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Cheap current transformers - tests and applications

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  • Cheap current transformers - tests and applications
    The split-core current transformer shown in the photo is available at a low price on many auction portals and online stores. The transformer has the parameters of 100A / 50mA. So when 100A alternating current flows through the cable through the transformer, the AC milliammeter at the transformer output will show 50mA. The current ratio of the transformer is 2000. The output signal is led out on a mini jack connector. The transformer casing enables the core to be opened and the conductor to be measured is enclosed.

    To begin with, we connect a 140W 230V bulb, a current of 0.66A flows in the circuit, the milliammeter shows 0.31mA.
    According to the transmission 2000, a current of 0.62A should flow in the tested circuit. The multimeter shows 0.66A, however the measuring range is very large (100A) and measurement errors will always occur. Here more about the transformers and available models:

    Cheap current transformers - tests and applications

    By checking the measurement results at several points, you can see the linear characteristics of the transformer:
    Cheap current transformers - tests and applications
    Iin [A] Iout [mA] x2000
    0.42 0.19 0.38
    0.66 0.31 0.62
    0.85 0.41 0.82
    1.49 0.73 1.46
    4.74 2.34 4.68
    5.08 2.49 4.98
    5.65 2.78 5.56

    For the measurement of smaller currents, it is worth using a transformer with a smaller measuring range. For the test, we put 5 turns on the transformer core, which changes the ratio and output currents,

    Cheap current transformers - tests and applications

    Cheap current transformers - tests and applications

    Iin [A] Iout [mA] x400
    0.42 1.03 0.41
    0.66 1.61 0.64
    0.85 2.09 0.84
    1.49 3.68 1.47
    4.74 11.59 4.64
    5.04 12.46 4.98
    5.64 13.94 5.58

    Another type of transformers are those with the output voltage depending on the current flowing in the tested line.
    An example of a 30A / 1V transformer. A separate description of the transformer here SCT013 30A / 1V . The input current in A divided by 30 should produce the corresponding output voltage in V.
    Cheap current transformers - tests and applications

    Alternating current of 0.66A causes the appearance of a voltage of 21.32mV at the output.

    Cheap current transformers - tests and applications

    Iin [A] Uout [mV] x30
    0.42 13.64 0.41
    0.66 21.32 0.64
    0.85 27.52 0.83
    1.49 48.8 1.46
    4.74 154.7 4.64
    5.08 166.1 4.98
    5.65 184.5 5.54

    By recording the output voltage of the transformer, we can, for example, examine the current consumption in the home electrical installation.

    What applications do you see for cheap transformers with limited measuring accuracy?

    Cheap current transformers - tests and applications Cheap current transformers - tests and applications

    The transformer can be transformed into a primitive oscilloscope probe for observing the waveform of current changes.
    Below is the shape of the current consumed from the network by a 100W bulb and a 13W LED:

    Cheap current transformers - tests and applications Cheap current transformers - tests and applications

    Inrush currents of, for example, different types of light sources can be observed:

    The simultaneous recording of the voltage and current waveform allows to determine the received active and reactive power,
    examples of differences between power supplies with and without APFC:

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  • #2
    Moderator of Designing
    The cheapest transformer is a ferrite core with a wound winding and terminated with an appropriate resistor value. The fact that the disadvantage of such a solution is the lack of a constant component, but the price varies between PLN 5-10 in the retail.

    If there is interest, I can compare such a rather primitive solution with the I-prober 520 probe.
  • #3
    @_lazor_ necessarily, such materials are lacking in articles, most things can now be obtained as modules, but such low-level DIY can come in handy in many cases.
  • #4
    Level 33  
    I am thinking about the construction of a measuring device which could measure the current consumed by a typical electric motor simultaneously on all phases, e.g. in order to detect e.g. a weaker current path in a contactor. It doesn't have to be super precise, my motors are from 4 to 55kW.
    For this 3 simple voltmeters and I would have a set.
    Of course, there are devices for this purpose, I even have such a recorder in my company, but there are 12 service technicians and one recorder.
  • #5
    Moderator of Designing
    I did some transformer measurements comparing to the waveform from the I-prober 520 probe.

    In a word of introduction, the measurements are illustrative and it seems to me that I plugged in with the probe badly, but unfortunately I do not have time to repeat the measurements.

    The transformer I test is the CS1050L:

    terminated resistor 15ohm 1% LR2F15R:

    Measuring system:
    Cheap current transformers - tests and applications

    As you can see from the diagram, the CS1050L transformer has a rectified signal, I had such a circuit at hand, so I used it.

    And now the meat, i.e. the measurement results:

    inverter supply voltage 10V
    I run a series of 8 periods for an RLC chip with a frequency of 80kHz

    Cheap current transformers - tests and applications

    yellow - CS1050L
    blue - I-prober 520

    I-prober - 520 1v / A = Imax = 1.86A
    CS1050L - 300mV / A = Imax = 2.10A

    Second measurement:

    inverter voltage 20V, the rest unchanged

    Cheap current transformers - tests and applications

    I-prober 520 1v / A = Imax = 3.48A
    CS1050L - 300mV / A = Imax = 3.93A

    measurement error = around 12%

    Such a large error could result from incorrect calibration of the current probe, unfortunately this is a new purchase and I have not read the manual well and suspect that I set the offset incorrectly.
    All anomalies result from the lack of any filter on the measuring resistor, and the I-prober 520 probe has only a 5Mhz band, so it is possible that it cuts out all disturbances.

    Sorry to be so quick and a bit careless, but sometimes better than not.


    Of course it is possible to implement the arrangement you see in the results above.
  • #6
    Level 31  
    I have been working on the construction of an electricity meter (3 phases) for a long time. I have already purchased most of the elements on the occasion of another order, unfortunately the lack of time prevents the design and construction of such a device. I haven't bought transformers yet and I was wondering about these. The absolute error is around 1% so it's not too bad. The price is also affordable because on Chinese portals with the shipment it is about PLN 25 per item. Where did you find such transformers for PLN 10-12 in retail?
  • #7
    Moderator of Designing
    Unfortunately, I am describing here transformers to be used up to 10A, in your case you need much larger transformers and thus more expensive. However, LEM transformers may still be an alternative to classic transformers for you.
  • #8
    Level 14  
    I wonder how they work with distorted and impulse waveforms.
  • #9
    Moderator of Designing
    What I tested was a typical pulse waveform with quite a lot of harmonics because the RLC circuit operated in the capacitive range. The CS1050L documentation says that it is designed to work above 20kHz and that was my purpose.