F.W. Bell 7030, Instruction Manual

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F.W. BELL Model 7030 Gauss/Tesla Meter Instruction Manual
A-2 Appendix A – Understanding Flux Density
MEASUREMENT OF FLUX DENSITY :
A device commonly used to measure flux density is the
Hall effect sensor. A Hall effect sensor is a
thin slice of a semiconductor material to which four leads are attached at the midpoint of each edge,
as shown in
Figure A-2 .
Hall Effect
Sensor
Figure A-2 Hall Effect Sensor
A constant current (Ic) is forced through the material. In a zero magnetic field there is no voltage
difference between the other two edges. When flux lines pass through the material the path of the
current bends closer to one edge, creating a voltage difference known as the Hall voltage (Vh). In
an ideal Hall generator there is a linear relationship between the number of flux lines passing
through the material (flux density) and the Hall voltage.
The Hall voltage is also a function of the
direction in which the flux lines pass through the material,
producing a positive voltage in one direction and a negative voltage in the other. If the same
number of flux lines pass through the material in either direction, the net result is zero volts. This
sensitivity to flux direction makes it possible to measure both static (dc) and alternating (ac)
magnetic fields.
The Hall voltage is also a function of the
angle at which the flux lines pass through the material.
The greatest Hall voltage occurs when the flux lines pass perpendicularly through the material.
Otherwise the output is related to the cosine of the difference between 90
°
and the actual angle.
The sensitive area of the Hall effect sensor is generally defined as the largest circular area within
the actual slice of the material. This
active area can range in size from 0.2 mm (0.008”) to 19 mm
(0.75”) in diameter. Often the Hall effect sensor assembly is too fragile to use by itself so it is often
mounted in a protective tube and terminated with a flexible cable and a connector. This assembly
is known as a
Hall probe . For more information on Hall Probes see Section 3 – Probes .