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DRV425 Fluxgate Magnetometer Based Current Probe

Who needs an AIM TTI I-prober 520 when you can make one yourself!

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Calibrated measurement of current through a PCB trace normally requires current to be passed through a closed magnetic loop, or conversely, measured with the use of an external shunt resistor placed in series with the PCB trace. Both of these methods involve physically breaking the circuit in order in order to perform these measurements. Physically altering the circuit introduces unwanted circuit impedance when a Shunt Resistor or jumper wire is added in series with the existing trace. These methods also raise the probability of human related error when soldering/de-soldering on the PCB. In order to avoid the aforementioned issues, we will construct a compact hand-held probe to measure current flow through a PCB trace by “Non-Contact” Probing.

This device is a simple but practical way to visualize and measure various small magnetic fields, as well as roughly measuring the current flow through wires. 

Typically current is measured using "Shunt Resistor's" in series with the desired PCB trace or “Split Clamp” device, or a typical multimeter in ammeter mode. Whereas this is suitable for individual wires, it can be a pain to implement on a PCB, as both methods involve physically breaking the circuit in order in order to perform these measurements. Physically altering the circuit introduces unwanted circuit impedance when a Shunt Resistor or jumper wire is added in series with the existing trace.

Using the DRV425 IC by Texas Instruments we can probe a PCB track and the current flowing through the copper trace can be observed and measured.  Using Ampere's Law, which states that a current carrying wire produces a magnetic field being proportional to the current flow. 

The field sensor must maintain the shortest possible distance from the PCB trace as possible; as Ampere's Law states this Magnetic Field exponentially decreases with the square of distance (to a first order approximation) as described by Ampere’s law for current carrying wires. The integrated fluxgate magnetometer sensor is manufactured into the silicon dye, along with all necessary amplifiers and circuitry.

Figure 1: Diagram of DRV425 measuring the magnetic field of PCB trace.

Figure 2: Table of DRV425 measuring the magnetic field of PCB trace at variable distances in millimeters.

As you can see from the above table. If the probe tip is 1mm away from the trace being measured we can see that there is a roughly 50% loss in the measured field strength.

Fluxgates operate on Faraday's principle of electromagnetic induction. Current is induced by the primary and secondary coil(s) being driven in and out of saturation in order to achieve an equilibrium. When an outside magnetic field is introduced to this system in equilibrium, the net change in flux will cause the inductive coils to produce current due to Faraday’s law. This current, while small, can be amplified and measured in order to extrapolate external field strengths. 

Figure 3: DRV425 Integrated Circuit showing internal fluxgate

Figure 4: Simplified Block Diagram for DRV425 Integrated Circuit

Texas instruments supplies a formula that allows the user to extrapolate the output of the sensor in Volts, to a readable magnetic field value in Tesla. The first element is the gain of the compensation coil which is defined as “The compensation current is proportional to the external magnetic field and its value is 12.2 .” [1] This compensation current generates a voltage drop across an external shunt resistor, , shown in figure 1. “An integrated difference amplifier with a fixed gain of 4  measures this voltage and generates an output voltage that is referenced to REFIN and is proportional to the magnetic field.” [1] The value of the output voltage at the  pin (VOUT) is calculated using Equation 1:

The Arduino Due is the best choice of micro-controller over the UNO, Leonardo or other cost comparative devices as we can achieve the best possible sampling rate and resolution. The Arduino DUE has a 12-bit integrated ADC (Analog to Digital Converter). This eliminates the need for an external ADC, and increases measurement accuracy. Typical cheap ADC's found on Amazon, Ebay, ADAfruit ect. use I2C serial communication which unnecessarily slows the rate at which we can sample. The fastest external ADC I found could sample at 188.9K Samples per Second at 12-bit resolution, which is almost the same price as the DUE with less than 1/5th of the possible sampling rate.

By setting the Arduino DUE to "free-running" mode we can achieve...

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ino - 2.11 kB - 09/14/2019 at 04:10

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  • 1 × Arduino Due
  • 2 × DRV425
  • 1 × Breadboard Electronic Components / Misc. Electronic Components
  • 1 × Wires
  • 1 × Laptop/Computer

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  • 1
    Gathering Components

    To begin making this probe you need to purchase( if you have not already) an Arduino Due, 2x DRV425EVM Integrated Fluxgate Magnetometer Evaluation modules, breadboard wires and a breadboard.Make sure that you have the latest version of the Arduino IDE installed on your computer.

    A soldering Iron is recommended in order to modify the jumper pin of one DRV425, referenced in the below step.

  • 2
    Modifying one of the DRV425 Boards

    It is important that you modify one of the DRV425 evaluation modules to have the jumper pins soldered on the opposite side of the board. With the differences shown in the below before/after picture.

    Picture 1 - Before

    Picture 2 - After

    As you can see the jumper is soldered on the opposite side of the board. Please be aware of the board level pin-outs when doing step 4.

  • 3
    Constructing the Probe

    You are allowed some freedom in designing your probe. I simply decided to use a piece of foam which I carved into a somewhat ergonomic shape. I then used electrical tape to secure the modified DRV425 to the bottom of the foam. 

    The second DRV425 must be able to rotate on a swivel, much like a joystick of a video game controller. To accomplish this I cut the swivel part off of a car phone holder and inserted the piece into the top of the foam. After removing the clasp which secures the phone in place, i was left with a small base to tape the second DRV425.

    I used silicon mounting tape to help hold the DRV425 still as well as be able to see the board to determine if each board is parallel to one another. I used a small level in order to aid when calibrating (which is optional).

    Finished Probe is shown above.

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Owen wrote 09/25/2019 at 01:49 point

I don't know a great deal about fluxgate measurements, but from your description it appears as though it is quite sensitive to how far away from the trace the probe is.
Have you done any investigation into how the measurement varies if the probe end is a little dirty (eg: a hair) or if the probe is not held exactly vertical and does not have the same stand-off distance to the trace as last time?
Also, how does it change if the angle between the probe and trace vary (ie: probe still vertical, but twisted by a few degrees)? Sine function - or something more exotic?

  Are you sure? yes | no

ensgoldmine wrote 09/27/2019 at 12:39 point

Yes the probe tip is very sensitive to distance and angle variations. I added a table of the distance variation to the details of the project, shown in figure 2. If the probe tip is 1mm away from the trace being measured we can see that there is a roughly 50% loss in the measured field strength. The angle is a more complex calculation, but the probe tip's fluxgate needs to be perpendicular to the trace.

If the internal fluxgate sensor is parallel to the trace there will be no measured field as no magnetic field lines will pass through the fluxgate. I assume there would be a linear decrease due to angle variations from the needed orthogonal position in measured field strength. 

  Are you sure? yes | no

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