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Current transformer for coupling noise into a cable

  1. Feb 5, 2017 #1
    I want to couple white noise to a communication cable. I am using a 50/5A CT. My idea is to connect the noise in the secondary and to couple that noise into the cable. I want to know will my idea work? Does the CT what I am using will be suitable?
    TAPE-INSULATED-CT11-380x300.jpg
     
  2. jcsd
  3. Feb 5, 2017 #2

    Paul Colby

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    what kind of cable? coax?
     
  4. Feb 5, 2017 #3
    Yah coax, I am now using cat5
     
  5. Feb 5, 2017 #4

    Paul Colby

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    so i expect the answer is no since a balanced cable will link no flux.
     
  6. Feb 5, 2017 #5
    But I have seen people using current probs for BCI(Bulk Current Injection) tests..
     
  7. Feb 5, 2017 #6

    Paul Colby

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    In cables like CAT5 and coaxial cables the transformer winding will excite the common mode while the cable usually is used for differential mode.
     
  8. Feb 5, 2017 #7

    Paul Colby

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    What I mean by this is the transformer winding will induce current in the same direction in both inner and outer conductors. This induces 0 volts across the line or no signal.
     
  9. Feb 5, 2017 #8
    Actually I have two devices which are communicating with modbus protocol, I need to analyze the efficiency of the communication under the effect of noise, which are very common in industries. For that I need to inject the noise into the cable either capacitively or inductively. As CT can be easly used I am using that for this purpose
     
  10. Feb 5, 2017 #9

    Paul Colby

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    Okay, The cable is passed through the winding hole in the device you've shown. Get a scope and a signal source and do a test. If it works you're golden. If not then what I've said holds.
     
  11. Feb 5, 2017 #10
    Ohk..Thanks
     
  12. Feb 5, 2017 #11

    Paul Colby

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    Of course if one splits the cable conductors and passes only inner (or outer) conductors through the CT then the difference mode would be excited and the signal will be injected. Somehow I expect this is not what you're after.
     
  13. Feb 5, 2017 #12
    Actually in industrial environment the noise/interference will be couple as common to both line, I can not go for differential mode . In fields we have already faced issue of data loss, so I need to know how much is the noise effect on the data loss. I can not perform this now in industrial environment.
     
  14. Feb 5, 2017 #13

    Paul Colby

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    If you need to measure common mode noise then your method will work just fine.
     
  15. Feb 6, 2017 #14

    Baluncore

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    A CT will not be operating as a CT in that application. It will be working more like a voltage transformer with a one turn secondary.

    The CT will inject common mode noise onto the bundle of conductors in the cable.

    A CT usually has a laminated core that is designed for the bandwidth of the power being measured. You need to use a transformer that will pass the bandwidth of the noise you want to inject. Consider using an iron powder or ferrite toroid.
     
  16. Feb 6, 2017 #15

    jim hardy

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    Common mode interference gets transformed into differential mode by unbalanced impedance to earth. So an experiment will show how well your CT works on that particular system.

    Baluncore is right, for "white noise" you want a wideband current transformer that uses a core suitable for a high frequency .
    We used a big one around the neutral of our main generator to listen for 1 mhz noise , evidence of internal arcing.

    Search on wideband current transformer turns up several vendors
    http://www.lilco.co.uk/ was first on mine

    but i'll bet you could make one out of the inductors inside a line filter.
    Take apart a dead PC power supply. They're rich with inductors.

    old jim
     
  17. Feb 6, 2017 #16

    Svein

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    Noise injectors...

    Way back when I started out in professional electronics, our main noise generator was the old Weller soldering irons. Turning them on and off generated wideband noise.

    Another version thought up by one of the guys: Get hold of a coarse file and a high voltage capacitor (about 10nF). Put on insulating gloves, connect one mains phase to the file and the other to one lead of the capacitor. Rub the other lead of the capacitor against the file.

    I also once found a switching supply with too long leads in the PCB layout. A short-circuited oscilloscope probe detected noise spikes one meter away.
     
  18. Feb 6, 2017 #17

    berkeman

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    Cat5 is twisted pair, not coax, AFAIK.

    https://en.wikipedia.org/wiki/Category_5_cable

    It sounds like you are wanting to do a conducted common-mode noise test similar to the industry standard EN 61000-4-6 test:

    http://www.elmac.co.uk/pdfs/PNP_61000-4-6.pdf [Broken]
     
    Last edited by a moderator: May 8, 2017
  19. Feb 6, 2017 #18

    tech99

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    I have used a ferrite RF toroid surrounding a cable to measure common mode RF current. It should work in reverse.The toroid requires a few turns of wire depending on frequency of interest. You can calibrate it using a signal generator or a transmitter. It is possible to split the toroid so it can be placed around a cable, then clipped together. The CT looks to be designed for power frequencies.
     
  20. Feb 6, 2017 #19
    Thanks for the answer, the CT what I have is 50/5A, 50/60Hz. What is mean by 50/60Hz which is written over the CT? Does it mean the bandwidth is 60Hz? My noise will have frequency upto 20KHz, in that case will this transformer works?
     
  21. Feb 7, 2017 #20

    Baluncore

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    Your CT has a steel laminate core that is designed to work on a 50/60 Hz power supply. It will probably work only up to the 5th harmonic at 300 Hz. You will need to find a core that is designed to work at 20 kHz and build your own transformer. Find one in a switching power supply or the horizontal output transformer of an old CRT TV.

    If the ends of the cable are transformer coupled you should inject a common mode voltage difference into the bundle. The turns ratio of the voltage transformer will depend on your signal source and the voltage you want to induce. You may gain an advantage by putting more than one pass of your cable through the transformer.
     
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