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DRV5056: non-invasive current sensing in a cable

aron kain
Prodigy 30 points
Part Number: DRV5056
Other Parts Discussed in Thread: DRV425

Hi we are looking to sense the magnetic field from a cable. The sensing must be non-invasive from the whole cable. This means that (1) we cannot open the cable and have access to the actual current to measure,  (2) we cannot disconnect the cable, slip a core over it, and then reconnect the cable, and (3) the magnetic measurement will be for the cable as a whole, i.e. that has both forward and return current in it so that, ideally, the net magnetic field is 0 at the cable surface-- standoff magnetic field sensing seems to be the only route to go-- Can you provide guidance or reference designs for accomplishing the measurement?

  • 0
    TI__Mastermind 32220 points

    Hello Aron,

    You could use the DRV425 but distance and magnitude will determine if you can use it.  There will be an issue that you will need to negate the earths magnetic field and anything else near that is generating a magnetic field.  Once a concentrator is removed you have other things to worry about.   To make sure the DRV425 could work for you I would need the magnitude, frequency of the current and the physicals dimensions of the cable.   I would then run a simulation on a free simulator FEMM to verify the fields seen.  The DRV425 can measure upto 2mT.  The downside is it will saturate easier than a hall sensor due to the 2mT range.  The DRV5056 can measure as well but this also depends on the magnitude.  The DRV5056 change its sensitivity based on temperature to compensate the behavior of a magnet.  For this application you do not want that feature.

  • 0
    Prodigy 30 points

    for right now, i would assume 14 gage wire (0.080" diameter copper), AC up to 130Vrms, 60Hz, and total cable diameter of 0.250"-- maximum current 5A.

  • 0 in reply to aron kain
    TI__Mastermind 32220 points

    Hello Aron,

    I assume the image is like it is below.  For that you can use the calculatyion of B = µ0*I/(2*π*r).  Where µ0= 4*π x 10^-7 and r is radius from center of conductor. If the return path is in the same cable they will cancel each other out and would not be able to make the measurement.

    From your description I see that the r = 0.125" but converting to meters = 0.003175 and a current of 5A this calculates to about 315µT at the outside of the conductor.  This could be done with the DRV425 but the sensor is about 0.4mm from the top of the package so you would need to account for that.  With this you would also need to understand that the earth field will give you a DC offset but as long as you filter non-60Hz you can remove it.  Another thing to consider is placement of the DRV425 will be key as well as orientation.  Also the DRV425 has a 8µT offset you will need to account for but it will be DC.  

    Any other 60Hz signal that is near could also be an influence in your signal.  Any external magnetic components could also manipulate your field of interest.

    I would start with a DRV425EVM

  • 0
    Prodigy 30 points

    technically, that is only true between the wires of the cable. however, on either side of the wires, the field from 1 wire does not fully cancel the field of the other, as the point of measurement is slightly more distant, and hence lower field, than the closer wire to the point of measurement. it is this difference, although slight, that would be the basis of measurement.

  • 0 in reply to aron kain
    TI__Mastermind 32220 points

    That is correct but your signal will be much less as they will cancel out.  Also placement of the sensor will matter as the orientation as you will only measure the tangential field of the outside insulator.  I did an assumption of placement of the internal conductors but the variation in this will also limit the field you will see.  Below is my simulation with forward and return current.  Below that is the tangential field strength which the DRV425 can measure.

    This is why it will be difficult to do it in this manner because the conductor placement is not controlled.