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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 10mT calibrated reference magnet

SDAP-RM-50 50mT calibrated reference magnet

SDAP-RM-100100mT calibrated reference magnet

Section 3 - Applications

3-1

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.

The Hall voltage can be calculated fromV

Hall

σB where:

Hall

=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

of chemical samples. The development of indium arsenide

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

the popularization of silicon semiconductors led to the

first combinations of Hall elements and integrated ampli-

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.

VHNo Magnetic

Field

VHSouth

Magnetic Field

VHNorth Magnetic

Field

VDD Output

GND

Digital Hall Effect Switch

V+

Differential

AmplifierSchmidt

Trigger

Hall

Plate

Output

GND

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

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

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

A-01 A-02

Figure 1

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.

Section 3 - Applications

3-3

NA-03

Figure 2A

Linear Slide-By

-100

100

200

300

400

500

600

700

800

0 50 100 150 200 250 300 350

Distance in mils (thousandths of an inch)

Flux Density in Gauss

.050" Airgap

.125" Airgap

.250" airgap

NA-04

Figure 2B

Linear Slide-By, Alnico8

Proximity Switch

The proximity configuration is the simplest, though it requires the greatest amount of physical movement. It

is also less precise in terms of the position that results in turning the sensor ON and OFF. The magnetic field

intensity is greatest when the magnet is against the branded face of the Hall IC and decreases exponentially as

the magnet is moved away.

Figure 3, Proximity Switch

In Figure 3A, the South magnetic pole is close to the Hall IC, so the switch turns ON. In Figure 3B, the

South magnetic pole has moved too far away, so the switch turns OFF.

Section 3 - Applications 3-4

200

400

600

800

1000

1200

1400

0 100 200 300 400 500 600

Distance in mils (thousands of inch)

Flux Density in Gauss

Head On Gauss

A-05

A-06

Figure 3A

Figure 3B

Linear Slide-By, Alnico8

An invisible or sealed switch may be made with either configuration. The Hall IC may be inside a sealed

container to shield it from oil or water, while the magnetic field penetrates or “sees” through the sealed

enclosure. Refer to Figure 4.

Figure 4, Sealed Box

The Hall IC can be shielded from

the elements and remain

sensitive to magnetic fields.

Interrupt Switch

When the Hall IC and magnet are fixed, the Hall IC can be activated using a ferrous vane. This system,

composed of a Hall IC, magnet and ferrous vane is called an interrupt switch. In the interrupt switch the

magnet is positioned so the South pole turns ON the switch while the Hall IC and magnet positions are

fixed relative to each other. When a vane made of a ferrous material is placed between the magnet and

Hall IC, the magnetic field is shunted or reduced to a very small fraction of the maximum field, turning

the switch OFF. This vane is shown in Figure 5 as a notched interrupter. This switch is an effective way

to sense position.

Figure 5, Interrupt Switch

In Figure 5A, the South magnetic pole is exposed to the Hall IC through the vane, so the switch turns

ON. In figure 5B, the switch turns OFF because the magnetic field is blocked by ferrous material.

Section 3 - Applications

3-5

A-07

A-08

A-09

Figure 5A Figure 5B

Rotary Interrupt Switch

The interrupt switch can be incorporated in applications of speed or position sensing, generally of rotat-

ing objects. The Rotary Interrupt Switch, in Figure 6, uses a toothed ring to interrupt the magnetic field

reaching the Hall IC. When a solid piece of steel (ferrous vane) blocks the magnetic field, the switch turns

OFF. During the gaps, or spaces in the steel, the South magnetic pole turns ON the switch. This is the sys-

tem commonly used for automotive ignition and many industrial applications, where accurate position is

critical.

Figure 6, Rotary Interrupt Switch

Figure 6 uses a notched

interrupter on a rotating

shaft to activate the device.

Section 3 - Applications 3-6

A-10

Figure 6

Rotary Slide-by Switch

Figure 7, Rotary Slide-by Switch

The Rotary Slide-by Switch in Figure 7 is generally used to measure rotary speed to synchronize switch-

ing with position. The Hall IC is activated by a rotating magnet. When the South pole passes by the Hall

IC, the IC is switched ON. As the North pole passes, the Hall IC is switched OFF. The solid circular mag-

net, shown in Figure 7A, is called a Ring Magnet. A ring magnet has alternating North and South poles.

Ring magnets may have from two poles to thirty-six or more, depending on size. Graph 1, below illus-

trates the transition between North and South polarity at various air gaps. Notice the transition point is

similar at the various gaps.

Graph 1, Rotary Slide-by vs. Air gap

Section 3 - Applications

3-7

SNSN

A-11

A-12

Figure 7A

6 Pole Ring Magnet

-150

-100

-50

100

150

0 50 100 150 200 250 300 350 400

Rotation in Degrees

Flux Density in Gauss

0 Airgap

0.025" airgap

0.050" Airgap

0.100" Airgap

0.150" airgap

Figure 7B

Working With Magnetic Fields

How Do They Work?

A magnetic field will convert electrical energy to

mechanical energy, attract ferromagnetic objects

and serves as an input for Hall-Effect Sensors. A

magnetic field is described in terms of flux. Flux

lines are imaginary lines of magnetic force, orig-

inating at the North pole of a magnet and ending

at the South pole of the magnet. These lines rep-

resent the physical force exerted by the magnet.

When these magnetic flux lines pass through a

plate of semiconductor material, electrons are

forced to one side of the plate resulting in a volt-

age potential. This phenomenon is known as the

Hall-Effect.

Flux density is measured in units of Gauss or

milliTesla. The intensity of the magnetic field depends on many variables, such as cross-sectional area, length,

shape, material and ambient temperature. Each one of these variables must be considered when designing the

Hall Effect sensor integrated circuit and magnetic system for your application. The following section is intend-

ed to explain some fundementals which are useful in Hall Sensor designs and applications.

Figure 9, Magnetic Spectrum

Figure 8, Flux Paths

Hall-Effect Sensors

Magnetoresister

Inductive Sensors

Giant Magnetoresister Nuclear Magnetic Resonance

Fluxgate Magnetometer Reed Switch

(Superconducting Quantum Interface Devices)

Ferrite Magnets

Alnico Magnets

Rare Earth Magnets (Neo, SmCo)

Ceramic Magnets

.000001 .001 .01 .1 1 10 100 1K 10K 100K

Gauss

10K1K100101.1.01.001.0001.0000001

milliTesla

Bio

Signals

Earth's

Field

Strongest

Electro

Magnet

Solar

Flares

SQUIDs

Evolution of Magnetics

Modern society would not exist in its present form if not for the development of permanent magnet technology.

Many of the major advances in the last century can be traced to the development of yet better grades of magnet

materials. The earliest magnets were naturally occurring iron ore chunks mostly originating in Magnesia hence

the name magnes. We now know these materials to be Fe3O4, a form of magnetite. Their unique properties were

considered to be supernatural. Compasses based on these magneswere called lodestonesafter the lodestaror

guidestar. They were highly prized by the early sailing captains.

The Pioneers

More sophisticated magnets did not come into use until the 15th century when William Gilbertmade scientific

studies of magnets and published the results. He found that heating iron bars and allowing them to cool while

aligned to the earth's field would create a stronger magnet than a naturally occurring lodestone. His magnet tech-

nology however remained a curiosity until the 19th century when Hans Christian Oersteddeveloped the idea

that electricity and magnetism were related. He was the first to determine that magnetic fields surround a current

carrying wire. It would require the development of atomic particle theories before scientific explanations of per-

manent magnets made further advances. The practical applications for magnets continued throughout the 19th

century.

Magnetism in a solid object seems to defy rational explanation. The magnetism is developed in a manner simi-

lar to electrons moving through a coil of wire, magnetic fields are created by electrons in motion around the atom-

ic nucleus. This nuclear model of an atom with electrons spinning in orbit around a nucleus provides a source of

charges in motion. In most materials however, the number of electrons moving in one direction equals that mov-

ing oppositely and hence their magnet fields cancel. This results in no overall magnetic field for the material. It

takes many electrons spinning in the same direction to generate a measurable field. Unfortunately there are kinet-

ic forces at work causing atoms to constantly vibrate and rotate resulting in random alignment. The higher the

temperature the more kinetic energy and the more difficult it is to maintain alignment. Fortunately soldsme mate-

rials exhibit an electrostatic property known as exchange interaction which serves to maintain parallel alignment

of groups of atoms. This force only works over short distances amounting to a few million billion atoms. This

may sound like a large quantity but on an atomic scale it is a relatively small amount. These groups are known

as dipoles and are the fundamental building blocks that determine the properties and behavior of permanent mag-

net.

