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Which method should I do prefer to sense current ?

Discussion in 'Homework Help' started by Seyit Yıldırım, Oct 27, 2016.

  1. Seyit Yıldırım

    Seyit Yıldırım New Member

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    I want to sense current through grid line by uC.
    Because of cost I use current sense resistance. not hall efect sensor or others.
    There are two basic method: Low-side and High-side

    According to me, low-side is little dangerous. But it is simply.
    High-side is more complex. Also, High Vdd is reguired

    All IC I examine support until 100V max. And grid line voltage is 220Vrms in Turkey.

    How to sense current by high-side method ?
     
  2. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Welcome to ETO!
    What value do you place on your life? For safety's sake, isolate your measuring system from the grid, e.g. by using a current transformer (which can easily be home made if cost or availability is a real issue).
     
  3. Seyit Yıldırım

    Seyit Yıldırım New Member

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    Thank you for reply alec_t
    Generally current sense resistance is used in application. I saw that in my research. Cost is not only criteria. circuit size is also important. In addition, measurement accuracy is ciritic.

    Note: My english is not very good. If I make mistake, dont heed :)
     
  4. dave

    Dave New Member

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  5. spec

    spec Well-Known Member Most Helpful Member

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    Hi SY,

    Your English is much better than my Turkish.:D

    As Alec said in post #4, directly measuring mains voltages of 220V is dangerous and the best approach would be to use a current transformer to isolate you measuring electronics from the mains supply. A current transformer can be very accurate, but it all depends on what current range you want. 0 Amps to 1A upwards should not be a problem.

    But if you must use a resistor in the grid line there are two methods, as you say:

    (1) Low side

    (2) High side

    I am not sure why you say that low side is more dangerous; I would have thought that high side was the more dangerous, and more difficult to implement.

    The best approach, in my opinion is to have the current measuring circuit, either high side or low side referenced to the mains supply, and completely protected from human touch.

    The current measuring circuit then communicates with your single board computer (SBC) via a serial link, normally I2C or SPI, but possibly asynchronous serial.

    The communications link between the sense circuit and the SBC would then be isolated either optically, capacitively, or inductively.

    There are low cost and simple to use chips for all these functions.

    One small complication is that you will need a precision rectifier to convert the AC current into a DC voltage.

    If you favor the above approach let us know and we can take it from there.

    spec
     
  6. Seyit Yıldırım

    Seyit Yıldırım New Member

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    Thanks for reply spec
    I agree with you about using current measuring circuit. But current measuring circuits support maximum 100V. I saw so that which i study application note of Microchip, Linear Technology.. vs..
    Even if I use current measuring circuit, i must determine where i connect it? according to place, it will low side or high side :) in all situations it will be low side or high side :)
     
  7. spec

    spec Well-Known Member Most Helpful Member

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    No problem SY.:)
    :wideyed: That does not stop the circuits from measuring current at any voltage (within reason).
    You can use either low or high side; it makes very little difference. But I would be inclined to use low side to start with because it will be easier to understand and develop.

    spec
     
  8. spec

    spec Well-Known Member Most Helpful Member

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    Here is an outline sketch illustrating the principle of a mains-voltage, low-side, AC current monitor. Note that this is not a practical circuit for construction.

    spec


    2016_10_27_ETO_CURRENT_SENSE_LOW_SIDE.jpg
    NOTES
    (1) The circuit operates from a 5V supply line which is derived from 300V half-wave rectified mains
    (2) The mains current flowing through 'R sense' generates a proportional voltage.
    (3) The first stage, at the left, is a peak detector made from an LM393 comparator, which has an open collector output.
    (4) The second stage is a microcontroller unit (MCU) which converts the 0V to 5V input from the peak detector into a digital stream (I2C, SPI, async), even wireless.
    (5) The third stage is an isolator (optical, capacitive, inductive) which outputs the digital stream isolated from the mains supply.
    (6) In a practical implementation, for safety, human contact with the above circuit would be prevented.
     
    Last edited: Oct 27, 2016
  9. Seyit Yıldırım

    Seyit Yıldırım New Member

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    spec Thank you for your concern and explanations

    I need to analog measurement not peak detector.


    that is not practial because of without isolation ?
     
  10. spec

    spec Well-Known Member Most Helpful Member

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    No sweat SY.:happy:

    If your mains supply is AC and not DC, you need to convert the voltage drop across the sense resistor, which will also be AC, into DC so that the analog to digital converter (ADC) in the processor can digitize the signal. A peak detector is a precision rectifier which accurately converts an AC voltage into a proportional DC voltage.

    You have the wrong end of the stick.:)

    The circuit I did is an outline circuit to show to you how a mains current monitor can work and to give you an idea of the size and complexity. It also shows how low voltage components can be used to measure currents at high voltage; the circuit could be configured for high side current monitoring too.

    The circuit does have isolation; that is not why it is not a practical circuit. It is not a practical circuit because I haven't worked out the details. It just illustrated a principal.

    In design there are various stages: concept > architecture > outline circuit > first draft circuit > circuit development > optimization, and so on. Of course, the stages vary according to the type and complexity of the circuit.

    Don't worry.:D

    spec
     
    Last edited: Oct 28, 2016

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