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4-20mA current sensing with passive device

265
Prodigy 40 points
Other Parts Discussed in Thread: DRV425, DRV425EVM

I would like to measure 4-20mA process current by passive means (without disconnecting wires). The inductive clamps that I have seen are expensive. Is there a TI inductive device that I can consider that would get me the required resolution (4.01mA for example) to two digit accuracy past the decimal? This clamp (or other)arrangement would be connected to a data logger. I don't mind building a circuit or doing some microcontroller programming. It would be nice to get the output signal in mA.

  • 0
    TI__Mastermind 20985 points
    Hi Rodger, we don't have a suitable Hall effect based device for your need. I'll move this thread to the current sensing forum so they can take a look at this.

    Best regards,
    RE
  • 0 in reply to RE
    TI__Mastermind 22875 points
    Hi Rodger,

    Could we get a little more information on your system/design? You say passive, but you also say you just don't want to disconnect wires. Would a current shunt monitor be applicable to your design, as once you design it in, you would not need to remove it again from the system. If not, then I can loop in Javier to see if our magnetic solutions could work for you. Any additional information would be appreciated. Thanks.

    Carolus
  • 0 in reply to Carolus Andrews
    Prodigy 40 points
    My goal is to have a removable device that can be transferred from one process loop to another. A current shunt monitor would not work. Thanks for any advice.
  • 0 in reply to 265
    TI__Mastermind 20985 points
    RAH, you could consider the DRV425 fluxgate.

    Best regards,
    RE
  • 0 in reply to RE
    Prodigy 40 points

    Thanks, I'll look at this. - RAH

  • 0 in reply to 265
    TI__Mastermind 32410 points

    RAH,

    The DRV425 has a full scale sensing range of approximately 2mT with an offset max of 8uT.  The range you are needing is very small and you would need to take a few thing into account if using the DRV425.  First if you place your current of interest above the  DRV425 you would place the part the sensor is 0.4mm below

    B = Uo*I / (2*Pi*r)

    Uo = 4*Pi*10^-7

    r = distance from current

    With I = 0.004A and r = 0.0004m  we get a field equal to 2uT.  This number is above the DRV425 offset of 8uT but it can be calibrated out.  The other issue with this level of magnetic fields is the earths magnetic field is about 60uT.  So depending on the orientation you could measure the full strength of the earths magnetic field or none at all.  This can be also calibrated out if you use a second DRV425 to use as a way to cancel out the earth's magnetic field or common fields seen by both devices.  These signal are still low and can get errors in the measurement by any neighboring magnetic field.  You can order an DRV425EVM board to attempt these measurements.  You may also realize you may need to shield external fields from your measurements but I am not sure of your mechanical limitations.  You could also gain up the magnetic signal by placing winding of the current of interest around the DRV425.  Please let me know if you think the DRV425 will meet your needs and I will help you more along the way.

    Regards,

    Javier

  • 0 in reply to Javier Contreras4
    Prodigy 40 points

    There could be some space limitations in some cases. There are many conductors nearby in some areas that carry signals sensitive to equipment operation. With a magnetic field shielding arrangement, could there be a problem with superposition in regards to other close conductors? I have another option to consider, but will read more about your device, too. Thanks.