Some netizens asked whether the metal shell of the switching power supply module should be grounded, and some netizens asked how to use the shielded wire of the copper mesh. Some netizens even asked how to make their equipment anti-interference.
In fact, in the middle school physics course, we have learned that in the state of electrostatic equilibrium, there is no net charge inside the conductor, which means that if there is charge on the conductor, the charge is distributed on the surface of the conductor.
Figure (01) Cheng Shouzhu's "General Physics" Vol. 2, p. 93
In general physics, we have also learned that in the state of electrostatic equilibrium, the charged body outside the cavity conductor will not affect the electric field distribution inside the cavity; for a grounded cavity conductor, the charged body inside the cavity will not affect the objects outside the cavity. As shown in Figure (01).
This phenomenon of making the electric field inside the conductor cavity unaffected by the outside world or using the grounded cavity conductor to isolate the charged body inside the cavity from the influence of the outside world is called electrostatic shielding .
Figure (02)
The previous sentence "In the state of electrostatic equilibrium, the charged body outside the cavity conductor will not affect the electric field distribution inside the cavity" can be illustrated by Figure (02). In Figure (02), there is no cavity conductor in the space, but there is a uniform electric field (electric lines are parallel to each other). Then we put a cavity conductor with no charge inside into this space, and the electric field is deformed after it is put in, as shown in Figure (02).
In Figure (02), we can see that the electric field outside the cavity conductor is no longer a uniform field, and the electric field is deformed. The
electric field is deformed because the external electric field redistributes the charges on the cavity conductor until these charges are no longer affected by the electric field force, as shown by the red and blue symbols in the figure. The movement of these charges on the cavity conductor generates a new electric field (not shown in the figure). This newly generated electric field is superimposed on the original uniform electric field, which on the one hand deforms the original uniform electric field, and on the other hand makes the electric field inside the cavity conductor zero.
The electric field inside the cavity conductor is zero, which can be easily proved by the zero force on the charge on the cavity conductor.
When the external electric field is not a constant electric field but an alternating electric field, the conclusion that the electric field inside the cavity conductor is zero is no longer valid, because the redistribution of the charge on the cavity conductor shell takes time and it is impossible to reach equilibrium immediately. However, as long as the frequency is not too high, the time required for the redistribution of the charge on the cavity conductor can be ignored, and the conclusion that the electric field inside the cavity conductor is zero is still approximately valid. In fact, if the conductor shell is not as thin as the nanometer level, the electric field inside the cavity conductor is still very small even if the frequency is as high as tens of GHz.
The sentence underlined in blue in Figure (02) "For a grounded cavity conductor, the charged body inside the cavity will not affect the objects outside the cavity" is also valid only in the case of an electrostatic field. If the charged body in the cavity is moving, such as Figure (03), the charged body is rotating at high speed, then the movement of the charged body will have an impact on the outside of the cavity, because it takes time for the charge on the cavity conductor to redistribute. However, as with the underlined red part, as long as the frequency is not too high, the conclusion that the internal charged body has no effect on the outside of the cavity conductor is still approximately valid. However, it should be noted that this conclusion is only valid when the cavity conductor is grounded. If the cavity conductor is not grounded, the charged body inside the cavity conductor will still have an effect on the outside, even in the case of static electricity.
Figure (03)
The interference to electronic equipment can be divided into electric field interference, magnetic field interference, electromagnetic field interference and conducted interference. This post only talks about electric field interference.
Electric field interference is caused by the distributed capacitance between the interference source and certain circuits of the interfered electronic equipment. These distributed capacitances may be difficult to distinguish for beginners because they are invisible. But this distributed capacitance must exist.
Figure (04) uses two conductor plates A1 and A2 to represent the distributed capacitance between the interference source S and the interfered device R. A1 may be a wire in the interference source, and A2 may be a wire or a component on the circuit board of the interfered device. A1 and A2 may not have such obvious volumes as shown in Figure (04). Note that the interference source S and the interfered device R have a common point.
Figure (04)
clearly shows that A1 and A2 form a capacitor. If we follow the drawing method of electrical schematics, we can draw it in the form of Figure (05).
Figure (05)
Figure (05) shows the form very clearly. The interference signal is divided by capacitor C and resistor R, and R gets a part of the S signal voltage. The larger C is, the larger R is, the larger the voltage is divided by R, and vice versa. For the same C and R, the higher the frequency, the larger the voltage is divided by R. This is exactly why high-frequency electric field interference is often stronger.
From the above description, the lower the input impedance of the interfered device, that is, the smaller R is, the less susceptible it is to electric field interference. Is this true? It is indeed true. The lower the input impedance of an electronic device, the less susceptible it is to electric field interference. However, low-impedance devices may be more susceptible to magnetic field interference. This is what we need to pay attention to in production.
