The theory and practice of magnetic shielding, the basic principles and applications of magnetic shi

The basic principles and applications of magnetic shielding are as follows:

1) Electrostatic shielding.

Surrounded by a complete metal shield, the same amount of negative charge will be induced with the charged conductor on the inner side of the shield, and the same amount of positive charge appears on the outer side with the charged conductor, if the metal screen is grounded, the positive charge on the outside will flow into the earth, and there will be no electric field on the outside, that is, the electric field of the positive conductor is shielded in the metal shield.

2) Alternating electric field residual shielding.

In order to reduce the coupling interference voltage of the alternating electric field to the sensitive circuit, a metal shield with good conductivity can be set between the interference source and the sensitive circuit, and the metal shield can be grounded.

The magnitude of the coupling interference voltage of the alternating electric field to the sensitive circuit depends on the product of the alternating electric field voltage, the coupling capacitance, and the ground resistance of the metal shield. As long as the metal shield is well grounded, the coupling interference voltage of the alternating electric field to the sensitive circuit can be minimized. The electric field shield is mainly reflective, so the thickness of the shield does not have to be too large, and the structural strength is the main consideration.

Alternating magnetic field shielding.

Alternating magnetic field shielding is divided into high and low frequencies. Low-frequency magnetic field shielding is the use of high permeability materials to form a low reluctance path, so that most of the magnetic field is concentrated in the shield. The higher the permeability of the shield, the greater the thickness, the smaller the magnetic resistance, and the better the effect of the magnetic field screen cavity.

In harmony with the weight of the device, of course. Shielding of high-frequency magnetic fields is achieved by counteracting interfering magnetic fields using the reverse magnetic field of eddy currents generated by materials with high conductivity.

The principle of electromagnetic wave shielding is to use the reflection, absorption and guidance of the electromagnetic energy flow by the shielding body, and the charge, current and polarization phenomena induced on the surface of the shielding structure and inside the shielding body. The following is the relevant introduction of electromagnetic wave shielding: 1. Application range:

Shielding conductive paint is a kind of paint that can be used for spraying, and after the paint film is formed, it can play the role of conducting electricity, so as to shield electromagnetic interference. Conductive paint is a paint coating that can be sprayed by adding conductive metal powder to a specific resin raw material. 2. Definitions:

Electromagnetic shielding is a measure used in an area of space to attenuate the field strength caused by certain sources. In the vast majority of cases, the shield can be made of metals such as copper, aluminum, steel, etc., but for constant and very low frequency magnetic fields, materials such as ferrite can also be used as the shield.

The purpose of magnetic shielding is to prevent some high-frequency electronic devices from being interfered with by external magnetic fields, or from causing electromagnetic interference to external communications.

Usually the barbed wire covers the part sensitive to magnetic interference, the permeability of the iron relative to the air is quite high, about 5000 times that of the air, when there is a magnetic field to pass through, he will directly pass through the barbed wire to form a magnetic loop, without passing through the part to be protected, for example, the current flows through an almost zero wire in parallel with an insulated circuit, the current will not flow through the insulator and directly through the conductor. In this way, the part to be protected is magnetically shielded.

The magnetic shield puts two media with different permeability into the magnetic field, and the magnetic field at their interface will change abruptly, and then the magnitude and direction of the magnetic inductance intensity b will change, that is, the refraction of the magnetic inductance line will be caused.

When two media with different permeability are put into a magnetic field, the magnetic field will change abruptly at their interface, and the magnitude and direction of the magnetic field strength b will change, that is, the refraction of magnetic inductance lines will be caused. For example, when the magnetic inductance lines enter the iron from the air, the magnetic inductance lines deviate greatly from the normal and therefore contract strongly. As shown in the figure on the right, it is a schematic diagram of magnetic shielding.

A in the figure is a cover made of a soft magnetic material with a large permeability (such as permalloy or iron-aluminum alloy) placed in an external magnetic field. Due to the starvation permeability ratio of the hood. It is much larger, so most of the magnetic field lines pass through the walls of the hood, and there are very few magnetic inductance lines in the cavity inside the shell.