Relative Magnetic Properties

Magnets and magnetic materials are classified by many terms which describe many different properties, some

of which are explained and used in this book. Perhaps the most commonly asked question about a magnet is

“How strong is it?” Although this can lead to a complex explanation, Figure 9 is an excellent guide to the rela-

tive strength of magnetic forces, from strongest magnetic forces known such as solar flares to the nearly unde-

tectable magnetic signals passing through the neuro network of our bodies.

The Hysteresis Curve

A solid block of magnetic material is composed of multiple dipoles wherein the alignment of all of the dipoles

results in a constant field of maximum value. This maximum field attainable is known as the saturation field. This

condition is obtained by placing a sample of material in a sufficiently strong electromagnetic field and increas-

ing the electric current through the magnetizing coil. As the samples dipoles begin to align a function for the rela-

tionship between the magnetizing field and the field in the sample becomes apparent. In the low field levels the

slope of the curve is very steep.

This relates to the rapid alignment with the magnetizing field of a majority of dipoles. As current levels increase

linearly the number of dipoles aligning decreases. The result is a shallow slope to the function curve. At some

point, related to the material properties, increases in current through the magnetizing coil will not increase the

value of the field in the magnet. This is the saturation valuefor the material. When the external magnetizing

field is removed the magnetic field value of the sample "relaxes" to a steady state known as the B

value, or resid-

ual flux value.

An analogy to charging a battery is appropriate. At some level the battery is fully charged and will not accept

any more energy. It is an amazing thing however that the magnet will never lose its charge unless it is subjected

to a larger field of opposite polarity, or if the temperature is raised above the point known as theCurie

Temperature.This temperature varies depending on the material and is specified in all manufacturers data

sheets.

In summary we have discussed two of the three forces at work, one the magnetizing force measured in oersteds

with cgs units or ampere turns/meter in the SI system. The second is the resultant or induced field in the sample,

this is measured with gauss in cgs units and Teslas in the SI system (see Tables 1 and 2, below).

Table 1, Magnetic Units Comparison

Table 2, Magnetic Units Conversion

The third is reluctanceor its' reciprocal permeability, think of this as the magnetic resistance per unit volume of

the sample being magnetized. Now that we have a magnetized magnet we can consider what occurs when forces

act to de-magnetize it. If we reverse the direction of current flow in the magnetizing coil a negative field is cre-

ated. As the negative current is increased the dipole alignment is reversed or undone. A curve results which is

similar to the magnetizing curve but in mirror image form. When the samples' flux value is completely demag-

netized the demagnetizing force at that instant is the coercive force -HC. This force is also measured like the

magnetizing force in Oersteds. Increasing the negative current level in the magnetizing coil.

Unit Symbol cgs System SI System English System

Flux ΦMaxwell weber Maxwell

Flux Density BGauss Tesla lines/in2

Magnetizing Force HOersted ampere turns/m ampere turns/in

Multiply By To obtain

lines/in20.155 Gauss

lines/in21.55 x 10 -5 Tesla

Gauss 6.45 lines/in2

Gauss 10-4 Tesla

Oersteds 79.577 Ampere turns/m

Ampere turns/in 0.495 Oersteds

Ampere turns/in 39.37 Ampere turns/m

This brings us once again to Br or the residual flux value, the pole orientation is now opposite the first satura-

tion state. Finally reversing the current back to its original direction we can exercise the sample through the curve

once more and pass through the +HC value to arrive once again at the Br value. We have now completed the hys-

teresis loop for the material and can draw a curve relating B to H as shown in figure "y".

Figure 10, Hysteresis Loop

This curve is fundamental to characterizing and comparing classes and grades of magnetic materials.

An important aspect of magnetic materials behavior is dependent on the physical arrangement of the magnet in

the application. In a motor the permanent magnet is operating in a magnetic circuit with mostly low reluctance

paths for the field to circulate through. In many sensor applications however the magnet operates with little or

no magnetic circuit. This operating condition is known as open loop operation.

Before magnetic force is applied (current),

domains are randomly oriented and no

energy is created.

When magnetic force is applied, domains

become oriented in the direction of the

applied field.

When magnetic force (current) is removed,

domains don't completely randomize,

therefor retaining some of the energy.

B+

When magnetic force (current) is reversed,

and released, the inverse of the above

occurs, creating the complete hysteresis

loop.

The hysteresis loop is unique to all magnetic

materials. This diagram does not illustrate

any specific material or magnetic circuit

B-

H+H-

Br = residual inductance

Hc = coercive force

A magnet in a closed high permeability magnetic circuit (an iron bar connecting the north to the south pole)

will operate at or near the Br value. A magnet with no pole pieces will operate with a flux density down the

demagnetization curve from the Br value, how far down is dependant on the aspect ratio or the ratio of the

length to the diameter. Short wide magnets will generate lower flux than tall skinny magnets of the same vol-

ume.

The concept of the load line and the operating point on the demagnetization curve will influence many magnet-

ic parameters. These include the flux density available to actuate a sensor and the reversible temperature coeffi-

cient.

Temperature Effects

Graphical representations are often used to determine the operating point on the demagnetization curve.

Temperature effects on permanent magnets are dependent on the type of material considered. Manufacturers will

specify various figures of merit to describe the temperature performance of magnet materials. Among these are

the Reversible losses that are represented by Tc. The term refers to the losses in the Br and the Hc. A calcula-

tion can show that for every incremental change in temperature the magnet will lose a proportion of its strength.

This loss will be recovered completely so long as the temperature does not exceed the Tmax or maximum prac-

tical operating temperature in air. The Tmax value is dependent on the magnets operating point on the demag-

netization curve. A magnet operating closer to Br can have a higher Tmax. Irreversible losses are described as

losses that can only be recovered by re-magnetizing the sample to saturation with an electromagnetic field. These

losses occur when the operating point falls below the "knee" on the demagnetization curve. This can occur due

to temperature and inefficient magnetic circuit design. An important feature of magnet materials is the Curie tem-

perature, TCurie,. This is a temperature at which the metallurgical properties of the sample are adversely effect

ed. In most applications the ambient temperature can never approach the Curie temperature without completely

destroying the electronic components first.

Losses Over Time

Time has minimal effect on the strength of permanent magnets. Long term studies in the industry have shown

that at 100,000 hours the losses for Rare Earth Samarium Cobalt magnets were essentially zero and for Alnico

5 were less than 3%. In the case of Rare Earth Neodymium materials the losses are compounded by internal cor-

rosion.

Corrosion & Coatings

It is often necessary to provide coatings to these materials to minimize the corrosion that results from the Iron

content. We lay-people refer to this stuff as rust. The options for coatings include epoxies, zinc and nickel. The

best of these is nickel however it is slightly magnetic and marginally reduces the available field. Coatings can

also be useful with Rare Earth Samarium to minimize "spalling" or the fracture of tiny slivers from the corners

of this brittle, hard material.

In many sensor applications these characteristics are of little significance but as with all engineering tasks it is

up to the design engineer to know what can safely be ignored and what must be consider for the projects suc-

cess.

Many texts are available to aid in a complete understanding of magnets. The Magnetic Material Producers

Association is a trade group that establishes and maintains standards for basic grades and classes of materials.

Their reference booklets are an excellent source for detailed technical data on the various generic classes of mate

rials. Certain manufacturers also provide excellent databooks with helpful applications and design sections.

These include Arnold Engineering Company, Magnet Sales & Manufacturing, Magnetfabrik Schramberg,

Hitachi Metals; Magnetic Materials Division and Widia Magnettechnik.

