63. Can AC filter and DC filter be used interchangeably? Generally speaking, AC line filters can be used in DC applications, but DC line filters must not be used in AC applications. Why?
Answer: The bypass capacitor used in the DC filter is a DC capacitor, which may be damaged due to overheating under AC conditions. If the withstand voltage of the DC capacitor is low, it will be broken down and damaged. Even if these two situations do not occur, the common-mode bypass capacitor in the general DC filter has a large capacity, and excessive leakage current will occur when used in AC applications, which violates the regulations of safety standards.
64. In a boxed device, such as an Ethernet switch or a PC, there are chassis ground and circuit ground. I found that some devices connect the two grounds with capacitors, some with 0 resistors, and some with ferrite Connection, which one is right?
Answer: We generally use 102 high voltage ceramic capacitors.
65. What does "institutional protection" mean? Is it the protection of the casing?
Answer: Yes, the casing should be as tight as possible, use less or no conductive materials, and be grounded as much as possible.
66. What is the impact on the ESD protection of the product if all the products use metal as the shell (such as aluminum, stainless steel, etc.)? How to deal with it better? Answer: All the products use metal shells. If the grounding is not good, it is of course not conducive to
ESD Protection, but as long as the grounding is done well, there will be no problem. As for how to ground, it depends on the specific conditions of the equipment. If it is a large-scale equipment, it can be directly connected to the ground through the equipment, and the effect will of course be ideal.
67. Why can't the spectrum analyzer observe transient disturbances such as electrostatic discharge?
Answer: Because a spectrum analyzer is a narrowband sweep receiver, it only receives energy within a certain frequency range at a certain moment. The transient interference such as electrostatic
discharge is a kind of pulse interference, and its spectrum range is very wide, but the time is very short, so what the spectrum analyzer
observes when the transient interference occurs is only a small part of its total energy, which cannot reflect the actual interference situation.
68. When diagnosing electromagnetic interference problems on site, it is often necessary to use a near-field probe and a spectrum analyzer. How to make a simple near-field probe with a coaxial cable?
Answer: Strip the outer layer (shielding layer) of the coaxial cable to expose the core wire, wind the core wire into a small ring (1~3 turns) with a diameter of 1~2 cm, and weld it on the outer layer.
69. Measuring the biomagnetic information of the human body is a new medical diagnosis method. The measurement of this kind of biomagnetism must be carried out in a magnetic field shielding room. This shielding room must be able to shield the alternating electromagnetic field from the static magnetic field to 1GHz. Please propose this Shielded room design.
Answer: First consider the selection of shielding materials. Since the magnetic field with a very low frequency needs to be shielded, materials with high magnetic permeability, such as
permalloy, should be used. Since permalloy has been processed, its magnetic permeability will decrease, so it must be heat treated. Therefore, the shielding room should be assembled
and assembled from plates. Each plate is processed according to the design in advance, then heat treated, transported to the site, and
installed with great care. The joints of each sheet are overlapped to form a continuous flux path. The shielding room constructed in this way can
have better shielding effect on low-frequency magnetic field, but the gap will produce high-frequency leakage. In order to make up for this deficiency, the outer layer of the permalloy shielding room
is welded with an aluminum plate to form a second layer of shielding, which plays a role in shielding the high-frequency electromagnetic field.
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