Grounding is one of the important means to suppress electromagnetic interference and improve the electromagnetic compatibility of electronic equipment. Correct grounding can not only suppress the influence of interference, but also suppress the external radiation interference of the device; on the contrary, the wrong grounding will introduce serious interference, and even the electronic equipment can not work normally.
1, the concept of grounding
"Ground" in electronic devices usually has two meanings: one is "earth" and the other is "system baseline." Grounding is the establishment of a low-resistance conductive path between a selected point in the system and a potential reference. "Connecting to the earth" is based on the earth's potential as a reference, and the earth's metal casing, the selected point of the line, etc., connected to the earth through a grounding device composed of a grounding wire or a grounding pole. “System reference ground†refers to the reference conductor of the signal loop (electronic equipment usually uses metal base, casing, shield or thick copper wire, copper strip as the reference conductor), and the reference conductor potential is relatively zero potential, but not Earth zero potential, referred to as system ground. There are two purposes for grounding: one is for safety, called protective grounding. The metal casing of the electronic equipment must be grounded to avoid the danger of high voltages on the metal casing caused by accidents and endanger the safety of operators and equipment. The second is to provide a low impedance path for the current to return to its source.
2 Type of grounding
In fact, there are electrical or physical connections between various ground lines, and there is not necessarily a clear division. In the ground system, sometimes a place bears both the protection ground and the role of lightning protection; or it bears both the work place and the protection ground. The connection of different functions is different for the electrical objects, and the focus of the processing methods will be different.
a. Protective grounding
The protective grounding is to protect the safety of equipment, devices, circuits and people, to prevent lightning strikes, static electricity from damaging the equipment, or to protect personal safety in the event of equipment failure. Therefore, the grounding of the equipment, equipment, circuit chassis and metal casing must be taken.
The protection ground protection principle is: by short-circuiting the equipment casing with fault voltage to the earth or ground terminal, the short-circuit current generated during the protection process disconnects the fuse or the air switch, thereby achieving the protection of equipment and personnel safety.
b. Working ground
The working ground is the equipotential reference point or reference plane of the signal between the board, motherboard or system, which provides a low impedance path for signal reflow. Signal quality depends to a large extent on the quality of the working ground. Due to the characteristics of the grounded material and other technical factors, the connection or overlap of the grounding conductor is always good, and there is always a certain impedance. The return of the signal will cause a voltage drop on the working ground to form a ground ripple. Signal quality has an impact; the weaker the signal, the higher the signal frequency, the more severe this effect. Nevertheless, it is still important to minimize the impedance of the working ground conductor during design and construction.
Basic concept of filter circuit
The filter circuit is a frequency selective network composed of an inductor, a capacitor, a resistor, a ferrite bead, and a common mode coil. The low pass filter is a filter commonly used in electromagnetic compatibility suppression technology. In order to reduce the external radiation of the power supply and signal cable, the interface circuit and the power supply circuit must be filtered.
The performance of the filter circuit depends on the impedance characteristics of the two sides of the filter circuit. In the low-impedance circuit, the simple inductor filter circuit can obtain 40dB attenuation, but in the high-impedance circuit, it has almost no effect; in the high-impedance circuit, the simple capacitor The filter circuit can achieve good filtering effects and hardly works in low impedance circuits. In the filter circuit design, the capacitor is close to the high-impedance circuit design, and the inductor is close to the low-impedance circuit design.
The insertion loss of the capacitor increases with increasing frequency until the frequency reaches the self-resonant frequency, and since the inductance of the wire and the capacitor electrode is in series with the capacitor on the circuit, the insertion loss begins to decrease.
Power EMI filter
The power EMC filter is a passive two-way network that has one end connected to the power supply and the other end connected to the load. The power EMI filter can be considered an "impedance mismatch network" in the higher frequency band of the attenuation band of interest.
Power filter products
Circuit schematic
The network analysis results show that the greater the port impedance mismatch on both sides of the filter impedance, the more effective the attenuation of electromagnetic interference energy is. Since the common mode impedance on the power line side is generally low, the impedance on the power supply side of the filter is generally high. In order to obtain a better filtering effect, a low-impedance power supply side should be equipped with a high input impedance filter; for a high input impedance load side, a low output impedance filter should be used.
The ordinary power supply filter has a good filtering effect on the interference signals of several tens of megahertz or less. In the higher frequency band, the filtering performance will be degraded due to the inductance effect of the capacitor. For higher frequency interference situations, a feedthrough filter is used. Due to its structural characteristics, the filter has good filtering characteristics, and its effective frequency band can be extended to GHz, so it is used more in wireless products.
The most important issue with the use of filters is the grounding problem. Only a well-grounded filter can perform its filtering, otherwise it is of no value. Pay attention to the following issues when using filters:
1) The filter is placed at the inlet of the power supply;
2) The feedthrough filter is to be placed on the metal wall of the chassis (cabinet);
3) The filter is directly connected to the cabinet, and no protective paint can be applied under the filter;
4) The input and output leads of the filter cannot be paralleled and crossed.
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The centerline of the jet is in the same plane as the runner because the uses the dynamic force of the high pressure water stream to rotate the runner and forces the rotor to work while it is rotating.
Turbines have the advantages of dense construction, stable running and easy operation. Small turbines that are usually organized with horizontal shafts and 1-2 nozzles Normal size and larger turbines are most often organized by vertical shafts and 4 or 6 to get more force Equipped on the nozzle.
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