Table 3, Magnetic Suppliers

Company Name Address Phone & Fax Types of Magnets

Arnold Engineering

Company 300 North West St., Marengo Il

60152 (815) 568-2000

(815) 568-2236 Alnico, Ceramic, Multipole Ring Magnets,

Flexible Magnets

Boxmag Magnets Chester St., Aston, Birmingham

B6 4AJ, United Kingdom (+44) 121-3595061

(+44) 121-3593501 Injection Molded Rings, NdFeB

Crucible Magnetics 101 Magnet Drive, Elizabethtown,

KY 42701 (502) 769-1333

(502) 765-3118 Alnico (Cast), Rare Earth

Dexter Magnetic Materials

Division 48460 Kato Road, Fremont CA

94538 (510) 656-5700

(510) 656-5889 Magnetic Material Distributor, Ceramic,

Alnico, Rare Earth

Electrodyne Company 4188 Taylor Rd., Batavia, OH (513) 732-2822

(513) 732-6953 Flexible Magnets, Multipole Ring Magnets

Hitachi Magnetics Corp. 7800 Neff Rd., Edmore, MI

48829 (517) 427-5151

(51) 427-5571 Alnico, Cermaic,NdFeB,Samarium Cobalt

Louis Magnet Supplies

Ltd. Hong Kong, China 011-852-2482-33290

11-852-2482-0806 NdFeB

Magnet Applications 415 Sargon Way, Suite G,

Horsham, PA (215) 441-7704

(215) 441-7734 Ceramic, Alnico,NdFeB,Samarium Cobalt

Magnet Sales &

Manufacturing 11248 Playa Court, Culver City,

CA 90230 (310) 391-7213

(310) 390-357 Ceramic, Alnico,NdFeB, Samarium Cobalt

Magnetfabrik Schramberg Max Planck Strasse 15, D-78713

Schramberg-Sulgen, Germany (+49) 7422-5190

(+49) 7422-51960 Hard Ferrite, Samarium Cobalt, NdFeB,

Plastic Bonded Ferrite

Magnequench

International 6435 Scatterfield Rd, Anderson,

IN 46013 (317) 646-5000

(317) 646-5060 NdFeB

Neomet Corporation PO Box 425, Route 551,

Edinburg, PA 16116 (412) 667-3000

(312) 667-3001 NdFeB

Polymag 685 Station Rd. Bellport, NY

11713 (516) 286-4111

(516)286-0607 Alnico, Flexible Magnets, Magnet Sheeting,

Ceramic

SG Magnets Tesla House, 85 Ferry Lane,

Rainham, Essex RM13 9XH,

United Kingdom

(+44) 1708-558411

(+44) 1708-554021 Ferrite, Ceramic, Molded NdFeB, Alnico

SG Armtek 4-6 Pheasent Run, Newtown, PA

18940 (215) 504-1000

(215) 504-1001 Ferrite, Ceramic, Molded NdFeB, Alnico

Shin Etsu Magnetics 2362 Quame Dr., Suite A, San

Jose CA 95131 (408) 383-9420

(408) 383-0203 NdFeB, Samarium Cobalt

Stackpole Magnetic

Systems 700 Elk Avenue, Kane, PA

16735 (814) 837-7000

814) 837-0203 Ceramic, Ferrite, Alnico

Sumitomo Special Metals

America 23326 Hawthorne Blvd., Suite

360, Torrance CA 90505 (310) 378-7886

(310) 378-0108 NdFeB, Samarium Cobalt

TDK Corporation of

America 1600 Fehanville Dr. Mount

Prospect, IL 60056 (708) 390-4374

(708) 803-6296 NdFeB, Samarium Cobalt

Tengam Engineering 545 Washington St., Otsego, MI

49078 (616) 694-9466

(616) 694-2196 Plastic Barium Ferrite, Strontium Ferrite,

Custom Injection Molded Magnets

Ugimag 405 Elm St., Valparaiso, IN

46383 (219) 462-3131

(219) 462-2569 Alnico, NdFeB

Vacuumschmelze 186 Wood Avenue South, Iselin,

NJ 08830 (908) 494-3530

(908) 603-5994 Alnico, NdFeB, Samarium Cobalt

Widia Magnettechnik Muncher Str. 90, D-45145 Essen,

Germany (+49) 201-7253348

(+49) 201-7253925 Alnico, NdFeB, Samrium Cobalt, Sintered

and Bonded Ferrite

Xolox Corporation 6932 Gettysberg Pike, Fort

Wayne, IN 46804 (219) 432-0661

(219) 432-0828 Barium Ferrite, NdFeB, Injection Molded

Magnets

Section 3 - Applications 3-8

Choosing A Magnet

Common Magnetic Materials

There are four classes of commercial permanent magnet materials. They are:

Ceramic

Alnico

Neodymium Iron Boron

Samarium Cobalt

Depending on the application at hand, the use of one material over another may have its benefits. The two

sample magnets provided with The Melexis Hall Effect Sensor Design Kit are both Neodymium 35 mag-

nets. They have very high flux densities for their size and should be handled with caution. These magnets

have been provided to assist in the design and construction of a Hall Effect Sensor System. The square

magnet will be referred to as magnet A, while the cylindrical magnet will be referred to as magnet B.

Graphs 1 through 4 are constructed using magnets A and B. South poles are marked with a do of red paint.

Melexis provides two types of neodymium magnets in the design kit chosen for low cost and high perfor-

mance, up to80

C. Neodymium is preferred for small-size magnets typically used with Hall ICs. If larger

magnet sizes or higher temperature ranges are necessary, Alnico or ceramic would be a better choice of

material.

Figure 11, Sample Magnets

4.4mm

3.0mm

4.6mm

Magnet A, Square Magnet B, Cylindrical

dia 9.5mm

10.0mm

A-15

Section 3 - Applications 3-10

Rare-Earth Magnets

Neodymium Iron Boron

Attributes of Neodymium

Low cost

Very high resistance to demagnetization

High energy for size

Good in ambient temperature

Material is corrosive and should be coated for long-term maximum energy output

Low working temperature

Applications of Neodymium

Magnetic separators

Linear actuators

Servo motors

DC motors (automotive starters)

Computer rigid disk drives

Samarium Cobalt

Attributes of Samarium

High resistance to demagnetization

High energy (magnetic strength is strong for its siz

Good temperature stability

Expensive material

Applications of Samarium

Computer disk drives

Automotive high-temperature environments

Traveling-wave tubes

Linear actuators

Satellite systems

Alnico Magnets

Attributes of Both Cast and Sintered Alnico (Large Magnets)

Very stable, great for high temperature applications

Maximum working temperature 524

C to 549

May be ground to size

Does not lend itself to conventional machining (hard & brittle)

High residual induction and energy product, compared to ceramic material

Low coercive force, compared to ceramic and rare-earth materials (more subject to demagnetization)

Most common grades of Alnico are 5 & 8

Applications of Alnico Magnets

MagnetosSecurity systems

Coin acceptorsClutches and bearings

DistributorsMicrophones

DC motors

Ceramic Magnets

Attributes of Ceramic Magnets

High intrinsic coercive force

Tooling is expensive

Least expensive material, compared to Akbuci and rare-earth magnets

Limited to simple shapes, due to manufacturing process

Lower service temperature than Alnico,.greater than rare-earth magnets

Finishing requires diamond cutting or grinding wheel

Lower energy product than Alnico and rare-earth magnets

Most common grades of ceramic are 5 & 8 (1-8 possible)

Grade 8 is the strongest ceramic material available

Applications of Ceramic Magnets

Speaker magnets

DC brushless motors

Magnetic Resonance Imaging (MRI)

Magnetos used on lawnmowers and outboard motors

DC permanent-magnet motors (used in cars)

Separators (separate ferrous material from nonferrous)

Used in magnetic assemblies designed for lifting, holding, retrieving and separating

Section 3 - Applications

3-11

Section 3 - Applications 3-12

Table 4, Magnetic Characteristics

Magnetic Material Density Maximum

Energy

Product

BH(max)