This achieves the purpose of magnetic shielding. In order to prevent the interference of external magnetic fields, a magnetic shielding cover is often added to the outside of the focused part of the electron beam in the oscilloscope tube and picture tube, which can play the role of magnetic shielding.

Electromagnetic shielding has nothing to do with whether the shield is grounded or not. There are only two factors that really affect the shielding effectiveness of the shield: one is that the entire surface of the shield must be conductive and continuous, and the other is that there can be no conductors that directly penetrate the shield.

There are many conductive discontinuities on the shield, the most important of which are the non-conductive gaps formed at the junction of different parts of the shield.

These non-conductive gaps create electromagnetic leaks, just as fluids leak from gaps in containers. One way to solve this leakage is to fill the gaps with conductive elastomeric material, eliminating non-conductive points. It's like filling the gaps in a fluid container with rubber.

This elastic conductive filler material is called an electromagnetic sealing gasket. In many literatures, electromagnetic shields are likened to liquid-sealed containers, and it seems that electromagnetic wave leakage can only be prevented if the gaps are sealed with conductive elastomeric materials to the extent that they are watertight.

Electromagnetic shielding is a shielding that uses conductive materials to reduce the penetration of alternating electromagnetic fields into a specified area. The principle of electromagnetic shielding is to use the reflection, absorption and guidance of the electromagnetic energy flow by the shield, which is closely related to the charge, current and polarization phenomena induced on the surface of the shield structure and inside the shield.

Electromagnetic shielding detection is a point that is paid more attention to at present, and electromagnetic pollution has become the fourth major pollution after air pollution, water pollution, and noise pollution. Therefore, many electromagnetic shielding materials were born, but they all need to be tested professionally.

In order to judge the shielding effectiveness of the material. So what are the functions of electromagnetic shielding detection?

First of all, our product testing is divided into two types: commissioned testing and supervision and spot checks.

In fact, the difference between the two can also be found through the literal meaning, and the entrusted testing is that the entrusting party sends it for inspection by itself to inspect the quality of the product; Supervision and spot check is a random spot check on the product by the relevant departments in accordance with the quality law.

Electromagnetic interference can be said to be a considerable phenomenon, and there are many ways to generate it, such as contact, friction, sharpness, etc. between items. Static electricity is characterized by long-term accumulation, high voltage, low charge, low current, and short duration of action.

The hazards generally produced are as follows:

1. The contact between one's own actions and other things or living things is the static electricity of the product, and some can even reach tens of thousands of volts

2. It often causes unstable operation or even damage to electronic and electrical products.

3. The main measures of electromagnetic interference protection in the production process are electrostatic leakage, dissipation, neutralization, humidification, shielding and grounding.

In fact, the main purpose of the electromagnetic shielding test is mainly several points: under what circumstances do you need to take physical protection in case of electromagnetic interference; Provide data support for the technology and use effect of electromagnetic interference; Provide technology for the classification and management of materials for electromagnetic interference, and so on.

Let me give you an example, for a hollow ball with inner and outer diameters r1 and r2 respectively, in a uniform external magnetic field h, the magnetic permeability of the ball is , which is explained by the magnitude of the magnetic induction intensity b in the sphere. For the invariant ratio r1 r2, the larger the size, the weaker b becomes, the stronger the electromagnetic shielding effect of the spherical shell, and when >>0 (vacuum permeability), b 0.

The level is limited, so I can only talk about so much.

The magnetic field lines are passed directly through the material with high permeability. As a result, the magnetic field lines passing through the shielded object will drop significantly, so as to achieve the purpose of shielding.

Magnetic screen bai

The purpose of the shield is to prevent some high-frequency DU electronics.

Zhi Zhi is subjected to an external magnetic field

or electromagnetic interference caused by external communications.

Usually the barbed wire covers the part sensitive to magnetic interference, the permeability of the iron relative to the air is quite high, about 5000 times that of the air, when there is a magnetic field to pass through, he will directly pass through the barbed wire to form a magnetic loop, without passing through the part to be protected, for example, the current flows through an almost zero wire in parallel with an insulated circuit, the current will not flow through the insulator and directly through the conductor. In this way, the part to be protected is magnetically shielded.

In the case of superconductivity, magnetic field lines flow along the superconducting shell, resulting in the absence of magnetic field lines inside.