MAGNETS OVERVIEWIntroduction
Purpose› Explore the magnet technology and how they are used in theelectronics industry
Objectives› Define key terms of magnets› Describe what magnets are and what they can do› Describe what and how they are used

Introduction
Let’s pose a few questions›

What is magnetic? Where does it come from?

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What is a magnet?

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What is a dipole?

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Where does the energy come from?

›What are a magnet’s properties - and why do we care?

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What affects magnets? How does one make an artificial magnet?

›How does one make a magnetic field?

›What is the Curie effect?

›Are there different types of magnets?

What is magnetic?

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The magnetic effect is created at thesub-atomic level
An atom has a nucleus composed ofprotons and neutrons. Electrons
encircle the nucleus
Two things occur in an atom that
produce a magnetic field
Both by the negatively charged
electrons

What is magnetic?

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Electrons by their very nature spin
Also electrons circulate around thenucleus of the atom
As electrons circulate around thenucleus they generate an angularmomentumNet effect of spin and the angularmomentum produces what is called adipole

What is magnetic?

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The electron’s spin and angularmomentum produces a tinymagnet within the atom and it
is called a dipole
The dipole’s magnetic field isvery small

DipoleWhat is magnetic?

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Most atoms have more than one
electronTwo electrons in a atom will forma pair or two dipoles
The two dipoles are equal andopposite in magnetic strength
The magnetic fields of twoelectron pairs cancel each otherout

Dipoles are equaland opposite

What is magnetic?

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With each additional pair of electrons within the atoms
there will be an accompanying pair of dipoles
Elements dipoles align themselves with an equal andopposite dipoles, canceling out overall magnetic effect.

What is magnetic?

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In several atoms the net magnetic
effect does not cancel
This is the case with many metals
When the atoms are grouped
together many billions of times
over there is still no net magnetic
field in the bulk material.
This is because in most metals thedipoles remain locked in placeThe arrows represent
random singular dipoles

What is magnetic?

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Iron, nickel and cobalt are
ferromagnetic elements that canoccur naturally magnetized innatureThis effect is shown to the rightwhere the dipoles are all aligned
These are called permanent
magnets
Most of the magnets we use inelectronics are made into magnets
by magnetic technology

What is magnetic?
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There are a few elements, wherethe dipoles are free to move whensubject to a magnetic field
The dipoles will align themselvesand can become ‘locked in place’ inthe crystal’s lattice structure
There are other ferromagnetic
materials where the dipoles alignwhen subject to a magnetic field
However, when the magnetic fieldis removed the dipoles revert totheir random structureMagnetic Properties

ALL MAGNETS HAVEDIFFERENT PROPERTIES

THE PROPERTIES OF EACH
ALLOW US TO SELECTIVELYUSE THEM IN DIFFERENTAPPLICATIONSMagnetic Properties – Used in a Relay or Sensor
Magnetize and demagnetize easily
Temperature stability
Magnetic strength
Magnet size

Magnetic Properties
Curie Temperature
Stability
Shock
Cost and availability

Temperature Effects

At temperatures >150°C use
high temperature stability
magnets such as AlNiCo series
and rare earth samarium
cobalt (SmCo)

Most magnets are relatively
stable at temperatures 0°C
and belowTemperature Effects
MagnetTypeLow
TemperatureHigh
Temperature

Comments

SmCo Magnets

Stable to 4ºK

Stable to 250ºC

Below 20ºC magnetic strength willrise slightly

NdFeB

Stable to 15ºK

Stable to 160ºC

Below 20ºC magnetic strength willrise slightly

Alnico magnets

Stable to near 0ºK

Stable up to550ºC

Most stable of all magneticmaterials

Ferrite magnets

Stable to -10ºC

Stable to 250ºC

At -20ºC they suffer a permanent
loss of magnetism

Magnetic Strength››

Magnetic strength determines distance where reedsensor will close and open
The effects of other magnetic fields or ferromagnetic
materials from nearby components may affect othermagnetic components

Magnet Size››

Size also determines theoperate points of sensor
The greatest sensing distanceis achieved when matching
size and strength of themagnet

Images courtesy of amazingmagnets.com

Curie Temperature››

The Curie temperature of a magnet or ferromagnetic
material is that temperature where the magnetic
properties are lost
The temperatures are usually quite high; however,they can and are reached in several applications

Curie Temperature›

Listing of different
magnet types and theirassociated Curie
temperatures

Material

Curie
Temp.
(K)

Curie
Temp.
(°C)

Curie
Temp.
(°F)

Co

1388

1115

2039

Fe

1043

770

1418

FeOFe2O3

858

858

1085

NiOFe2O3

858

585

1085

CuOFe2O3

728

455

851

MgOFe2O3

713

440

824

MnBi

630

357

674

Ni627

354

669

MnSb587314

597MnOFe2O3573

300

571

Y3Fe5O12560

287

548

CrO2386

113

235

MnAs

318

45

113

Gd

292

1918

Dy

88-185-301

EuO

69-204-335

Magnetic Stability››

Depending upon the application, stability can be an
important parameter affecting the sensing distances
for a given sensor
Careful evaluation of the magnet specification needs
to be consideredShock01

02

03Strong shock can change
the magnetic strength
for a given magnet.

