Lake Shore Model 421, Руководство пользователя

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Lake Shore Model 421 Gaussmeter User’s Manual
Hall Generator C-1
APPENDIX C
HALL GENERATORS
C1.0 GENERAL
This chapter provides theory of operation, specifications, mechanical drawings, and definition of
terminology. Hall Generator theory of operation is detailed in Paragraph C2.0. Generic Hall generator
hookup is detailed in Paragraph C3.0. Hookup to a Model 421 Gaussmeter is discussed in
Paragraph C4.0. Specifications of the various available Hall generators are detailed in
Paragraph C5.0. Finally, the HALLCAL.EXE program is detailed in Paragraph C6.0. Additional
installation and calibration information is available in Lake Shore Document Number F075-00-00 –
Hall Generator Application Guide.
C2.0 THEORY OF OPERATION
The Hall effect was discovered by E. H. Hall in 1879. For nearly 70 years it remained a laboratory
curiosity. Finally, development of semiconductors brought Hall generators into the realm of the
practical.
A Hall generator is a solid state sensor which provides an output voltage proportional to magnetic flux
density. As implied by its name, this device relies on the Hall effect. The Hall effect is the
development of a voltage across a sheet of conductor when current is flowing and the conductor is
placed in a magnetic field. See Figure C-1.
Electrons (the majority carrier most often used in practice) “drift” in the conductor when under the
influence of an external driving electric field. When exposed to a magnetic field, these moving
charged particles experience a force perpendicular to both the velocity and magnetic field vectors.
This force causes the charging of the edges of the conductor, one side positive with respect to the
other. This edge charging sets up an electric field which exerts a force on the moving electrons equal
and opposite to that caused by the magnetic-field-related Lorentz force. The voltage potential across
the width of the conductor is called the Hall voltage. This Hall voltage can be utilized in practice by
attaching two electrical contacts to the sides of the conductor.
The Hall voltage can be given by the expression:
V
H
=
γ
B
B sin
θ
where: V
H
= Hall voltage (mV)
γ
B
= Magnetic sensitivity (mV/kG) (at a fixed current)
B = Magnetic field flux density (kilogauss)
θ
= Angle between magnetic flux vector and the plane of Hall generator.
As can be seen from the formula, above, the Hall voltage varies with the angle of the sensed
magnetic field, reaching a maximum when the field is perpendicular to the plane of the Hall generator.
C2.1 ACTIVE AREA
The Hall generator assembly contains the sheet of semiconductor material to which the four contacts
are made. This entity is normally called a “Hall plate.” The Hall plate is, in its simplest form, a
rectangular shape of fixed length, width and thickness. Due to the shorting effect of the current supply
contacts, most of the sensitivity to magnetic fields is contained in an area approximated by a circle,
centered in the Hall plate, whose diameter is equal to the plate width. Thus, when the active area is
given, the circle as described above is the common estimation.