According to the classification of the shielding box, there are three types: electrical shielding, magnetic shielding and electromagnetic field shielding. The essence of electrical shielding is to reduce the influence of electric field induction between two devices (or two circuits, components, components). It includes electrostatic shielding and shielding of low-frequency electric fields from high-impedance electric field sources in the near field. Electrical shielding should have two basic points, namely, a perfect shield and a good grounding. Shield the interference of the electrostatic field source to the outside world: Use a shield to enclose the electrostatic field source, and ground the shield to seal the power line generated by the electrostatic field source inside the shield.
Shield the interference of the external electrostatic field: the grounded metal shield can realize effective shielding. Shielding of low-frequency electric field: The shielding body must be made of materials with good conductivity, and it must be grounded. Only in this way can the interference be effectively reduced. If the shielding body is not grounded or the grounding is poor, the interference will become greater after shielding due to the effect of the distributed capacitance.
Magnetic shielding includes the shielding of a constant magnetic field and the shielding of the near field of a low-impedance magnetic field source. Magnetic shields generally use ferromagnetic materials with high magnetic permeability such as steel, iron, permalloy, etc. This kind of magnetic shield allows most of the magnetic flux to flow through in the air, thereby achieving the purpose of weakening the magnetic field.
Electromagnetic field shielding is to select materials with high magnetic permeability to increase the wall thickness of the shield to reduce the magnetic resistance of the shield. The shielded object should be placed in the center of the shielding body, and the magnetic flux should not pass through the shielded object as much as possible to avoid reducing the shielding effect. Pay attention to the structure of the shield. All gaps, ventilation holes, etc. should be distributed along the direction of the magnetic field, and the passage of magnetic flux should not be blocked as much as possible.
For strong magnetic fields, a double-layer shield structure can be used. When a strong external magnetic field is to be shielded, the outer shield is required to use materials that are not easily saturated, such as silicon steel; the inner layer is made of high-permeability materials that can easily reach saturation, such as slopes. Mo alloy and so on. Conversely, the order in which the shielding materials are used is also reversed. When installing the two-layer shield, pay attention to the magnetic circuit insulation between each other. If the shielding body has no grounding requirements, it can be supported by insulating materials; if grounding is required, the two-layer shielding body can be supported by non-ferromagnetic metals.