AC Magnetic Field



A magnetic field detector is a sensitive indicator of a moving or dynamic ac field. This detector might be of interest if you are concerned about radiation from magnetic fields for safety reasons. The detector can also indicate if a relay is energized or if a high-voltage transformer is operating. Any device with a transformer may be detected readily. The ac-magnetic field detector measures a single-axis field. Therefore, position the detector in each desired axis or build two similar coils and either switch them into the circuit or build three display units to monitor all three axes at once.

The experimental field detector begins with a sense coil. The sense coil consists of 100-200 turns of 28-gauge enameled wire wound on a plastic form covered with aluminum foil and placed inside a plastic film container. The coil is coupled to an LM324 opamp via a 1-k resistor. A 100-kQ feedback resistor is connected between pins 1 and 3. By switching in different feedback resistors, various sensitivity ranges can be selected. The heart of the ac field meter is two LM3914 DOT/BAR display driver chips, wired so that the first chip drives the second one for a 20-LED sequential display. Two 10-position LED display packages can simplify construction. The LM324 was used because it is a single-supply device. Power is applied to pins 4 and ground to pin 11. The gauss meter can be powered by a 9-V transistor battery.

Calibration of the ac field meter is accomplished by connecting an ac milli ammeter, an 8.2 Vac transformer, and a 100-Q potentiometer in series with a calibration coil. A calibration coil consists of 110 turns of 26-gauge enameled wire. The sensor coil is placed inside the calibration coil. The calibration formula is: gauss equals turns per meter times the ac amperes of coil current. Then, coil amperes equals gauss divided by the number of turns per meter, multiplied by 20.

For example, 110 turns x 20 = 2,200turns per meter.

(Note: a 5-cm coil is 5% of a meter.) Now, 2,200 turns per meter times the ac current in milli amperes equals the field in gauss. The gauss meter can detect the presence of magnets, but, since it is not a moving field, you have to sweep the magnet or the coil past each other to see an indication.


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