Current Transducer

Discussion in 'General Electronics Chat' started by scorca, Jan 7, 2009.

  1. scorca

    Thread Starter Active Member

    Oct 31, 2008
    36
    0
    Hi all,

    We are building a MPPT charger for solar panel rated at 300 W, 15 V @ 20 A.
    At the measuring part, we use a current transducer (ct) LEM HXS 20-NP/SP 3 (datasheet:http://www.farnell.com/datasheets/61020.pdf). We have it at the input and output. At the input has to measure about 20 A and at the output has to measure about 5-6 A. Somehow, we tested the ct and unfortunately the output of ct was not changing when the current was changing. We tested 3 same ct and those gave me the same output. Does anybody know why? Thank you in advance.


    Regards,
    Ben
     
  2. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Describe or post your CT test circuit. What voltages or current are connected to which pin?

    Have you connected the three capacitors shown in image? Vref is the internal reference voltage and can be replaced by an external one if required but in your case, only a single 47nF capacitor is needed.

    [​IMG]

    I have not used this type of transducers before but someone else must have so they can give you suggestions.
     
    Last edited: Jan 7, 2009
  3. scorca

    Thread Starter Active Member

    Oct 31, 2008
    36
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    This is my testing circuit....
     
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  4. scorca

    Thread Starter Active Member

    Oct 31, 2008
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    Sorry, I drew it wrong. This one is the right one.
     
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  5. italo

    New Member

    Nov 20, 2005
    205
    1
    Since you are asking question I wander if the output was ties to any load? Without a load you will never get any current to register.
     
  6. scorca

    Thread Starter Active Member

    Oct 31, 2008
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    I suppose those 2x39 ohm parallel resistance are the load.
     
  7. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    What value of current is flowing in the CT primary circuit?

    How many amperes?
     
  8. scorca

    Thread Starter Active Member

    Oct 31, 2008
    36
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    When we were testing it was not more than 2 amps.
     
  9. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Referring to the formula for the CT output as given in the datasheet:

    [​IMG]

    Ipn depends on the input connections configuration and in your case the CT is wired up for 20A nominal. Thus Ipn=20A in the formula.

    At 2amp primary current, CT Vo=2.5V-(0.625*Ip/Ipn) = 2.5 - 0.625*2/20 = 2.4375V.

    If you reduce the primary current to 1A, the output voltage = 2.5 - 0.625*1/20 = 2.4688V. The difference is only 0.03V. Some DVM will automatically changes to use the 20V range to display an input voltage larger than 2V. You would need a DVM with a range other than fsd 20V to actually see the change more clearly.

    If you have connected a DVM to the output of the CT in your previous test, does the reading not change, even in the last digit?
     
    Last edited: Jan 7, 2009
  10. Alberto

    Active Member

    Nov 7, 2008
    169
    36
    From your schematic connection are wrong!.
    Input should be on the side you are using as output (see spec under remark). So very likely you have a negative output.
    Just reverse connection and it should work.

    Alberto
     
  11. scorca

    Thread Starter Active Member

    Oct 31, 2008
    36
    0
    When there is no Ip, the output of my ct shows 7.4 mV only. It doesnt show 2.5 V.
    And when we increased the Ip, the output of the ct was increasing. But when we decreased the Ip, the output of the ct was still increasing slowly.
     
  12. scorca

    Thread Starter Active Member

    Oct 31, 2008
    36
    0
    to Alberto,

    It was my fault. I drew it wrong, but actually my connections are good.
     
  13. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    There is something not right here.

    Have you disconnected all the loading on the CT output before measuring the voltage? The datasheet requires CT output loading be larger than 2KΩ.
     
  14. scorca

    Thread Starter Active Member

    Oct 31, 2008
    36
    0
    I tested also another current transducer HX 20-P/SP2 (datasheet:http://www.farnell.com/datasheets/118834.pdf) and it was working (without Vref). I used 12 ohm Rload. The datasheet says it has to be > 2 kOhm.

    I changed the Rload to 2.35KOhm. Unfortunately, I didnt see any differents.
    I'm hopeless now.
     
  15. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    If one does not follow the limits stated explicitly in the datasheet, then the part will not perform correctly or satisfactorily or will end up having a short service life.

    Say goodbye to your CTs if you insists on using 12Ω output loading. You might have already killed them. :(
     
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