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DRV425EVM: How to measure 100 milliamps DC current with DRV425EVM ?

Part Number: DRV425EVM
Other Parts Discussed in Thread: DRV425, DRV421, TIPD205

I need to measure a DC current of 100 milliamps with the DRV425 EVM kit.  I would like to know how this component works.  I am not familiar with the concept of this product.  I have noticed an instability with the output voltage when the current is 0.  Ususally the output voltage is stable when the current is null.  May be I am not understanding the theory behind this product.

  • Hello Fouad,

    The DRV425 measures magnetic fields using a flux gate sensor inside the device.  You can measure a current by observing the magnetic field it produces.  So the sensor location relative to the current source is important and also direction of the current relative to the axis of sensitivity the fluxgate sensor.  There are some more drawings that may help you understand in our Bus Bar Theory of Operation.  Even though it is not the same application some of the information is related and can help in the understanding.

    For your measurement of 100mA and we get as close to the sensor as possible (See images below for numbers) we get a magnetic field of  ~50uT. 

    B = µ0*I/(2*π*r)

    µ0 = 4π X 10^-7

    I= 0.1 A

    r = 0.0004 m

    So the current will generate 50uT and the earths magnetic field could vary your output by ±60uT depending on the orientation of the sensor to the earths magnetic field.  Now the DRV425 will pick up any magnetic field it observes so this is not limited to the earths magnetic field.  To make a small magnetic field measurement you would need to shield them from the DRV425.  Shielding requires a ferromagnetic material to divert fields you do not want to observe with the DRV425.  

    Please let me know if you have any follow up questions.

  • Dear Javier,

    Thank you for your informations.

    Let me study all your material.

    What I can add to my application, is that I need to measure this 100 mA DC in a high voltage substation !

    If you have any new recommandation, let me know.

    I will let you know my results.

    Best regards

    Fouad Brikci

  • Is there any way to gain up the magnetic signal by looping the current around a magnetic core or around the DRV425?  Or our these conductors carrying high currents as well. 

    If you use a magnetic concentrator and force the flux through the DRV425 this can increase your magnetic signal.   The DRV421 would be similar but this one is a closed loop system that would be more accurate but you need a compensation coil around the core.  

  • Dear Javier,

    Thank you for this information about DR421.

    As I am new in this kind of design, I do not feel comfortable to work with the DR421 because of the closed loop system.

    I prefer to continue my effort with the EVM425 kit. 

    We have found that the slope of sensitivity is 10 mV/100mA in placing the fluxgate sensor horizontally and parallel to the current direction.

    I have also ordered your second evaluation kit ''TIPD205 ±100-A Busbar Current Sensor Reference Design Using Open-Loop Fluxgate Sensors'' to learn how to place the fluxgate sensor and evaluate if we could not get a better signal in a differential configuration.   

    In fact, I have 2 questions :

    1- how to increase the signal output in playing only with the resistor values ?

    My concern is the very low value variation of the signal output because of the noisy environment. 

    What I understand is that the earth magnetic field will be a serious obstacle in our application.

    2- could you show me in a picture how to mount the ferrite ?

    Could you recommend a ferrite ? we have access to digikey, mouser or Newark easily.

    I have never played with this kind of component.

    For your information, the current has to be measured in a bus bar (wire of 1 inche diameter or rectangle of 4 inches wide or cylinder of 2 inches diameter). 

    The installation is in a high voltage substation and of course we cannot modify the structures to do our measurements.

    Of course, any answer or partial answer will be much appreciated.

    Thank you.

    Best regards.

    Fouad Brikci

     

  • Fouad,

    The TIDP205 is used to actually lower the magnetic field seen and use two DRV425 to measure a differential field.  This is explained in the documentation I referenced earlier (Bus Bar Theory of Operations).  The signal you are trying to measure is much lower and needs to be gained up.

    You can increase your output signal with the resistor setting but you will be limited and you will still see external magnetic fields.  You can download our DRV425 System Parameter Calculator to help.  The obstacle may not only the earths magnetic field but any external field you are not trying to measure.

    Unfortunately the magnetic concentrator or shield you need may not be off the shelf.  The concentrator will also act as a shield.  I will give you some drawings with simulation information on how I would suggest you doing it.  Could you please provide a drawing of the conductor.  I want to make sure I understand and there is no confusion on the conductor the current will be measured on.

  • Find a picture attached of a high voltage breaker where I plan to use the fluxgate sensors.

    I have drawn a red circle where the sensor could be attached easily on the bus bars.

    Of course we cannot do any hole in these high voltage breakers.

    I have decided to inject a current of 3 amps that I can detect easily with the DRV425.  

    I need also to measure very low current at the same time.

    I have 2 problems to solve :

    1- be able to detect very low current

    2- be able to calibrate the sensors because the output signals given are very different from a position to an other one.  It seems to me that I need to do before each measurement an ''offset test''.

    I do not know how to calibrate these sensors.  If you have some examples it will be very helpful.

    I have seen your evaluation board with the bus bar at 100 amps.  My application will be similar except that I am not allowed to do any hole in my bus bar.

    Thank you.

    Best regards

    Fouad Brikci

  •  Sorry I did not attach the picture

  • Fouad,
    Will the device be rigidly affixed in place on/near the conductor, or will it be more of a probe-instrument?

    The DRV425 bus bar application works because of the hole in the bar where the fields are large and close by. Sensing a small field in a magnetically noisy environment can be challenging, but we do have some ways to work around that.

    Our responses for the next week or two will probably be slow/delayed due to the holidays. Apologies in advance.
  • Dear Jason

    The fluxgate sensor will be a part of a battery clamp that we plan to design.

    We cannot do a hole but we can attach solidly through the battery clamp or any other clamp to get the direction we want exactly for the fluxgate sensor.

    I need really help and examples to understand how to calibrate the flux gate sensor.

    Thank you for your help

    Fouad Brikci

  • Fouad,

    The biggest challenge will be calibrating the gain of the system if the mechanical distance will change.  I mentioned a magnetic core that can concentrate the flux but this will be limited and not forgiving to changes in shape for a constant gain.  A clamp will change mechanically and will change your magnetic field seen for a specific current, basically changing your gain.  Calibrating this would prove difficult.  One way is to force a known to the accuracy required when the device is installed and then calculating the gain of the system.  If you are measuring an AC signal you can then eliminate the DC offset as well.