Br Reversible

Coefficient Residual

Induction

Coercive

Force

Intrinsic

Coercive

Force Hci

Maximum

Operating

Temperature

Curie

Temperature

lbs/in3g/cm

3MGO %/oCGauss Oersteds Oersteds oFoCoFoC

SmCo 18 0.296 8.2 18.0 -0.04 8700 8000 20000 482 250 1382 750

SmCo 20 0.296 8.2 20.0 -0.035 9000 8500 15000 482 250 1382 750

SmCo 24 0.304 8.4 24.0 -0.035 10200 9200 18000 572 300 1517 825

SmCo 26 0.304 8.4 26.0 -0.035 10500 9000 11000 572 300 1517 825

Neodymium 27 0.267 7.4 27.0 -0.12 10800 9300 11000 176 80 536 280

Neodymium 27H 0.267 7.4 27.0 -0.12 10800 9800 17000 212 100 572 300

Neodymium 30 0.267 7.4 30.0 -0.12 11000 10000 18000 176 80 536 280

Neodymium 30H 0.267 7.4 30.0 -0.12 11000 10500 17000 212 100 572 300

Neodymium 35 0.267 7.4 35.0 -0.12 12300 10500 12000 176 90 536 280

Alnico 5 (cast) 0.264 7.3 5.5 -0.02 12800 640 640 975 525 1580 860

Alnico 8 (cast) 0.262 7.3 5.3 -0.025 8200 1650 1860 1020 550 1580 860

Alnico 5 (sintered) 0.250 6.9 3.9 -0.02 10900 620 630 975 525 1580 860

Alnico 8 (sintered) 0.252 7.0 4.0 -0.025 7400 1500 1690 1020 550 1580 860

Ceramic 1 0.177 4.9 1.05 -0.20 2300 1860 3250 842 450 842 450

Ceramic 5 0.177 4.9 3.4 -0.20 3800 2400 2500 842 450 842 450

Ceramic 8 0.177 4.9 3.5 -0.20 3850 2950 3050 842 450 842 450

Section 3 - Applications 3-28

Magnetic Design

Input Characteristics

Digital Hall-Effect Sensors have specific magnetic response characteristics that govern their actuation

from OFF to ON. These characteristics are classified in terms of operate point, release point and differ-

ential. The operate point, commonly referred to as BOP, is the point at which the magnetic flux density

turns the Hall Sensor ON, allowing current to flow from the output to ground. Conversely, the release

point, commonly referred to as BRP, is the point at which the magnetic flux density turns the Hall Sensor

OFF. The absolute difference between BOP and BRP is referred to as Hysteresis, Bhys. The purpose of

hysteresis is to eliminate false triggering, which can be caused by minor variations in input, electrical

noise and mechanical vibration. There are three basic types of Digital Hall Sensors commonly used, as

listed below:

Switch - (unipolar) Operates with a single magnetic pole. Guaranteed not to latch ON in the absence of

a magnetic field. Opposing field has no effect. Generally used for mechanical switch replace

ment.

Latch - (bipolar) responds to both magnetic poles. Turns on in the presence of North or south pole, and

turns off only when the opposing field is sufficiently strong. Guaranteed to latch. Used primary

ily in brushless DC motor applications.

Bipolar Switch - (unipolar or bipolar) described as a device which responds to the zero-crossing from

North to South poles

The Hall-Effect Latch

The latch is a type of Hall IC which remains in either state (output ON or Off) until an opposite pole mag-

net is applied. A South magnetic pole turns the device ON (BOP). The device will stay ON until a North

magnetic pole is applied and turns it OFF (BRP). Melexis manufactures two types of Hall Effect latches.

designated for .2.2V to 18V operation. The US2880 series of Hall Effect Latches are designed for high

sensitivity. For more information refer to the data sheet section of this manual.

The Hall Effect Switch

There are two types of Hall Effect Switches, unipolar. The unipolar switch is normally “OFF” in the

absence of a magnetic field. The device turns ON (BOP) in the presence of a sufficiently strong South

magnetic pole, and turns OFF BRP) in the presence of a weaker South magnetic pole. MELEXIS manu-

factures the US5881UA and US5881SO Hall Effect Switches. For more information refer to the data sheet

section of this manual.

Magnetic Design Considerations

When designing a magnetic circuit, there are five considerations to be covered:

1. Cost of Hall IC, Magnet and Assembly

2. Temperature Range

3. Position Tolerance of Assembled Parts

4. Position Switching Accuracy

5. Tolerance Buildup

3-29

Cost

Hall IC cost will vary depending on the temperature specifications of BOP, BRP and Bhys. A loosely

specified device may easily be one half to one third the cost of a tightly specified device, yet perform the

same job. By providing steep slopes of flux density vs. distance and using strong magnets, the Hall IC

cost may be reduced.

Temperature Range

Hall Effect Sensors are categorized into different temperature ranges for the use in application-specific

design. It is very important that the Hall IC you select complies with your system’s ambient temperature.

Position Tolerance

Depending on the application and how it is assembled, the position of components, such as the magnet,

Hall IC and mechanical assembly, will determine the mechanical variations of the system. Some systems

are more tolerant of changes in air gap and lateral motion than others.

Position Switching Accuracy

The requirement in angular (degree) or linear position ultimately governs the magnetic circuit and Hall

IC specifications. That is if switching must repeat +0.1250in. or +0.1mm then the Hall IC specification

will be much tighter than if the specification is +1.00 or +1.0mm.

Tolerance Buildup

Tolerance buildup is the sum of all the variables that determine the operate point and release point of a

Hall IC. These variables include position tolerance,temperature coefficient, wear and aging of the assem-

bly and magnet variations.

Total Effective Air Gap

As mentioned previously, both Magnet A and Magnet B in the design Kit are composed of the same mate-

rial. Although the two magnets have similar characteristics, due to the difference in size and shape

total Effective Air Gap (TEAG) will have different effects on each magnets’ flux density vs. distance

curve.

TEAG is defined as the sum of active area depth and the distance between the Hall IC’s branded face to

the surface of the magnet. TEAG = Air Gap + Active Area Depth. Active area depth is simply the dis-

tance from the branded face of the sensor to the actual Hall Cell within it. The TEAG should be as small

as the physical system will allow, after taking into consideration factors such as the change in air gap with

temperature due to mounting, vane or interrupt thickness and wear on mounting brackets.

Graph 2 is given to show the effects of air gap on the slope of a graph using a single-pole slide-by con-

figuration with magnet A.

Section 3 - Applications

Section 3 - Applications 3-30

Graph 2, Slide-by Method with Magnet A Steep Slope vs. Shallow Slope

Tolerances Build-up

The following examples incorporate many different factors in order to show how tolerance buildup can

affect a Hall Effect system. Air gap tolerance, temperature range, and the temperature coefficient of the

magnet will cause the activation distance of a US5881 EUA to vary, thus impacting switching accuracy.

Tolerances:

Air gap Tolerance = 3mm. +1 mm. and 6mm. +1mm.

US5881EUA BOP/BRP Range = 95G min. BRP to 300G max. BOP

(IC Temperature Selection (EUA) = -40oC to 85 oC)

Temperature Range = -40oC to 85 oC

Magnet Temp. Coefficient = -0.1098%/oC

The US5880 Hall-Effect Switch has a maximum BOP of 250 Gauss and a minimum BRP of 140 Gauss.

If this part were to be actuated by sample Magnet B at an air gap of 3mm and 6mm, the following results

would occur (See Graphs 3 and 4). Each graph has an air gap tolerance of +1mm which could be due to

a loosely fitted mechanical assembly. Notice the difference in distance and slope between Graphs 3 and

Graph 3, Slide-by With Magnet A, Shallow Slope

Graph 2 shows what

happens to the slope of a

flux density vs. distance

graph by using different air

gaps with the same magnet

configuration.

Distance (mm)

1 2 3 4 5 6 7 8 9 10

400

200

Flux Density (Gauss)

600

800

1000

1200

Air Gap = 2.5mm

Air Gap = 5mm

200

400

600

800

1000

0 2 4 6 8 10 12 14 16 18 20

Distance (mm)

Flux Density (Gauss)

Air Gap = 5 mm

Air Gap = 6 mm

Air Gap = 7 mm

Always "OFF"

Always "ON"

3-31

Graph 4, Slide-by With Magnet B, Steep Slope

The effects of mechanical tolerance on a Hall Effect System have just been illustrated. Temperature range

can also affect this system. Temperature will expand or contract the mechanical assembly, but will also

affect the field strength of the magnet and the distance from BOP t BRP of the Hall IC. The Neodymium

sample magnets used in this kit have a temperature coefficient of -).1098%/0C. This means that as tem-

perature increases by one degree Celsius, the Flux Density will decrease by 0.1098%.

