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Sensing the flow of current

Discussion in 'Electronic Basics' started by Daniel Rudy, Aug 1, 2004.

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  1. Daniel Rudy

    Daniel Rudy Guest

    Hello Everyone!

    I've ran into a situation where I need to sense a very large flow of
    current (100A) at 24VDC or so. This is for a motor supply feed. The
    problem that I'm running into is isolation because there is a
    microcontroller that is part of the control circuit that electrically
    needs to stay away from this line as its powered from a different
    source. So far, there are two ways that I'm aware of to do this:

    1. Use a very low value resistor (I would need a few in parallel),
    sense and amplify the voltage across that. Then feed that into a
    voltage to frequency converter so it can be sent across an opto-isolator
    and then on to a timer on the microcontroller.

    2. Wind a couple of turns of the main feed wire onto a torridal core
    form. The torrid has a gap cut into it. Inside that gap is a hall
    effect device that is epoxied into place. This inherently provides the
    galvanic isolation that is required. After signal conditioning, the
    analog voltage can be sent directly to a ADC input on the controller.

    I would prefer to use #2 as that would be easier to implement, and it
    seems to have a lower parts count. Accuracy in the area of 1 amp
    resolution or better, if possible, would be good. Any advise, links,
    etc. is appreciated. Thanks.
     
  2. It would seem that a 24VDC power supply and a microcontroller
    power supply could be configured so that the grounds are connected.
    If this were possible, then you could just put your resistors between
    the low side of the power circuit and ground and then connect the high
    side of the resistors to the ADC on the microcontroller. Even 8 bits
    should give you the resolution you are asking for. If the isolation
    is definitely required, there are other (more expensive) solutions.


    So far, there are two ways that I'm aware of to do this:
    International Rectifier has some prepacked chips that can do
    this. They are generally used for higher voltage systems, but should
    give you the accuracy you specify.
    Allegro Microsystems has prepackaged sensors which operate on
    this principal. (I am not sure if they go up to 100 amps.) Another
    option is to use a magnetoresistive current sensor. F W Bell and
    others offer these units.
     
  3. scada

    scada Guest

    "Daniel Rudy"
    http://www.allegromicro.com/sf/0750/ Has hall device ICs that are good for
    100A. I just received some samples, but have not yet designed with them.
    They look promising, high isolation and 0-5V output.
     
  4. CFoley1064

    CFoley1064 Guest

    Subject: Sensing the flow of current
    Good morning, Dan. If you need isolated sensing of AC current, you would use a
    current transformer. For DC current, the best way might be to use a hall
    effect sensor. Several manufacturers make hall effect sensors built onto a
    zero ohm shunt with integrated electronics. The current is then sensed by the
    electrically isolated circuit, which gives you an analog output voltage
    proportional to the DC current.

    Allegro makes integrated current sensors which are good to up to 100 amps,
    available off the shelf. One product you might want to look at is the
    ACS750SCA-100, which operates on your microcontroller 5V supply. These are
    available from stock at newark.com and other sources.

    http://www.allegromicro.com/sf/0750/

    Good luck
    Chris
     
  5. I think that you can buy these sensors ready-made from people like LEME who use
    them in their DC clamp meters.

    We had a similar problem with some data loggers, but were able to use an RS 232
    Galvanic Isolator between the loggers and the PC. The loggers were battery
    powered. We had problems with currents flowing down the RS232 cable between the
    PC ground and the equipment ground.

    Peter
     
  6. Got names mixed up there, it was HEME (that was subsequently bought by LEM)

    www.lem.com

    Peter
     
  7. Daniel Rudy

    Daniel Rudy Guest

    And somewhere around the time of 08/01/2004 08:11, the world stopped and
    listened as scada contributed the following to humanity:
    I went looking and found the ACS750ECA-100. Arrow has 6 peices
    available for $8.43 each.
     
  8. Daniel Rudy

    Daniel Rudy Guest

    And somewhere around the time of 08/01/2004 08:04, the world stopped and
    listened as Gary Reichlinger contributed the following to humanity:
    The problem has to do with the surge current. There can be a 70A or
    higher current pulse on the surge when the motor kicks on under load.
    Furthermore, that much current being suddenly drawn causes inductive
    voltabe spikes along the wiring/pcb traces of the motor power circuit.
    It is these spikes that I'm trying to keep away from the controller.
    I found some, they do.
     
