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PERMANENT MAGNETS LIMITED Phone no: +91 22 28454164 Fascimile: +91 22 29452128 Email: sales@pmlindia.com |
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Magnetic Shields
Introduction of
Shields
We are surrounded by
magnetic fields (both AC and DC) from the earth’s magnetic field to
man-made sources such as magnets, motors and transformers. To avoid effect of
this field on sensitive equipments, magnetic shielding is required. e.g.
cathode ray tubes, photomultiplier tubes, audio transformers, scanning electron
microscopes, position sensors.
The shield is made of soft
ferromagnetic material with a high Permeability (μ). 36%, 48% &80%
NiFe are most commonly used soft magnetic alloys. In order to have a low
Hysteresis, the shield is annealed after shaping. Any stress after annealing will
deteriorate the performance and hence should be avoided.
How does a magnetic
shield work?
There is no known
material that can block magnetic fields without itself being attracted to the
magnetic force. A magnetic shield acts as a kind of sponge redirecting the
magnetic field around the shield instead of passing through the sensitive
instrument which is being shielded. A good magnetic shielding material must
have high permeability which means that the magnetic field lines are strongly
attracted to the shielding material. If the magnetic field is too high for the
material chosen it will saturate and become ineffective. In this case, a multi
layer shield with a combination of the above alloys can be used.
Shielding Factor:
The Shielding Factor is
defined as the ratio between the external and the internal field when the
shield is placed in an area with a homogenous magnetic field.
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The Shielding Factor is
defined as the ratio between the external and the internal field when the
shield is Shielding Factor = External Field / Internal Field.
Saturation:
The magnetic saturation of
the shield is given by the magnetic properties of the shield material.
Ferromagnetic material
saturates between 0.5 and 2 Tesla. Once the shield is saturated, its apparent
Permeability will decline to the value 1, meaning that the shield acts like
air. A saturated shield does not guide nor attract magnetic flux lines.
Therefore, the shield design has to be chosen such that the field generated by
the current and the field generated by external sources do not saturate the
shield material.
General Principles of shield Designing:
In the following sections
we will describe how the different geometries influence the Shielding Factor,
Magnetic gain and Saturation level.
Influence of Width W:
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Influence |
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W has an influence on
the magnetic gain and also on the Shielding Factor. We recommend choosing W
as small as possible. The magnetic gain and the Shielding Factor are reciprocally proportional to W. |
Influence of Length L:
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Influence |
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L has an influence on
the Shielding Factor. We recommend choosing L as big as possible. The
absolute minimum of L is the length of the part (visible in blue). The maximum is open, but
8mm more than L is sufficient. The Shielding Factor
increases with the length of the shield L. Once the shield is long enough,
approximately the length of the part ( l +8mm), a longer shield will not
improve the Shielding Factor significantly. |
Influence of Height H:
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Influence |
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H has an influence on
the magnetic gain and on the Shielding Factor. We
recommend making H’ (from the top of the PCB surface to the end of the
shield) about 10mm. The absolute minimum of H’ is about 6mm (visible in
blue). The maximum is not relevant, but the shielding effect will not
increase significantly for a shield with H longer than 15mm. It is important
that both sides of the shield have the same height H. The Shielding Factor increases with the height of the shield H. Once the shield is high enough, approximately 15mm, a higher shield will not improve the Shielding Factor significantly. |
Influence of the Thickness t :
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Influence |
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t has an influence on
the saturation level of the shield. We recommend making t minimum 0.8mm. Once
the shield is thick enough, the increase of the thickness will not improve
the saturation level anymore. In the most of the case a shield thicker than
2mm is not needed. The saturation level
increases with the thickness t of the shield. Once the shield is thick enough
a increase of t will not improve the saturation level. |
Magnetic Shielding Definitions
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Property |
Unit |
Formula |
Definition |
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1 |
Attenuation (g) |
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g= Ho/ Hin |
Ho : Field intensity outside in Oersteds
(Oe), Hi : Field intensity inside in Oersteds (Oe) |
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2 |
Shielding Efficiency (SE) |
dB |
20log o. g |
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3 |
% shielding |
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(1-1/g) x 100 |
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4 |
Attenuation in Uniform DC field ( g DC) |
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(µ/4) x (1-a²/b²) |
µ -
Permeability of Material in Gauss |
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a
- Inner radius of shield |
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b
- Outer radius of shield |
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t -
thickness of shield |
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(µ/4) x (T/b) (2-T/b)≈µT/2b |
Shielding effectiveness depends only on the
permeability of the material and the ratio of wall thickness to outer radius.
This holds true for cylinders with a length to diameter ratio of 4 or more |
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5 |
Attenuation in AC field g (AC) |
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Designing for a DC field provides a
maximized shield in AC fields of equal density ( AC Peak) |
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Field Strength (H) |
Oersteds(Oe) |
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lines/cm² in Air |
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Flux Density(B) |
Gauss (G) |
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lines/cm² in Air |
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Permeability (µ) |
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µ=B/H |
measure of material's capability to conduct
magnetic lines of force or flux |
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Magnetic saturation level |
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The flux level at which the material can no
longer conduct any additional lines of force. |
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Reluctance ( R ) |
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R = l/µA |
The measure of material's resistance to the
passage to magnetic flux. l = flux path length (cm), A = cross sectional area
(cm²) |
What PML
can Offer:
Shields which are punched &
bent or punched and deep drawn for various applications like electricity
meters, electronic control protections in automobiles, avionics, medical
devices, audio devices.
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