Graph 5, Effects of Temperature on Flux Density

Due to the temperature coefficient, the magnet will have a difference in flux density of 30% and a change

in activation distance of 1mm over this range of temperature. This may not appear tobe a significant dif-

ference in field strength or distance, but in conjunction with other mechanical factors, temperature could

become a factor.

Design Example 2: Now that Design Example 1 illustrated the effects of mechanical, magnetic and Hall

IC tolerances within a Hall Effect System. Design Example 2 illustrates how they produce tolerance

buildup.

Tolerances:

Air Gap Tolerance = 2mm +0.5mm

US5881EUA BOP/BRP Range = 90G min. BRP to 300G max. BOP

(IC Temperature Selection (EUA) = -40oC to 85oC)

Temperature Range =-40oC to 85oC

Magnet Temp. Coefficient = -0.1098%/oC

Section 3 - Applications

100

200

300

400

500

600

0 2 4 6 8 10 12

Distance (mm)

Flux Density (Gauss)

Air Gap = 2 mm

Air Gap = 3 mm

Air Gap = 4 mm

Always "ON"

Always "OFF"

Always "ON"

Always "OFF"

100

200

300

400

500

600

700

0 5 10 15 20

Distance mm

Flux Density (Gauss)

Flux Denisity @ 25C

Flux Denisity @ -40C

Flux Denisity @ 85C

Section 3 - Applications 3-32

Sample Magnet A is used in the double-pole slide-by method to show the variation air gap may have in

a mechanical system. These changes in air gap may be caused by factors such as vibration, wear, etc...

Note the differences in each air gap’s slope and maximum flux density. As the air gap distance becomes

larger, a decrease in flux density slope will cause less accurate switching in a Hall Effect System.

Graph 9

Graph 6, Slide-by with Sample Magnet A Air Gap Tolerance

By zooming in on the previous graph, min. BRP and max. BOP of the Hall Effect System can be shown

in greater detail and will also reveal the differences in activation distance at each air gap. See Graph 10.

Graph 7, Slide-by with Sample Magnet A

Change in Activation Distance with Air Gap

The plotted lines in Graph 7 do not include the effects of temperature. Graph 8 shows how temperature

will change the magnet’s flux density characteristics over the selected operating temperature range (-400C

to 85 0C), thus affecting the activation distance at min. BRP and max. BOP. To simplify the graph, only

the 2mm air gap is plotted.

-1200

-1000

-800

-600

-400

-2000

200

400

600

800

1000

1200

-30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30

Distance (mm)

Flux Density (Gauss)

Air Gap = 1.5 mm

Air Gap = 2.0 mm

Air Gap = 2.5 mm

-400

-300

-200

-100

100

200

300

400

-1.5 -1 -0.5 00.5 11.5

Distance (mm)

Flux Density

(Gauss)

Air Gap = 1.5mm

Air Gap = 2.0mm

Air Gap = 2.5mm

Always ON

Always OFF

3-33

Graph 8,Slide-by with Magnet A

Change in Activation Distance With TC

There is a 17% difference in flux density over the entire temperature range compared with the fixed-tem-

perature case in Graph 7. Note that because of the negative temperature coefficient of the magnet, the neg-

ative temperature (-400C) adds flux density to the magnet, while the positive temperature (850C) reduces

flux density.

Graph 9 shows how the air gap tolerance and temperature coefficient add together and cause tolerance

buildup in the Hall Effect system.

Graph 9, Slide-by Method with Magnet A

Change in Activation distance with Air Gap & TC

The two plotted lines in Graph 9 show the minimum and maximum possible cases when temperature and

air gap are considered. The overall difference in distance between min. BRP and max. BOP is slightly

larger because of the sum of tolerances being considered. This design example has shown the tolerance

buildup of air gap, benefits of slope, Hall IC BOP to BRP variations and magnet Tc. We have not con-

sidered the variation of initial flux density, which you must obtain from a magnet supplier.

Section 3 - Applications

-400

-300

-200

-100

100

200

300

400

-1 -0.75 -0.5 -0.25 00.25 0.5 0.75 1

Distance (mm)

Flux Density

(Gauss)

Temp = 25

Temp = 85

Temp -40

-400

-300

-200

-1000

100

200

300

400

-1.5 -1 -0.5 00.5 11.5

Distance (mm)

Flux Density

(Gauss)

Air Gap = 1.5mm @ -40

Air Gap = 2.5mm @ 80

Section 3 - Applications 3-34

The Push-Pull Method

Graph 10 shows the push-pull method. A South magnetic pole, perpendicular to the branded face, is used

in conjunction with a North magnetic pole at the opposite face. The two magnets in the single-pole slide-

by configuration are moved in the X-direction with respect to a stationary Hall IC.

Graph 10, Push-Pull Activation Using Slide-by Method with Magnet A

The Push-Push Method

The push-push method is similar in configuration to push-pull, but requires a South pole located at the

branded face along with a South pole at the opposite side of the Hall IC. When the Hall IC is centered

between these two South magnetic poles, their flux density cancels out leaving zero flux density at this

position. If the two magnets maintain the same distance between each other and are moved in the head-

on method in either direction, the flux vs. distance graph will be linear.

Graph 11, Push-Push Activation Using Head-On Mode

The US3881EUA Hall Effect Latch has maximum BOP of 90 Gauss and a minimum BRP of -90Gauss

over the temperature range of -400C to 850C. Due to the temperature coefficient of -0.1098%/0C, the

magnet will have a difference in flux density of 8% over this distance range. The distance necessary to

fully actuate a US3881EUA Hall Effect Latch is approximately 0.35mm from max. BOP to min. BRP.

This is an extremely large increase in performance from the previous example.

-200

200

400

600

800

1000

1200

1400

-15 -13 -11 -9 -7 -5 -3 -1 1 3 5 7 9 11 13 15

Distance (mm)

Flux Density (Gauss)

Motion

Air Gap

Motion

The Air Gap for

both magnets is

approximately 6.3mm

-350

-300

-250

-200

-150

-100

-500

100

150

200

250

300

350

-1 -0.75 -0.5 -0.25 00.25 0.5 0.75 1

Distance (mm)

Flux Density (Gauss)

Temp = 25

Temp = -40

Temp = 85

Max. Bop

Min. Brp

Gauss BOP BRP Bhys

Maximum 90 -10 100

Minimum 10 -90 100

The Air Gap for

both magnets is

approximately 3mm

3-35

Biased Operation

Biased operation is a method of controlling the magnetic field surrounding a Hall IC and is quite similar

to the Push-Push Method. For example, if a South Pole were attached to the reverse side of a Hall Effect

Switch, the Hall IC would be held on the “OFF” position until a South pole of a larger magnitude is intro-

duced to the branded face of the sensor and cancels out the opposing magnetic flux. This can be a very

important concept if a Hall IC were located within an electronic system with other opposing magnetic

fields. It will ensure that the Hall Sensor cannot switch accidentally. Figure 11 is an example of the bias

method. In Figure 11, the push-button uses a bias magnet to ensure that the button is in the Off position

until being pressed. When the button is pressed, the magnet adjacent to the branded face moves in the

head-on configuration closer to the Hall IC. This positive flux density will cancel out the negative flux

density provided by the bias magnet, eventually turning the button ON. Once the button is released, the

two opposing South magnetic fields will repel each other and send the button to its original OFF position.

Figure 11, Push-button with Bias Magnet

If the US5881 Hall Effect Switch were tobe used in this bias magnet configuration, this switch would

remain in the OFF position until the button is

pressed. If the magnet adjacent to the branded

face of the Hall IC has an air gap of 2.0mm before

being pressed, the switch will be exposed to a flux

density of approximately - 200Gauss. the button

will need to be moved a distance of 0.75mm

inward to exceed 250 Gauss and turn ON. After

being released, the magnets will repel and turn

OFF the Hall IC at a distance of 1.7 mm away

from the branded face.

Section 3 - Applications

Flux Concentrators

A flux concentrator, or pole piece, is a ferrous material used to significantly increase the performance of

a Hall Effect Sensor System. When a flux concentrator is placed opposite the pole face of a magnet, the

magnetic field channels through the concentrator, thereby increasing the flux density between the con-

centrator and the pole face.

