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Useful Information for Designing Your Own Magnetic Shielding

In the event that you wish to design your own magnetic shielding, you may find the following information useful. MuShield engineers are available for consultation should you require assistance.

Magnetic shielding protects electronic circuits from magnetic field interference. Usually, sources of this interference include permanent magnets, transformers, motors, solenoids, and cables. Magnetic shields provide a path around sensitive areas to deflect magnetic flux. In addition, shielding may contain magnetic flux around a component that generates flux.

The ability to conduct magnetic lines of force is called permeability, and in a magnetic shield, the degree of permeability is expressed numerically. The standard or base line is free space with a rating of one, compared to shield materials which range from about 200 to 350,000.

Shields handle two basic needs. One prevents strong field radiation from sources such as transformers, magnets, and motors. The second shields instruments and devices from magnetic fields in an environment. Three types of materials are used for magnetic shielding - high permeability, medium permeability, and high saturation.

High permeability materials have a minimum permeability value of 80,000 at B-40 and a maximum of 350,000 with a saturation point of about 7,500 gauss after heat treating. Medium permeability materials are usually used with high-permeability materials and have values of 12,500 to 150,000 with a saturation point of about 15,500 gauss. High saturation materials have permeability ranging from 200 to 50,000 with saturation points between 18,000 to 21,000 gauss.

Definitions & Symbols

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Gauss = measure of magnetic flux density equal to one line of magnetic force per square centimeter.
Flux = the rate of flow of magnetic field
Saturation Field = field generated within the magnetic shield causing the permeability to asymptotically approach unity
B = flux density in the shield, in gauss
d = shield diameter (Note: in rectangular shields, use longest dimension)
Ho = external field (oersted)
µ = permeability of material
A = attenuation of field (ratio)
t = shield material thickness

Engineering Formulas

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Magnetic Fields: To determine the approximate field in the shield,

For example, a shield 1.5" in diameter made of material .060" thick in a field of 80 gauss has a flux density of 2,500.

Shield Thickness: For selection of shield thickness for fields less than 2 gauss,

For example, a shield 1.5" in diameter with a permeability of 80,000 and attenuation field of 1,000 to 1 would need a shield .019" thick.

Efficient Shielding: When considering the price of materials, maintaining the correct thickness keeps costs to a minimum. The magnetic shielding material must have an initial permeability of at least 80,000, otherwise the shield thickness is compromised.

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When a strong field is encountered, a thickness can be selected that develops maximum permeability in the material. For instance, a flux density in the shield of 2,300 to 2,500 gauss produces maximum permeability in the material. To determine the required thickness,

For a sense of scale, a shield 6" long and 1.5" in diameter in an 80 gauss field requires a shield .060" thick:

Field Attenuation: To determine the attenuation field (ratio),

Using the above figures, the attenuation of a field (ratio) is found to be 14,000 for a shielding material with permeability of 350,000.

Flux Density: To determine the flux density within the shield in gauss,

Here, a flux density of .0057 gauss is present within the shield when a field of 80 gauss exists outside the shield and an attenuation of 14,000 has been achieved;

Additional Design Points

  • Begin the design process by analyzing the interfering field and calculate its strength and frequency. Next, determine the interference level that can be tolerated.
  • Make multi-layer shields when shielding high field magnets, such as vac ion pumps. If possible, leave a 1/2" space between the inner shield and the magnet.
  • In shielding a vac ion pump, use low permeability material for the inside layer, medium permeability material for the intermediate shield, and high permeability material for the outer shield.
  • Use a single layer shield to shield a sensitive device such as a CRT. You should use a total enclosure for a CRT up to 5". On larger models, it may be necessary only to shield the neck section or the yoke assemblies.
  • For very low field chambers, use a 3-layer shield of high permeability materials with a copper shield on the outside of the inner shield. By passing a heavy AC current through the copper shield, you can degauss the inner shield. The copper will also shield electrostatic fields.
  • For shield construction, use overlap and spot welding where part configuration and material thickness permits. Laps should overlap by at least 3/8". "TIG" or Tungsten Inert Gas welding may be used to join corners or butt joints as required.

Tips on Using Magnetic Foil

If it is impractical to shield smaller components by fabricating rigid metal shields, foil makes an excellent alternative. When working with foil, the following information should be helpful.

  • To minimize fringing of fields, do not create sharp corners. If holes are required, use round holes or slots with generous radii on either end.
  • When covering a cylindrical object, overlap the foil by at least 3/4" each layer. Make the first two seams 180° a part. Make the next layer at right angles, the next at 180°, and so on.
  • To improve shielding, space the foil layers by three or four thickness' of masking tape.
  • Because foil has a high permeability, never wind it continuously in a spiral. If the material is spiraled, there is danger of creating a pole piece in the center of the shield.
  • When drilling foil, be sure that the drill is ground for cutting sheet metal and not for normal steel cutting. A normal drill will pick up foil with a corkscrew effect. This bending will reduce the foil's permeability.