,

    Thanks for your Update ..

    Yes I am working on 220V 50Hz AC .

    Concerning the amplifier, if its input is bipolar you will require a negative supply voltage. Furthermore you will likely need to add a reference voltage...this is typically done by using a difference amplifier or instrumentation amplifier.

    Yes I can create negative voltage . I am confused .. the need of  negative supply voltage ?.

    But I am using 10-ADC Bit  . I have detect only peak for 0 to 5V which is actually scale down from  CT output Voltage  (Am I right Here ?)

    I think i have missed below link in my first post . I just want to know that which topology between two (or any other) will work at my end .

    Guide me equations and value for other component ? Any AppNote or any Example Circuit suiable as per my applicaion?

    First Method

    www.crmagnetics.com/.../Precision Rectifier Circuit for CT Signal Conditioning.pdf

    Second Method

     

    Thanks

     

    --

    Kr

  • 0 in reply to Karan123
    TI__Mastermind 25805 points
    Hello Kr,

    Thank you for the additional information. Since you're using an ADC to quantify the output of the signal condition stage, I suggest using software for the peak detection (or any other algorithms you want to run on the current waveform).

    The input common-mode range and output swing of and amplifier (op amp or instrumentation amplifier) depends on the supply voltage(s). Please review this at www.ti.com/precisionlabs (specifically take a look at the Input and Output Limitations video). That is why it is so important to understand the voltage waveform on the secondary side of the CT.

    Once you provide us with the the voltage waveform present on the secondary side of the CT, we can develop the appropriate circuit and determine the necessary supply voltage(s), device(s), and topology.