Figure 13, Flux Concentrator

The reason this magnetic flux channels through the concentrator is because of the reluctance of the fer-

rous material. Reluctance is the resistance that magnetic flux lines experience as they flow from the North

pole into the South pole of a magnet. Ferromagnetic materials have lower reluctance than air, therefore a

pole piece provides an easier path for the flux to flow through, while increasing the flux density at the

same time. There are three benefits to adding a flux concentrator to a magnetic circuit. First, a less sensi-

tive Hall Effect Sensor can be implemented, a result of the increased flux density. The second benefit of

using a flux concentrator is that Hall Effect Sensor with a specific operate level can be actuated a greater

distance from the magnet, than if one were not in use. The addition of a pole piece also allows the use of

a magnet with a lower field intensity. A flux concentrator makes it possible to use a smaller magnet or a

magnet of different material to achieve the same operating characteristics as one with higher flux density

or a larger size. When choosing materials for a pole piece, pay attention to the following characteristics:

The permeability and reluctance of the material will affect its performance as a flux concentrator. Also,

pay close attention to the mechanical characteristics of machining and corrosion. These properties are

very important when selecting an alloy.

Section 3 - Applications

3-9

Magnet

Ferrous

Concentrator

Section 3 - Applications

3-13

Electromagnets

Another method of actuating Hall Effect Sensors is through the use of electromagnets. They are especial-

ly useful in circuit-breaking or current-sensing applications because their magnetism can be “turned on”

or “turned off” at will. An electromagnet consists of a coil of wire which may be wrapped around ferrous

material or core. The strength of this magnetic field is dependent on many variables, such as the perme-

ability of the ferrous material, the number of times the coil is wrapped around the material, the amount of

current flowing through the coil and the length of the core. These variables are related to each other in the

same way for all types of electromagnets, but the formulas differ slightly due to the variance of shape in

core material.

Figure 14, Common Electromagnet Shapes

Where:B = magnetic flux density

u0 = permeability of core material

N = number of turns made by coil

i = amount of current in coil

R = mean radius of toroid

Where:

B = magnetic flux density

u0 = permeability of core material

N = number of turns made by coil

i = amount of current in coil

l = length of cylindrical core

After choosing a core that will physically fit into the proposed current-sensing system, the values of length

of radius, permeability and at what value the current limiter is to operate will be known. The operate point

and release point for each Melexis Hall IC can be located in the datasheet section of this manual. By

knowing this information, it is now possible to calculate how many turns of wire will be required to pro-

duce enough flux density to actuate the Hall Effect Sensor. When designing an electromagnet use mate-

rials with the following properties: High saturation induction and high permeability will produce strong

magnetic fields resulting in a smaller device that will operate with little energy. When an electromagnet

is being switched on and off, use a material with a low coercive force for faster magnetization and demag-

netization. Also pay attention to magnetic aging and mechanical characteristics such as corrosion. This is

Measuring Flux Density

The Melexis Hall-Effect design kit is supplied with a linear Hal-Effect sensor which has been calibrated

to 1mV/G, making it very easy to build a circuit which will measure flux density. This is done with the

fully programmable MLX90215, programmed in this case to deliver exactly 2.5V at zero magnetic field

rising 1mV for every 1 Gauss applied.

If a field of 100 Gauss is applied, the output will increase 100mV. Similarly, if a field of -100 Gauss is

applied, the output will decrease 100mV. It is very important to maintain a regulated supply voltage of

5V because the output has no internal regulator and is ratiometric. The Hall IC's output is exactly V

/2,

so if the supply voltage changes the output voltage also changes.

Figure 15, Flux Measurement

Figure 9 shows a circuit which allows the Voltmeter to be calibrated to read in units of Gauss. Shifting the

ground reference of the Voltmeter to exactly the quiescent voltage of the Hall IC makes the 2.5V VOQ

appear to be zero. The potentiometer allows this to be set to exactly zero. When a magnetic field is

applied, the DVM will display units of Gauss in a range of +/- 2000 Gauss. The resistor network is to

divide the output voltage by 10 so it can be measured by

200mV meter. If your meter is 2V range, this divider is not

used. This circuit can be built into a small case with a battery

and voltage regulator for a very inexpensive flux measuring

device.

With no divider used:

Full Scale: (2.00V)+/-2000 Gauss

+/-200 mT

1 Gauss = 1mV

1 mT = 0.1mV

With 1/10 divider used:

Full Scale: (200mV)+/-2000 Gauss

+/-200 mT

1 Gauss = 0.1mV

1 mT = 0.01mV

1mT = 10Gauss, exactly

Section 3 - Applications 3-16

MLX

90215

2501000

5V+

200Ω

Potentiometer

zero adjust

9k*

1k* Attach to

Voltmeter

1µF.1µF

A-17

1/10

divider

(optional)

Output Voltage vs. Magnetic Flux Density

-200 -100 0 100 200

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

5.0

Output Voltage (Volts)

Flux Density (mT)

MLX

90215

2501000

DVM

2.00V Scale

.1µF.1µF

A-16

Figure 16, Flux

Section 3 - Applications

3-17

Applications

Balance, Level, Vibration, Acceleration

A magnet suspended from a pendulum, free to move in the X - Y plane, may be used to detect level posi-

tion, acceleration and vibration. Figure 12 shows a magnet with the center as a North pole and an outer

ring of South pole. In a level, stable condition, the North pole turns OFF the switch. Motion will cause

the magnet to move in such a way that the South pole is over the Hall IC, turning the switch ON.

Obviously, the mechanical configuration is not trivial, and will vary greatly depending on the application.

Switches like this are used in some washing machines.

Figure 17, Level and Motion Switch

A-18

SNS

The magnet is free to move relative to the Hall IC.

Any force disturbing the magnet may result in a South pole

facing the Hall IC, resulting in an ON condition

Section 3 - Applications 3-18

Programmable Isolated Current Sensor

Hall Effect Sensors can be used in conjunction with an electromagnet to make a very efficient, isolated

current-sensing device. This can be used to protect components from damage such as overheating.

The only components needed, as in Figure 13, are a Hall IC and a slotted ferrite toroid core driving an

indicator, relay or a logic-level fault signal. The ideal Hall IC in this application is a programmable linear

device, which would allow accurate calibration of the sensor and also versatility. For example, the dia-

gram below shows a toroid with 4 turns of wire. If a current of 10A was applied to the coil, the field at

the sensor would be about 6mT. The Hall IC can be programmed to give the desired response to this mag-

netic range. If the desired voltage swing is 0.5V (or 500mV) and the magnetic swing is 6mT, then the

device can be programmed to

83mV/mT to give the correct

response.

By varying the number of

windings and the programming

codes, rages as small as 100mA

and as large as 500A can be

achieved.

If used with a linear Hall-Effect

sensor, the output voltage will

be proportional to the current

flowing through the windings

on the toroid. This Voltage can

be used to indicate current

level, or trigger a shut-down

circuit.

Current Indicator/Limiter

If only a current indicator or limiter is needed, a simple circuit can be built with a Hall switch, and requires

no programming. In Figure 13, each complete turn of coil around the core, with a current of 1 Amp flow-

ing through it, will produce a flux density of approximately .6 mT upon the Hall Effect Switch. By adjust-

ing the number of coil turns around the core, the Hall Effect switch can act as either a current indicator or

current limiter.

Example:

Under normal operation a current of 10 Amps is delivered to a motor by some wire. Wrapping four turns

of the wire around the core will produce a magnetic field of 24 mT upon the Hall IC, which will activate

the device to ON. This can be used to illuminate an LED to symbolizing “normal” current. The LED will

remain ON until the field drops to about 18mT or about 7.5 Amps.

To use as a limiter, simply drop one turn of wire, which will require a higher current to turn the Hall switch

ON. With 1 turn removed, the switch will require 13 Amps to turn ON, and will remain on until the cur-

rent level drops below 1Amp.

PTCTM

Figure 18, Programmable Current Sensor

3-19

Flow Meter

One popular device that uses a Hall IC is a flow meter (Figure 15). The spoked wheel (paddle wheel), is

driven by some type of medium flowing through the pipe. A magnet is attached at the tip of each spoke.

