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A fluxgate magnetometer – a prestudy

Just a few days ago I read up about variations in the magnetic field of earth and how these can be used to detect northern lights. Since there have been sightings of northern lights quite recently even in major parts of Germany I became interested in the topic.

A common type of magnetometer is based on the fact that the magnetic core of a coil will go into saturation at a certain current through the coil and when it does, its permeability drops. This drop can be picked up by a second coil. By clever arrangements in the form of a toroid or two parallel rods with opposite magnetic flux, the influence of the primary current on the readout coil will cancel out. Only the slight asymmetry in the external magnetization of the core will lead to a slight shift between both halves of the sensor going into saturation and this gives the output signal.

I decided to do some tests with quick-and-dirty wound fluxgate structures. One is based on a toroidal ferrite core of completely unknown characteristics which I had lying around, the other is tubular, based on two rods which are composed of two ferrite beads each, Amidon FB-73-101. The ideas behind the designs can be found in Don Lancaster’s Hardware Hacker from December 1988 and in a building description from RHR Laboratories from 2000. In the latter emphasis was put on the importance of developing the right core ferrite – I used the first one which fell into my hands…

Here are the results:

Toroidal fluxgate

The fluxgate on a toroidal ferrite core of unknown characteristics. The primary coil consists of two parallel windings of about 90 cm of 0.15 mm copper wire through the core, the secondary of about 1.80 m of 0.15 mm copper wire around the core.

Response (lower trace) of the toroidal fluxgate to a sine wave of about 7 V amplitude (upper trace, heavily distorted because of the limitations of the signal generator) in the presence of an external magnetic field.

Response of the toroidal fluxgate to a sine wave of about 7 V amplitude in the presence of an external magnetic field of opposite polarity to the previous image.

Response (lower trace) of the toroidal fluxgate to a square wave of about 5 Vpp (upper trace, heavily distorted because of the limitations of the signal generator) in the presence of an external magnetic field.

Response of the toroidal fluxgate to a square wave of about 5 Vpp in the presence of an external magnetic field of opposite polarity.

Tubular fluxgate

The tubular flux gate. The primary coil consists of about 10-15 windings of 0.15 mm copper wire through the ferrite beads, the secondary of about 1.80 m of 0.15 mm wire around both cores.

Response (lower trace) of the tubular fluxgate to a sine wave of about 10 V amplitude (upper trace) in the presence of an external magnetic field.

Response of the tubular fluxgate to a sine wave of about 10 V amplitude in the presence of an external magnetic field of opposite polarity.

2 comments to A fluxgate magnetometer – a prestudy

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