If an application calls foran environment
involving shock, caremust be taken in
selecting the correctmagnet.

Shock can become afactor in a Form B orlatching relay where
relay handling at thecustomer site can cause
enough shock to alter
the operate points.

Types of Magnets

Most common - Iron (Fe), nickel (Ni), and cobalt (Co)Less common - Chromium (Cr) and manganese (Mn)
Very strong fields - Rare earth
Most popular - Neodymium (NdFeB) and Samarium (SmCo)
Magnets with combined elements (ex. AlNiCo)

Strong Magnetic Fields››

Much speculation about
large magnetic fields
physically distorting reedswitches
Once the reed blades aremagnetically saturated,
there is no increased fieldstrength

Using Magnets››A magnetic field can begenerated by passing acurrent through a wire
The simplest way to makea uniform magnetic field
is with a solenoid or coil

MAGNETSHOW THEY ARE USED IN THE ELECTRONICS INDUSTRYHow Magnets Are Used

Magnets areused in ReedSensors

Magnets areused in some
Reed Relays

MAGNETSUSED IN REED RELAYSReed Relays›››

For normally closed or latching reed relays a magnet
within the relay is required.
Ability to magnetize and demagnetize the magnetfor precise activate and deactivate points
Stable results require the magnet fully magnetizedthen demagnetized to the best operating point

Reed Relays››››

Magnet is placed inside the coil
Magnet centered on reed switch gap and in parallelwith the switch
The two reeds within the reed switch become
magnetized, with opposite polarity
Opposite polarity causes the reeds to close

Reed Relays

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Power is applied to the reed relay coil, the magnetic field
produced is equal and opposite in polarity
The magnetic field produced by the coil opens the contactsRemoving the coil power the contacts reclose
This completes the cycle of how a normally closed reed relay
works
Latching relays occurs in a similar way

Reed Relays›

The best magnet types to use for reed relays
The AlNiCo series are usually best
› Sintered magnets are also very good›

MAGNETSUSED IN REED SENSORSReed Sensors›››

Reed sensors require a magnet
Reed sensors use the most magnets of any sensing
technology
Reed sensors use all types of magnets

Reed Sensors

3 ways to usemagnets
NormallyNormallyopen sensor closed sensor
1 magnet

2 magnets

Latchingsensors
2-3 magnets

Reed Sensors – N.O. (Normally Open)

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Normally open reed sensors most popular
Reed switch in a protective package and mounted on
a PCB or directly wired
Magnet is mounted to a moving element where its
distance is being sensed
Magnet entering reed switch’s sphere of magnetic
influence will close contacts

Reed Sensors – N.C. (Normally Closed)›››

Normally closed reed sensors used in applications
where contacts will be closed for long periods oftime
No power used when the contacts are closed
Two magnets are required

Reed Sensors – N.C. (Normally Closed)

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A magnet is centered over the contacts closing the contacts
Another magnet is mounted to a moving element where its
distance is being sensed
Magnet entering reed switch’s sphere of magnetic influence
will open the contacts
Removing the second magnet will cause the contacts toreclose completing the cycle

Reed Sensors – Latching›

Again a magnet is centered over the contacts›

The magnet’s strength is not strong enough to close the contacts›››

A second magnet is mounted to a moving element where its distance isbeing sensed
When this second magnet enters reed switch’s sphere of magnetic
influence the contacts will close
Removing the second magnet the contacts will remain closed

Reed Sensors – Latching

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A third magnet with opposite polarity to the second magnet
is usedAlso, one can use the second magnet with its polarity
reversed
When this magnet enters reed switch’s sphere of magnetic
influence the contacts will open
Removing this magnet the contacts will remain open
completing the cycle

Summary

Magnets
have uniqueproperties inelectronics

Wide variety
of magnets
with host ofdifferent
properties

Magnetusage isincreasing inelectronic
applications

They allow
us to switch
reed sensors
with zeropower.

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