In the presence of moving vapor or liquid, the magnets spin at a speed related to the viscosity of the medi-

um flowing through the device. The spinning magnets will switch the Hall IC, producing a square wave

output, with a frequency proportional to the flow rate.

Figure 19, Flow Meter

Liquid or vapor entering in the

direction of the arrow spins the

paddle wheel switching the Hall

IC “ON” and “OFF”, creating a

square wave output

Power Control

Many switches must control significant power. A Hall IC can do this only through a relay or Power FET.

For a component cost of $1.00, an isolated 50 Volt, 50 Amp switch can be made.

Figure 20, Power Switch

Section 3 - Applications

U18

627

12V Load

Section 3 - Applications 3-20

Push-button

The common push-button switch may be rep;aced with a Hall IC and magnet, as shown in two configu-

rations, Figure 17. The slide-by case, Figure 17A, requires a mechanical return spring, returning the push

button after it is depressed, but Figure 17Buses repelling magnets to activate the switch and return the

push-button.

Figure 21, Push-button

Figure 21A

A mechanical spring is required in order

to eturn the button to its off location.

Figure 21B

Repelling South poles take the place of a

mechanical spring to return the button to

3-21 Section 3 - Applications

Liquid Level Detector/Alarm

By attaching a magnet to a float, as in Figure 18, the proximity method of actuation is used to turn on the

Hall IC as the liquid level rises within the housing.

Figure 22, Liquid Level Detector

Magnet

Float

Housing

Liquid

Section 3 - Applications 3-22

Position Sensor

As shown in Figure 19, a hydraulic or air-piston is moved downward until reaching the specified position

set by the Hall IC. This could be a useful application in robotic assembly machines, where accuracy of

position is extremely important.

Figure 23, Position Sensor

Sealed

Compression

Chamber

Piston

Magnet

Hall IC

3-23 Section 3 - Applications

Geartooth Sensor

Magnetic Geartooth Sensing

The need to sense speed and position of ferrous gears occurs in numerous industries. The ability to con-

vert the repetitive passing teeth to an electrical impulse has been sought for many decades. purely

mechanical systems have been used with the attendant issue of wear and failure limiting its use to low

speed and low duty cycle applications.

Hall-Effect geartooth sensing makes use of the Hall element to sense the variation in flux found in the air-

gap between a magnet and passing ferrous gearteeth. A modern approach is to convert the signal from the

Hall element to a digital value and then perform signal processing to create a digital output from that

effort. In the case of the Melexis geartooth sensing scheme each time the signal changes direction a

counter is reset. If the signal level changes beyond the preset magnitude from the positive or negative peak

the output level is changed. This creates a digital zero speed peak detection speed sensor. It is immune to

orientation requirements and cab follow the gear speed down to the cessation of motion. It will detect the

first edge of the next tooth after immediately after power on. The digital signal processing does introduce

an uncertainty from quantization that is greater at larger speeds. Extremely demanding timing require-

ments like those found in crank position sensors may suffer from the loss of accuracy at high speeds.

Figure 24 shows the Melexis MLX90217 geartooth sensor operation.

Figure 24, Geartooth Sensor

Gear Tooth Sensor Magnetics

In order to detect the passing gear teeth with a Hall effect sensor it is necessary to provide a source of

magnetic energy. The simple way to do this is to arrange a permanent magnet such that the axis of mag-

netization is pointing toward to surface of the gear teeth. As a tooth moves across the surface of the

magnet the flux will become attracted to the lower reluctance path provided by the ferrous steel struc-

ture. When this occurs the flux density measured by the Hall element between the face of the sensor and

the gear tooth increases. Many schemes have been developed and some patented that use the various

attributes of the vector flux field and its changing nature to create zero speed Hall effect gear tooth sen-

sors. Melexis has chosen to work with digital signal processing schemes and in this way minimize the

magnetic circuit manipulation required of the end user. Put simply, by applying silicon "smarts" the

magnetic subtlety and slight of hand is nearly eliminated.

Figure 25, Geartooth Flux Transitions

Magnetic modeling courtesy of AnSoft

TMmagnetic modeling software.

Section 3 - Applications 3-24

Brushless DC Motor Sensors

The use of Hall Effect Sensors in DC motors eliminates the friction, electrical noise and power loss asso-

ciated with other types of mechanical commutation, such as brushes. Hall ICs provide a long mainte-

nance-free life and offer greater flexibility with respect to direst interface with digital commands.

Figure 26, Brushless DC Motor Sensor

Figure 26, Brushless DC Motor Controller

The US8881 is a brushless DC motor driver that was designed to meet the needs of high volume, low

cost motors which do not require the expensive options needed for servo or other closed loop applica-

tions. The US8881 works with 3 HallIC latches, and provides all motor control via 6 external N-chan-

nel FETs. A complete datasheet for this device is in section 4 of this book.

Figure 26, Brushless DC

Motor Controller

Ω

BLDC Motor

Filter ISENSE

Resistor

Forward/Reverse

Thermal Switch

OSC

COSC

Speed Adjust

Ω

VREF

Ω

0.1

1000

V+ = 40 V

Brake

Counter Reset

0.1

Supply Voltage1Cap Boost "A"

VREF

Out

Hall "A" Input

Hall "B" Input

Hall "C" Input

FWD/REV Input

Speed Adjust Input

Oscillator Control

(+)Current Limit (Brake)

Analog Ground

(-)Current Limit (Reset)

Power Ground

Gate Top "A"

Feedback "A"

Gate Bottom "A"

Cap Boost "B"

Gate Top "B"

Feedback "B"

Gate Bottom "B"

Gate Top "C"

Feedback "C"

Gate Bottom "C"

Cap Boost "C"

UF4002

(3 Places)

VREF

REF

0.005 µf

10K Ω

1K Ω

0.1µ

1000

15V

500Ω

0.1

F20

3-25 Section 3 - Applications

Programmable Motion Sensors

The use of Hall-Effect Sensors as an alternative to resistive potentiometers is a popular trend because of

the advantages of non-contacting elements. in the past, linear Hall ICs were not very practical because of

bad temperature prformance and the ned for disctete trimming methods.

Melexis created it’s programmable linear Hall ICs to solve both problems, leaving the end-user a very

accurate sensor which is stable over temperature.

The configurations are infinite, shown are two basic magntic circuits which will allow literally thousands

of position sensing applications.

Rotary Motion

Figure 27 is the rotary method, where the Hall Ic is placed within a ring magnet, and the manetic field is

linear to the rotary motion. Depending on the magnetic elements, this method can be linear up to 160

, but

typically, as shown 45

-90

oof linearity can be easily achieved. This configuration is suitable for any rotary

position application.

Linear Motion

Figure 28 illustrates a linear position sensor, where a linear motion (as apposed to rotary motion) is trans-

lated to a liear voltage via the Hall IC. The magnetic circuit shown is linear for approximately 50% of the

entire length of magnet. For example, if the magnets were 1” long, they can be used to measure 1/2” of

motion.

Programming

To further enhance the system, the sensors are programmable to give optimal results. Details about pro-

gramming are available in the MLX90215 and MLX90237 datasheets in section 4 of this book

Figure 27, Programmable Rotary Potentiometer

PTCTM

Section 3 - Applications 3-26

The use of Hall Effect Sensors in DC motors eliminates the friction, electrical noise and power loss asso-

ciated with other types of mechanical commutation, such as brushes. Hall ICs provide a long mainte-

nance-free life and offer greater flexibility with respect to direst interface with digital commands.