Figure (06)
If we insert a conductor plate B between A1 and A2, and connect B to the common point of S and R, then the circuit formed by B, A1 and A2 is shown in Figure (07).
Figure (07)
If Figure (07) is drawn in the usual way, it will become Figure (08). Among them, C1 is the capacitor formed by A1 and B, and C2 is the capacitor formed by B and A2.
In Figure (08), we can see that the signal of the interference source S is short-circuited to the common point by capacitor C1, and there is no interference signal from the interference source on the interfered device R.
Figure (08)
Figure (08) is only an approximation of the real situation. In fact, after B is inserted, there is not no interference signal on R. After the conductor plate B is inserted in Figure (06), there is still distributed capacitance between A1 and A2 (not shown in the figure), but the distributed capacitance between A1 and A2 is greatly reduced compared to before B is inserted, but it is not zero. In order to reduce the distributed capacitance between A1 and A2 to zero and completely prevent R from being interfered with, the only way is to use a good conductor to completely wrap the interfered device R to form a cavity conductor, which is difficult to do. However, when the distance between A1 and B and between B and A2 is relatively small compared to the size of the board, inserting B can reduce the interference generated by S on R to one thousandth of the order of magnitude. This can be seen as the distributed capacitance between A1 and A2 is reduced to one thousandth of the order of magnitude after B is inserted.
This is an application of electrostatic shielding.
In fact, B is not necessarily a solid conductor plate. Even if B is a mesh with many holes on it, it can also play a good electrostatic shielding role. As shown in Figure (09).
Figure (09)
Figure (10) is a switching power supply module. We can see many holes on its shell. With these holes, air can circulate and help the switching power supply module dissipate heat, so these holes are called heat dissipation holes. Although there are many holes and one end of the module is not closed, this aluminum shell can still play a very good electrostatic shielding role.
Figure (10)
shows a wire that is insulated and wrapped in a layer of copper gauze. This type of wire is usually called a shielded wire. The number of wires in a shielded wire varies, from one to dozens. Figure (11) shows a shielded wire with three insulated wires wrapped in a copper gauze. The copper gauze of the shielded wire is the shielding layer.
Figure (11)
In production activities, we often use oscilloscopes. The input impedance of the oscilloscope is very high, usually megohms or even tens of megohms. Its sensitivity is also very high. Ordinary oscilloscopes can usually achieve 5mV/div or even 2mV/div. Therefore, the input end of the oscilloscope is very susceptible to electric field interference. For this reason, the oscilloscope probe must use a shielded cable, as shown in Figure (12).
Figure (12)
The copper mesh outside the connecting wire of an ordinary oscilloscope probe is connected to the nut of the oscilloscope BNC plug at one end and to the copper sleeve wrapped around the outside of the probe at the other end, so that all the components inside the probe are covered in the shield. These components are usually a resistor and a small variable capacitor. The variable capacitor can also be placed near the BNC plug, as is the case with the probe in Figure (12). The shielded wire used by ordinary oscilloscope probes has only one conductor inside.
In Figure (04), the interference source S and the interfered device R have a common point. However, in some cases we may not be able to find the common point between the interference source and the interfered device. Often, the interference is caused, but the interference source is not very certain, such as an oscilloscope, and the interference source is not known before use. Another situation is that it is known that this device is a strong interference source, such as a switching power supply module, but it is not known which device will be interfered. In this case, where should the electrostatic shield be connected?
From the descriptions of Figures (01) to (10), we can know that
if you are considering the possible interference to a certain device, the electrostatic shield should be connected to the "ground" of the device. For example, the electrostatic shielding layer of the oscilloscope probe shown in Figure (12), which is the copper mesh of the shielding wire, should be connected to the "ground" of the oscilloscope input amplifier. However, the "ground" of the oscilloscope input amplifier is not necessarily the real earth, but only the common reference point of the various circuits of the oscilloscope. Similarly, if the signal input end of the audio amplifier has an electrostatic shield, it should also be connected to the "ground" of the audio amplifier.
If you are considering that a certain device may interfere with other electronic devices, the electrostatic shield should be connected to the real earth. For example, the switching power supply shown in Figure (10) is a very strong interference source. Its metal shell should be connected to the real earth. As can be seen in the figure, there is a screw on the right side of the terminal block, which is connected to the metal shell. The metal shell should be connected to the earth through this screw. If the switching power supply module is placed on a rack, it should at least be connected to the rack.
This content is originally created by EEWORLD forum user maychang . If you want to reprint or use it for commercial purposes, you must obtain the author's consent and indicate the source
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