Page 215 - Understanding Automotive Electronics

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6 SENSORS AND ACTUATORS

Figure 6.10
Hall-Effect Position
Sensor

FPO

generation of a voltage that is dependent on a magnetic field—is called the
Hall effect.
In Figure 6.11b, the current, I, is represented by electrons, e, which
have negative charge, flowing from left to right. The magnetic flux flows
along the legs of the magnet as indicated and is generally perpendicular to
the face of the semiconductor Hall element. Whenever an electron moves
through a magnetic field, a force (called the Lorentz force) that is
proportional to the electron velocity and the strength of the magnetic flux is
exerted on the electron. The direction of this force is perpendicular to the
direction in which the electron is moving. In Figure 6.11b, the Lorentz force
direction is such that the electrons are deflected toward the lower sense
electrode. Thus, this electrode is more negative than the upper electrode and
a voltage exists between the electrodes, having the polarity shown in Figure
6.11b.
As the strength of the magnetic flux density increases, more of the
electrons are deflected downward. If the current, I, is held constant, then the
voltage, V , is proportional to the strength of the magnetic flux density, which,
o
in turn, is determined by the position of the tabs. This voltage tends to be
relatively weak so it is amplified, as shown in Figure 6.10.

202 UNDERSTANDING AUTOMOTIVE ELECTRONICS