Section 3 - Applications
3
Section 3 - Applications
Introduction
Solid-state switches have been available for many years. In various applications, Hall- Effect Sensors (Hall ICs)
have replaced mechanical contact switches completely. In the mid 1980’s the ignition points in automobiles
were replaced by Hall ICs. The automotive market now consumes more than 40 million Hall ICs per year.
Melexis has been manufacturing high quality Hall-Effect Sensors and signal conditioning ASICs for nearly a
decade, and has pioneered the next generation of programmable sensors and sensor interfaces.
This section contains some fundamental information about Hall-Effect sensors, magnetics, and the added value
of programmable sensors and sensor interfaces. It is intended to be useful for the novice as well as the expert.
Design Kit Materials
This section refers to magnets and devices which are included in the Melexis Hall-Effect Sensor Design Kit or
the MLX90308 demo kit. Contents of these kits are listed below. These items can be ordered directly from the
factory by contacting Melexis at (603) 223-2362.
Hall-Effect Sensor Design Kit
Square Neodymium, sample magnet “A” (approximately 200mT)
Cylindrical Neodymium, sample magnet “B” (approximately 380mT)
Gauss meter circuit diagram
MLX90215 linear Hall Effect sensor and calibration chart
Samples of various Melexis Hall ICs
Sensor Interface Demo Kit
MLX90308 demo board
Serial interface cable
MLX90308 programming software (31/2” Diskette)
Note: Kit requires IBM compatible PC with a free COM port
Melexis Reference Magnets
Melexis offers calibrated magnets for use as a reference magnetic field available in 3 ranges. These are for ref-
erence only, and are not calibrated from a traceable source nor are they intended for calibration of any type of
instrumentation. They are intended for programming MLX linear Hall ICs, and for general lab reference.
SDAP-RM-10
SDAP-RM-50
SDAP-RM-100
10mT calibrated reference magnet
50mT calibrated reference magnet
100mT calibrated reference magnet
3-1
Section 3 - Applications
The Hall-Effect
The Hall-Effect principle is named for physicist
Edwin Hall.
In 1879 he discovered that when a conductor or
semiconductor with current flowing in one direction was introduced perpendicular to a magnetic field a voltage
could be measured at right angles to the current path.
V
H
V
H
V
H
V
H
V
H
V
H
No Magnetic
Field
South
Magnetic Field
North Magnetic
Field
The Hall voltage can be calculated fromV
Hall =
σB
where:
VHall = emf in volts
σ
=
sensitivity in Volts/Gauss
B=
applied field in Gauss
I=
bias current
The initial use of this discovery was for the classification
V
+
Output
V
Output
of chemical samples. The development of indium arsenide
Differential
Schmidt
Amplifier
Trigger
semiconductor compounds in the 1950's led to the first
useful Hall effect magnetic instruments. Hall effect sen-
sors allowed the measurement of DC or static magnetic
fields with requiring motion of the sensor. In the 1960's
Hall
Plate
GND
the popularization of silicon semiconductors led to the
GND
first combinations of Hall elements and integrated ampli-
Digital Hall Effect Switch
fiers. This resulted in the now classic digital output Hall
switch. (right)
The continuing evolution of Hall transducers technology saw a progression from single element devices to dual
orthogonally arranged elements. This was done to minimize offsets at the Hall voltage terminals. The next pro-
gression brought on the quadratic of 4 element transducers. These used 4 elements orthogonally arranged in a
bridge configuration. All of these silicon sensors were built from bipolar junction semiconductor processes. A
switch to CMOS processes allowed the implementation of chopper stabilization to the amplifier portion of the
circuit. This helped reduce errors by reducing the input offset errors at the op amp. All errors in the circuit non
chopper stabilized circuit result in errors of switch point for the digital or offset and gain errors in the linear out-
put sensors. The current generation of CMOS Hall sensors also include, a scheme that actively switched the
direction of current through the Hall elements. This scheme eliminates the offset errors typical of semiconduc-
tor Hall elements. It also actively compensates for temperature and strain induced offset errors. The overall
effect of active plate switching and chopper stabilization yields Hall-Effect sensors with an order of magnitude
improvement in drift of switch points or gain and offset errors.
Melexis uses the CMOS process exclusively, for best performance and smallest chip size. The developments to
Hall-Effect sensor technology can be credited mostly to the integration of sophisticated signal conditioning cir-
cuits to the Hall IC. Recently Melexis introduced the world’s first programmable linear Hall IC, which offered
a glimpse of future technology. Future sensors will programmable and have integrated microcontroller cores to
make an even “smarter” sensor.
DD
How Does it Work?
A Hall IC switch is OFF with no magnetic field and ON in the presence of a magnetic field, as seen in Figure
1. The Earth’s field will not operate a Hall IC Switch, but a common refrigerator magnet will provide sufficient
strength to actuate the sensor.
Figure 1, How it Works
Figure 1
S
N
A-01
A-02
No magnetic field = OFF
South magnetic pole = ON
But How Much Do They Cost?
The cost of a Hall IC depends on the application. Automotive Hall ICs may cost $0.35 to $1.50 or more, while
Hall ICs for Industrial and Consumer applications, such as appliances, game machines, industrial manufactur-
ing, instrumentation, telecom and computers, cost $0.20 or less.
Automotive chip costs are higher because of the unique requirements for shorted loads, reverse battery, double
battery voltage, load dump, 100% test at three temperatures and temperature operation up to 200
o
C. Devices
that do not meet the stringent automotive specifications are more than adequate for other environments, such as
in industrial and consumer products. Melexis products are created primarily to meet automotive specifications,
with off-spec parts sold at a lower price. The cost directly reflects how well the part performs versus the sever-
ity of the operating environment.
Section 3 - Applications
3-2
Activation - Using Hall-Effect Switches
A switch requires a Hall IC, a magnet and a means of moving the magnet or the magnetic field. Figures 2, 3
and 4 show several ways by which a magnet can control the Hall IC switch. The following examples are simi-
lar in principle to most real applications. Slide-by, proximity and interrupt configurations represent the three
basic mechanical configurations for moving the magnet in relation to the Hall IC.
Slide-by Switch
In the Slide-by configuration, the motion of the magnet changes the field from North to South within a small
range of motion. This configuration provides a well defined position and switching relationship. The minimum
required motion may be as little as 1 or 2 mm.
Figure 2, Slide-by Switch
In Figure 2A, the South magnetic pole is too far away, so the switch stays OFF. In Figure 2B, the South
magnetic pole turns the switch ON.
Figure 2A
Figure 2B
S
N
A-03
S
Linear Slide-By
Linear Slide-By, Alnico8
800
N
A-04
700
600
Flux Density in Gauss
500
400
c
300
.050" Airgap
.125" Airgap
.250" airgap
200
100
0
0
-100
50
100
150
200
250
300
350
Distance in mils (thousandths of an inch)
3-3
Section 3 - Applications
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