  9. Jamie

    Jamie Guest

    for DC current and good isolation we used
    a Shut going through a torrid, the small
    windings around the toride get a heavy
    change effects in its inductance as the
    current in the field causes it to collapse.
    thus a DC current trough a torride can cause
    the inductance to change. use this as part
    of a Freq to Voltage converter circuit..
    we also use a small mini circuit using a
    shunt bar where both sides are input to the
    + and - side via resistors to an op-amp
    using a dual supply., this supply is fully
    ground isolated via a transformer type on the
    secondary side so that the op-amp components do
    not come in contact with any other electrical which
    could potentially over voltage the op amp./
    in this case the op-amp output drives an OP-coupler
    with transistor output.
    if you really want to get crude, you can use a
    low voltage incandescent lamp accrossed the shunt in a
    mini tube with a photo detector on the other end of the
    tube.
     
  10. You posed the question. I just answered it. I never made any
    claims as to the worldly importance of my comments.
    There are ways of offering protection to the microcontroller
    input that are much simpler and cheaper than full isolation. Usually
    the ADC input will have a protection diode built in so that just
    adding a resistor in series between the input and the test circuit
    will be sufficient (see the microcontroller datasheet). Zener diodes
    or a wide range of filtering circuits can also be used to protect the
    input. It would seem unlikely that full isolation should be necessary
    for a 24vdc system, but more details would be required to say that for
    sure.
     
  11. Tim Perry

    Tim Perry Guest

    "Daniel Rudy"
    assuming this is a power supply and not a 24V battery, and assuming it is
    unregulated: why not just put a standard toroidal pickup on the AC input to
    the transformer? the resulting output it then rectified, filtered, and
    calibrated.
    any quiescent current will be so small in relation to the 100A load as to
    be insignificant.

    of course it might be a 3 phase input which would add to the complexity.
     
  12. TP

    TP Guest

    Allegro Microsystems makes a 100A hall effect sensor ACS752xCA-100.
    The two heavy leads connect inline with your motor current.
    You apply 5 volts control supply. The zero-current signal is 2.5
    volts. The signal
    voltage increases for positive current and decreases for negative
    current. You can send that to an ADC.
    You can get a sample of the hall effect sensor from Allegro..

    http://www.allegromicro.com/sf/0752/

    TP
     
  13. Third way. Run the current thru a core of an inductor, which has the
    winding for an oscillator. The DC varies the coil's inductance which
    changes the freq of oscillation. Isolation is inherent as in the above.
     
  14. Jim Adney

    Jim Adney Guest

    These are available as a commercial product from a Swiss company
    called LEM. They make a good product line.

    -
     
  15. Ed Price

    Ed Price Guest

    It ought to be embarrassing for a poster to ask a serious question, have it
    seriously answered, and then reply with an automatic tag file that's
    sarcastic and demeaning.

    But I suppose Mr. Rudy is either too dumb to recognize his automated insult,
    or too sophomoric to care.


    Ed
    wb6wsn
     
  16. Daniel Rudy

    Daniel Rudy Guest

    At about the time of 08/01/2004 11:37, Gary Reichlinger stated the
    following:
    Sorry about that, no insult was implied or intended. It's one of those
    automated taglines I came up with a long time ago. Just haven't gotten
    around to changing it and completely forgot about it. It's changed now.
    The other reason that I'm going for full isolation is that the
    microcontroller and motor are powered from completely separate supplies.
     
  17. Robert Baer

    Robert Baer Guest

    Since you state DC current, and high voltage isolation, ther are two
    solution types:
    1) Hall effect transducer. These can do the job, but one cannot get 1%
    accuracy, linearity or temperature insensitivity that you seem to
    demand.
    But if you are willing to put up with about 12% nonlinearity with
    isolationof 5nA at 2000V, look at the Zetex ZMC20 (need to modify
    magnetic coupling to allow 100A sensing).
    2) Current transformer (fluxgate) as made by LEM. Expensive, but gives
    wideband and precision response. Perhaps you could "roll your own".
     
  18. John Doe

    John Doe Guest

    Hi Daniel,

    Another way: If you know the resistance/lenght of the wire, you may put
    another big resinstance in paralell and sense the current trough the
    later. I1=R2·I2/R1 where R2 is the small wire resistance, and R1 is the
    big paralell one. Anyhow, I recommend to isolate from there before
    entering the microcontroller.
     
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