Magnetic-Field Probe Requires Few Components

Allegro A1323

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Popularly known as “gauss meters,” various makes and models of magnetic field meters are available on the market at prices that make them unaffordable to many hobbyists and engineers. This Design Idea combines a commonly available DMM (digital multimeter) with a single semiconductor component to measure magnetic-flux density and, in turn, magnetic-field intensity.

Figure 1 illustrates the measurement equipment, comprising a probe, its battery pack, and a DMM. The probe's active element consists of a linear Hall-effect sensor. Although virtually any linear Hall sensor will work in this application, this version of the probe uses an Allegro MicroSystems A1323 sensor, which produces a voltage proportional to an applied magnetic field. Operating from a power supply of 4.5 to 5.5 V, the A1323's quiescent output voltage (zero-field output) rests at 50% of the supply voltage. Given its nominal sensitivity of 2.5 mV/gauss, the A1323 provides a full-scale range of 1800 gauss (4.5 V/2.5 mV/gauss = 1800 gauss) for a supply voltage of 4.5 V.

A digital multimeter and a Hall-effect sensor form an easily assembled magnetic-field probe.
Figure 1. A digital multimeter and a Hall-effect sensor form an easily assembled magnetic-field probe.

Applying a magnetic field oriented south of the sensor's face increases the sensor's output voltage in proportion to the applied field perpendicular to the sensor's branded face, and applying a magnetic field north of the same face causes a proportional decrease in output voltage. For a supply of 4.5 V, the sensor's quiescent output voltage of 2.25 V can increase to 4.5 V for a 900-gauss, due-south field or decrease to 0 V for a 900-gauss, due-north field. Although the sensor can detect the intensity and polarity of a dc magnetic field, its ac-field bandwidth extends to 30 kHz.

The probe's breadboard version comprises a small piece of pc board of sufficient length to fit the operator's hand (Figure 2). The sensor's leads connect to a length of high-quality, three-conductor shielded cable and two 10-nF surface-mounted decoupling capacitors. The sensor's power supply comprises three series-connected, miniature, 1.5 V batteries for a total of 4.5 V. For a larger full-scale-measurement range, use a 9 V battery to feed a 5 V regulator IC, such as a 7805 voltmeter and add an on/off switch if desired. Place the batteries near the meter. Otherwise, the batteries' steel cases will disturb the magnetic field under observation. Use 10-nF SMD capacitors to decouple the sensor's input and output pins. Although any DMM offering high dc accuracy and an ac bandwidth exceeding 50 kHz can display the sensor's output, a DMM with a REL Δ (“relative-difference-from-reference-reading”) function, such as a Fluke model 187 DMM, eases measurement and polarity detection of a dc magnetic field (Reference 1).

The digital multimeter's relative-change mode (REL Δ) displays a near-zero magnetic field reading and the sensor's nominal zero-field output voltage of 2.25 V.
Figure 2. The digital multimeter’s relative-change mode (REL Δ) displays
a near-zero magnetic field reading and the sensor’s nominal
zero-field output voltage of 2.25 V.

Remove the probe from the shielded enclosure and measure the magnetic field under observation. To achieve maximum sensitivity, place the sensor's face perpendicular to the field. If the field's direction is unknown, rotate the probe about its longest axis to search for maximum voltage. To calculate the magnetic-flux density, divide the output-voltage reading by the sensitivity (2.5 mV/gauss). For example, if the meter reads –1.9800 V, then the magnetic field is 792 gauss due north. For an ac-magnetic-field measurement, use the DMM's true-rms mode to read the sensor's ac output voltage.

You can calculate a magnetic field's intensity in air by applying the following formula:

B = µ0•H,

where B represents magnetic-flux density in teslas, H represents magnetic-field intensity in amperes per meter, and µ0 = 4π•10–7 H/m (the permeability of free space). Given that the tesla represents a relatively large measurement unit, a 1 T field is quite strong.

For greater measurement resolution, apply the following conversion factors to use the gauss, a more popular unit: 10,000 gauss = 1 T, 1 gauss = 79.6 A/m, 1.2560 mT =1 kA/m.

Applications for the magnetic-field sensor include troubleshooting moving-magnet linear-position detectors, fabrication of dc motors and audio speakers, investigation of low-frequency-magnetic-field interference, and designing and fabricating electromagnetic-interference shields.

References

  1. Model 187 & 189, True RMS Multimeter, Users Manual

Materials on the topic

  1. Datasheet Allegro A1323

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1-4 Layer PCBs $2

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