MEDER electronic
ABOUT MAGNETS
Magnets and their specifications
Magnets are available in multiple specifications on the
market. Almost all dimensions and geometries can be
realised. To activate the reed switch a magnet (magnet
field) is needed. The different magnet materials have
either more positive or negative specifications, depend-
ing on the dimension and geometries as well as on the
environment. Most preferred and used forms are cyl-
inders, rectangles, rings and discs. Depending on the
different requirements, magnets can be magnetised in
many different ways (see picture # 37).
Furthermore each magnet material has a different mag-
net force as well as a different flux density. Additionally
to dimension and material, other factors exist that define
the energy of a magnet. These are mounting position,
environment and other magnetic field witch influence
the interaction between reed sensor/switch and mag-
net. In applications were a magnet is used to activate
a reed sensor/switch, the environmental temperature
needs to be considered (in the application as well as
in storage). High temperatures can cause irreversible
damage and will have heavy impact on the magnetic
force and the long term stability. AlNiCo magnets are
best suitable for applications up to 450°C.
(Figure #37. An assortment of magnets are shown. Magnets can be formed and made into almost any shape.)
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41
ABOUT MAGNETS
General Information to magnet mate-
rial
Magnets have reversible and irreversible demagneti-
sation specifications. Be specially careful with shock,
vibration, strong and close external magnetic fields as
well as high temperatures. All these factors influence
the magnetic force and the long term stability in differ-
ent intensities. Preferably the magnet is mounted on the
moving part of the application. Professional tuning of
magnet and reed switch can improve the functionality
of the whole sensor-magnet system.
MEDER electronic
AlNiCo - Magnets
AINICo Features
Standard geometric and magnetization
Rectangle
Cylinder
•
working temperature
from -250 to 450
o
C
•
low temperature
coefficient
LOW
HIGH
costs
energy (WxHmax.)
working temperature
corrosion - resistant
opposing field - resistant
mechanical strength
temperature coefficient
Ferrite
Ferrite
NdFeB
NdFeB
AlNiCo
Ferrite
AlNiCo
AlNiCo
AlNiCo
Ferrite
SmCo
Ferrite
SmCo
SmCo
NdFeB
SmCo
SmCo
AlNiCo
NdFeB
NdFeB
NdFeB
SmCo
NdFeB
AlNiCo
Ferrite
SmCo
AlNiCo
Ferrite
Raw materials for AlNiCo magnets are aluminium nick-
el, cobalt, iron and titanium. AlNiCos are produced in a
sintering - casting procedure. The hard material needs
to be processed by grinding to be cost effective. Due to
its specifications, the best dimension is a remarkably
longer length than its diameter. In combination with reed
sensors / switches we recommend a length / diameter
ratio of more than 4. AlNiCo magnets have an excellent
temperature stability. Negative are the high raw material
prices. Cylindercal AlNiCo magnets can be used with all
MEDER reed sensors / switches without any problems.
AINiCo
Magnetic Values according to DIN
17410
(B x
H)
max.
Min.
Typ.
Max.
