20-04-2012, 12:36 PM
Hall effect sensors
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Introduction
The Hall effect was discovered by Dr. Edwin Hall in 1879 while he was a doctoral candidate at Johns Hopkins University
in Baltimore. Hall was attempting to verify the theory of electron flow proposed by Kelvin some 30 years earlier. Dr. Hall
found when a magnet was placed so that its field was perpendicular to one face of a thin rectangle of gold through which
current was flowing, a difference in potential appeared at the opposite edges. He found that this voltage was proportional to
the current flowing through the conductor, and the flux density or magnetic induction perpendicular to the conductor. Although
Hall’s experiments were successful and well received at the time, no applications outside of the realm of theoretical
physics were found for over 70 years.
With the advent of semiconducting materials in the 1950s, the Hall effect found its first applications. However, these were
severely limited by cost. In 1965, Everett Vorthmann and Joe Maupin, MICRO SWITCH Sensing and Control senior development
engineers, teamed up to find a practical, low-cost solid state sensor. Many different concepts were examined, but
they chose the Hall effect for one basic reason: it could be entirely integrated on a single silicon chip. This breakthrough
resulted in the first low-cost, high-volume application of the Hall effect, truly solid state keyboards. MICRO SWITCH
Sensing and Control has produced and delivered nearly a billion Hall effect devices in keyboards and sensor products.
Theory of the Hall Effect
When a current-carrying conductor is placed into a magnetic field, a voltage will be generated perpendicular to both the
current and the field. This principle is known as the Hall effect.
Figure 2-1 illustrates the basic principle of the
Hall effect. It shows a thin sheet of semiconducting
material (Hall element) through which a
current is passed. The output connections are
perpendicular to the direction of current. When
no magnetic field is present (Figure 2-1), current
distribution is uniform and no potential difference
is seen across the output.
When a perpendicular magnetic field is present,
as shown in Figure 2-2, a Lorentz force is exerted
on the current. This force disturbs the current
distribution, resulting in a potential difference (voltage) across the
output. This voltage is the Hall voltage (VH). The interaction of the
magnetic field and the current is shown in equation form as equation
Hall effect sensors can be applied in many types of sensing devices. If the quantity (parameter) to be sensed incorporates or
can incorporate a magnetic field, a Hall sensor will perform the task.
Analog output sensors
The sensor described in Figure 2-4 is a basic analog
output device. Analog sensors provide an
output voltage that is proportional to the magnetic
field to which it is exposed. Although this is a
complete device, additional circuit functions were
added to simplify the application.
The sensed magnetic field can be either positive or
negative. As a result, the output of the amplifier
will be driven either positive or negative, thus requiring
both plus and minus power supplies. To
avoid the requirement for two power supplies, a
fixed offset or bias is introduced into the differential
amplifier. The bias value appears on the output
when no magnetic field is present and is referred to
as a null voltage.
Digital output sensors
The preceding discussion described an analog output sensor
as a device having an analog output proportional to its input.
In this section, the digital Hall effect sensor will be
examined. This sensor has an output that is just one of two
states: ON or OFF. The basic analog output device illustrated
in Figure 2-4 can be converted into a digital output
sensor with the addition of a Schmitt trigger circuit. Figure
2-10 illustrates a typical internally regulated digital output
Hall effect sensor.
Input characteristics
The input characteristics of a digital output sensor are defined in
terms of an operate point, release point, and differential. Since
these characteristics change over temperature and from sensor to
sensor, they are specified in terms of maximum and minimum
values.
Maximum Operate Point refers to the level of magnetic field that
will insure the digital output sensor turns ON under any rated
condition. Minimum Release Point refers to the level of magnetic
field that insures the sensor is turned OFF.
the input characteristics for a typical unipolar
digital output sensor. The sensor shown is referred to as unipolar
since both the maximum operate and minimum release points are
positive (i.e. south pole of magnetic field).
A bipolar sensor has a positive maximum operate point (south
pole) and a negative minimum release point (north pole). The
transfer functions are illustrated in Figure 2-14. Note that there
are three combinations of actual operate and release points possible
with a bipolar sensor. A true latching device, represented as
bipolar device 2, will always have a positive operate point and a
negative release point.
Output characteristics
The output characteristics of a digital output sensor are defined as the electrical characteristics of the output transistor.
These include type (i.e. NPN), maximum current, breakdown voltage, and switching time. The implication of this and other
parameters will be examined in depth in Chapter 4.
Summary
In this chapter, basic concepts pertaining to Hall effect sensors were presented. Both the theory of the Hall effect and the
operation and specifications of analog and digital output sensors were examined. In the next chapter, the principles of magnetism
will be presented. This information will form the foundation necessary to design magnetic systems that actuate Hall
effect sensors.