Figure 28, Programmable Linear Potentiometer

PTCTM

Linear

Range

3-27

Other Applications

Some additional Hall Effect Sensor applications are listed below:

Application: Configuration: Refer to Figure:

Aircraft/Automotive:

Tachometer Ring Magnet, Gear Tooth Sensor 7

Speed Indicator Ring Magnet, Gear Tooth Sensor 7

Roll Indicator Linear, Pendulum 12

Planing Angle Indicator Linear, Pendulum 12

Acceleration Indicator, Linear Pendulum 12

Fuel or Liquid-Level Sensor Level Detector 18

Seat Belt Sensor Proximity Switch 3

Airbag Ejection Sensor Proximity Switch 3

Power Window Sensor Proximity Switch 3

Door-Ajar Sensor Proximity Switch 3

Appliances:

Water/Liquid Flow Digital, Ring Magnet 15

Washer Water Level Digital Float 18

Washer Tilt Sensor Digital, Pendulum 12

Washing Machine Motor Latch, DC Motor 20

Air conditioning Blower Latch, DC Motor 20

Refrigerator, Door Sensor Proximity Switch 3

Home, Tools and Security:

Security Door Sensor Digital Vane Switch 5

Security Window Vibration Linear, Pendulum 12

Circular Saw Motor Latch, DC Motor 20

Overload Protection Current Limiter 13

Digital Combination Lock Rotary Switch 14

Mechanical Jam Protection Current Limiter 13

Power controller Switch with Circuit 16

Exercise Machine Counters Slide-by Switch 2

Computer Key Pads Push-button 17

Pinball Machine Buttons Push-button 17

Section 3 - Applications

The Programmable Sensor Interface

A microcontroller sensor interface provides signal conditioning for a sensor element of any kind. Some types of

sensor elements that can be used are pressure sensors, strain gauges, load cells, thermistors, and potentiometers

(position sensing).

The MLX90308 sensor interface provides control over the offset, gain (or sensitivity), linearity and temperature

compensation of the sensor’s signal using a microcontroller.

In the past, such conditioning was done through discrete circuits. Such circuits required costly and unreliable

trimming methods, not to mention component count on the PC board. Because the MLX90308 is a microcon-

troller, calibration is done digitally through a standard PC. The MLX90308 contains an 8-bit RISC core micro-

controler, analog signal path, supply regulator, and EEPROM for storing compensation coefficients.

Figure 29 illustrates a fundamental application of the MLX90308 and a bridge type pressure sensor element. In

this application, the 90308 uses an external FET as a pass transistor to regulate the voltage to the sensor and the

analog portion of the IC. This is known as Absolute Voltage Mode, where voltage to the sensor and analog cir-

cuit is regulated, independent of the supply voltage. The 90308 can be operated in Ratiometric Voltage Mode,

where the output (VMO) is tied to an A/Dconverter sharing the same Supply and GND reference. A third wiring

IO1

IO2

TSTB

FLT

OFC

VBN

VBP

TMP

COMS

GND

CMN

CMO

VMO

VDD1

FET

VDD

MLX90308

External FET

for Regulation

Pressure

Sensor

Oil

Pressure

(psi)

Communications

GND

Signal Out

V+

Figure 29 - Oil Pressure Gauge Communications

Signal Out

GND

V+

option is Current Mode, which allows the user a 4mA to 20mA current range to use as a

2-wire analog sensor. These wiring examples as well as technical specifications are shown

in section 4 of this book.

The figures above illustrate to performance of an unconditioned sensor output and a conditioned sensor output

versus stimulus(pressure) and temperature. Notice that figure XXa has a range of only 200mV and has a non-lin-

ear response over a 0-100psi range. The sensitivity of the unconditioned output will also drift over temperature,

as illustrated by the three slopes. The MLX90308 corrects thes errors and also amplifies the output to a more

usable voltage range as shown in figure XXb.

Prototyping with the MLX90308

Melexis has available a MLX90308 evaluation kit

which contains an evaluation circuit board, serial

interface cable, and software diskette. The circuit

board provides the neccessary circuitry for all three

applications circuits shown in the MLX90308

datasheet (section 4 of this book). Also contained on

theboard is level shifting and glue logic necessary

for RS-232 communicatios.

The boaerd has a socket with a single MLX90308

installed, and direct access to the pins of the IC. The

user can easily attach bridge sensor to the board for

in-system evaluation. The serial interface cable

connects the evaluation board directly to a PC’s ser-

ial port for in-system calibration.

The software runs in the familiar Windows platform

and allows for programming and evalution of all

compensation parameters within the EEPROM of

the MLX90308.

Pressure (in psi)

0 100

Voltage (in mV)

200 Raw Sensor Output

measured between VBP and VBN

Pressure (in psi)

0 100

Voltage (in Volts)

5Conditioned Sensor Output

measured between VMO and GND

150 oC

25oC

-40oC

150 oC

25oC

-40oC

Figure 30a Figure 30b

Hall

Plate

Microcontroller Family Overview

Melexis is offering custom and semi-custom microcontrollers for automotive applications. Custom microcon-

trollers provide the most cost affective solution by exactly matching the designers needs. Off-the-shelf micro-

controllers typically force the customer to pay for features they don't need, or don't satisfy all of the system

requirements and drive up the component count. Benefits such as increased reliability and design flexibility are

realized using a custom microcontroller. Typical applications are small system control and smart sensor inter-

faces.

Melexis' custom microcontrollers are based on either an eight or sixteen bit RISC core. These can contain the

exact analog and digital periphery the customer needs. Melexis is also embedding sensors on the same die.

Chopper stabilized Hall effect sensors and temperature sensors are currently available. To withstand the auto-

motive environment, these microcontrollers are designed to withstand an 80V load dump, -40°C to +150°C oper-

ating die temperature, 6V to 26V supply and output shorts to ground or the battery.

Current applications include controllers for dashboard indicators, air conditioning, solenoid valve system, heat-

ing system, electric window, sunroof and head cushion position, intelligent relay control, and sensor signal con-

ditioning.

Memory

Versatile memory types and configurations are available. The microcontroller's I/O is entirely up to the cus-

tomer. Melexis has a number of standard digital and analog interfaces available. Custom interfaces can be devel-

oped if needed. Typical implementations include:

MX11 (8 bit core) MLX16 (16 bit core)

RAM, (bytes)128 256 - 512

ROM 2K 8K

E2PROM 32128 - 256

I/O(memory mapped)816

PROM 2K 8K

Digital Interfaces

Logic I/O, up to 256 eight-bit ports are possible. The outputs can have high voltage (up to 80V), high current

(up to 300 mA), or current limited capability. Also, digital outputs can have re-circulation diodes for driving

relays.

Other digital function that are available are PWM outputs (0% to 100% duty cycle), timers, UART, watchdog

timer and display interfaces.

Custom digital interfaces or functions can be implemented to meet the customer's needs. The logic can be

Analog Interfaces

There is a wide range of analog interfaces that can be integrated with the microcontroller. Multi-channel A/D

converters in both 8 and 10 bit resolution . Band gap references for absolute measurements. On chip tempera-

ture sensing for compensation of measurements, digital to analog conversion, phase locked loops (PLLs) as well

as operational amplifiers and comparators. Oscillators can be implemented either with external frequency defin-

ing components or completely internal. Oscillators that are completely internal have frequency controlled from

a value stored in EEPROM. Custom analog interfaces are also available to meet the customer's needs.

Power

Maximum power consumption depends on the microcontroller and its exact configuration, clock frequency,

loads, etc. The minimum power needed can be as low as 150

µA in a sleep or power down mode with the core

still active. Operating supply can be from 6 to 26 volts and load dump protection to 80 volts.

Development Tools

Melexis has developed a tools set and methodology which allows the user to develop their system concurrently

with silicon development.

Software

Melexis has a full set of software tools to expedite the firmware development. For developing assembly language

firmware, an assembler, linker and loader is used. A C compiler is also available for developing firmware. The

firmware development can also be expedited by the software simulator. The simulator allows the developer to

run or single step through the code as well as reading and forcing registers and memory locations.

Hardware

A development board is available which contains a processor core with all of the program memory and periph-

eral interfaces external to the processor. The processor core with the RAM and interrupt controller are contained

in one Melexis chip. The firmware is executed from a user programmed PROM or EPROM. Peripheral inter-

faces and functions are implemented within a Xilinx PLA and discrete components. The PLA can be repro-

grammed by the user to develop the exact logic and interfaces needed. In addition to the development board,

there is an in circuit emulator (ICE) available. The ICE replaces an external PROM/EPROM and allows control

over the program execution flow. The user can start, stop and single step though the program, similar to using

the software simulator.

Prototype Devices

Development is also made easier by prototype devices. These devices have the internal busses brought out to

package pins. This allows for use of external program memory or ICE while the peripheral interfaces are at their

final implementation