Units
Energy Product
Remanence
35
39.8
1210
0.02%
kJ/
m
3
mT
%/K
kA/m
kA/m
%/K
g/cm
3
450
o
Reverse Temperature Coefficients
of
B
r
Coercivity
Coercivity
Reverse Temperature
Coefficients of H
cJ
Density
Max. Operating Temperature
All details correspond to manufacturers information
H
cB
H
cJ
-
47.0
48.0
-
51.3
51.7
-
7.4
C
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42
MEDER electronic
Rare - Earth Magnets
ABOUT MAGNETS
SmCo Features
Disc
•
high energy density
•
small size
•
working temperature
up to 250
o
C
•
best opposing field-
resistance
•
available plastic
bounded
Standard geometric and magnetization
Rectangle
Cylinder
NdFeB Features
Disc
•
high energy density
•
small size
•
working temperature
up to 180
o
C
•
lower prices com-
pared to SmCo
•
available plastic
bounded
Standard geometric and magnetization
Flat Rectangle
Ring
Rare - Earth magnets like SmCo and NdFeB have the
highest energy density per volume and wight and also
the best demagnetisation resistance. Following below,
we compare other magnets with the same energy:
• Hartferrit
• AlNiCo
• SmCo
• NdFeB
=
=
=
=
Volumen 6 cm
3
Volumen 4 cm
3
Volumen 1 cm
3
Volumen 0.5 cm
3
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43
ABOUT MAGNETS
Both magnets are produced by sintering and can only
be processed by grinding, due to the strength and brittle
of the material. The temperature range goes up to +
250 °C. Very small magnets can be produced. Disad-
vantages are the high raw material prices and the lim-
MEDER electronic
ited availability of special alloys. The supply of different
geometry, size and magnetisation allow many creative
combination of reed sensor / swit and magnet and help
to find the best functionality of the sensor - magnet sys-
tem for each application.
SmCo
5
Magnetic Values according to
DIN 17410
Energy Product
Remanence
(B x H)
max.
B
r
Min.
Typ.
Max.
Units
160
900
170
925
-0.042
kJ/ m
3
mT
%/K
kA/m
kA/m
%/K
Reverse Temperature Coefficients of
B
r
Coercivity
Coercivity
H
cB
H
cJ
680
1750
2000
-0.25
Reverse Temperature Coefficients of H
cJ
Density
Max. Operating Temperature
All details correspond to manufacturers information
8.3
250
g/cm
3
o
C
NdFeB
Magnetic Values according to
DIN 17410
(B x H)
max.
B
r
Min.
Typ.
Max.
Units
Energy Product
Remanence
205
1180
295
1240
-0.1
kJ/ m
3
mT
%/K
kA/m
kA/m
%/K
g/cm
3
160
o
Reverse Temperature Coefficients of
B
r
Coercivity
Coercivity
H
cB
H
cJ
680
1750
920
1900
-0.6
7.6
Reverse Temperature Coefficients of H
cJ
Density
Max. Operating Temperature
All details correspond to manufacturers information
C
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44
MEDER electronic
Hard ferrite - Magnets
ABOUT MAGNETS
Ferrit Features
Disc
•
cheapest magnet
materia
•
working temperature
up to 300
o
C
•
many options in form
and magnetiuation
•
available plastic
bounded
Standard geometric and magnetization
Rectangle
Cylinder
Ring
Hard ferrite magnets are produced with iron oxide and
barium or strontium oxide. The raw materials are mixed
together and mormally pre sintered, to generate the
magnetic phase. The pre sintered mixture then gets
crushed. The resulting powder gets pressed together
(wet or dry) either in a magnetic field (an - isotropic)
or without a magnetic field (isotropic) and in the end
sintered. Proceedings are only possible by ginding. Due
to the low cost of the raw material, hard ferrite magnets
are the cheapest magnet type out of the actual sup-
ply of magnets. Ferrites have a very good electrical
isolation effect and are hard to demagnetise even in
strong external magnetic fields. Corrosion tendency is
low. Preferred shapes are long and thin but also round
forms are easy to produce. Disadvantages are the high
breakability and the low tensile strength. The strength
and brittleness of hard ferrites are similar to ceramics.
Furthermore the temperature resistance is limited and
they have only a low energy to volume ratio.
Ferrit 28/26
Magnetic Values according to
DIN 17410
(B x H)
max.
B
r
Min.
Typ.
Max.
Units
Energy Product
Remanence
28
385
30
395
-0.2
kJ/ m
3
mT
%/K
kA/m
kA/m
%/K
g/cm
3
250
o
Reverse Temperature Coefficients of
B
r
Coercivity
Coercivity
H
cB
H
cJ
250
260
265
275
0.3
4.85
Reverse Temperature Coefficients of H
cJ
Density
Max. Operating Temperature
All details correspond to manufacturers information
C
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