【 Weak current College 】 circuits grounding 】

Grounding techniques in the areas of modern electronics have been extensive and intensive applications. Electronic device "to" usually has two meanings: one is "Earth" (safely), and the other is the "base" system (signal). Grounding means in the system and a potential base surface between the low resistance of conductive pathway. "Earth" is received to the Earth's potential, and to the Earth as a zero potential, the electronic equipment of the metal casing, the circuit is connected with the geodetic datum points. Since the Earth capacitor is very large, it was generally felt that the Earth's potential to zero. At first, grounding techniques mainly used in electric power system, and later, grounding technology extends to the weak electricity system. In the weak electricity system grounding in General does not mean the real sense and Earth are connected to the grounding. For electric and electronic equipment will be grounded on the Earth directly attached or connected to a reference potential of conductor, when there is current through the reference potential, then place the circuit common reference point, this voltage 0V, circuit other points of the voltage at the reference point for the benchmark, usually in the circuit diagram of the identified points voltage data are relative to the size of the Earth-side, which can greatly facilitate the repair of voltage measurement. The same lines that received location is called the ground. The ground plane and the ground connection, are often motivated by the following considerations: improve device circuit system stability, electrostatic discharge, to provide security for staff. Grounding of purpose: security considerations, namely the protection of the Earth. It is a signal voltage to provide a stable reference point zero potential (signal or systematically) shield protection. I. the type and role of grounding
Different circuits are not the same ground, and electronic power equipment in the common ground approach include the following:
1. safety grounding
Safety grounding is about high-voltage equipment enclosures and earth connection. One is to prevent the accumulation of electric charges, chassis produced electrostatic discharge and endanger the equipment and physical security, such as computer chassis grounding, tank car that drag on the ground of its tail, are in order for the gathering together of the charge-discharge, prevention of accidents; and second, when the device insulation damage to shell the power of live, promote the protection and cut off the power to protect the security of staff, such as refrigerators, rice cookers shell. Third, you can shield device huge electric field, a protective effect, such as civil protection bar of transformers.
2. Thunder
When the power electronic equipment in case of lightning, whether direct or indirect lightning Thunder, if you lack the appropriate protection, power electronic equipment will be subject to considerable damage and even scrap. In order to prevent lightning strikes, we generally in high places (such as roofs, chimney top) sets the lightning rod and Earth are connected, in order to prevent lightning endanger the safety of equipment and personnel.
Safety grounding and lightning protection and grounding is to give e-power equipment or personnel to provide security protection measures used to protect the safety of equipment and personnel.
3. work grounding
Earth is a circuit to work and provide a benchmark potential. This benchmark potential General set to zero. The benchmark potential can be set to circuit system a certain point, a paragraph or a piece. When the benchmark potential not connected with the Earth, as a relative zero potential. But this relative zero potential is not stable, it will as external electromagnetic field changes, the system's parameters are changed, causing the circuit system instability. When the benchmark potential and earth connection benchmark potential as Earth's zero potential, not as external electromagnetic field. But unreasonable work would increase the circuit ground. For example, add location is incorrect due to interference, the common side of electronic equipment is not properly connected and interference.
In order to effectively control circuit to work in all kinds of disturbances, so that it can comply with the principle of electromagnetic compatibility. We designed the circuit, according to the nature of the circuit, the work can be divided into the following grounding into different categories, such as DC, AC, digital, analog, signal, power, power supply, etc. Different ground should be set. Not in a circuit which will they mixed in together, such as digital and analog cannot have a ground wire, two circuit will have a very strong interference, so that the circuit be paralyzed!
4. signal ground
Signal to a variety of physical quantity signal source zero potential public benchmarks. Because the signal is generally weak, vulnerable to interference, unreasonable have grounding makes the circuit interference, signal to the high demand.
5, analog
Analog is a simulation of the circuit zero potential ground public benchmarks. Analog circuits in the small-signal amplifier, multi-level zooming, Rectifier, regulator, inappropriately grounding can cause interference, circuit function. Analog circuit to ground on the entire circuit is a very big meaning, it is the whole circuit is one of the basis of the normal work. So analog circuit reasonable grounding on the role of the entire circuit can not be ignored.
6, digital
Digital is a digital circuit zero potential public benchmarks. Digital circuit in pulse State, especially the pulse of the steeper or before and after travelling along the frequency is high, will produce a lot of electromagnetic interference circuit. If the ground is not reasonable, will enable interference, so on digital access site selection and grounding installation to full consideration.
7. power supply to
Power is the power of zero potential public benchmarks. Because power is often at the electricity supply to individual cells in the system, and each unit requires power supply properties and parameters may vary greatly, so it is necessary to ensure reliable power supply, and also to ensure that other units is stable and reliable work. Power supply to the power of anode in General.
8. power to
Power to the load circuit or power drive circuit for zero-potential public benchmarks. Due to the load circuit or power-driven circuit current strong, the higher the voltage, if Earth's ground resistance is large, it will have a significant drop from the interference of the larger, so the power interference on the ground. Therefore, power and other weak and must be set to ensure that the entire system stability mayRely on the work.
9, shielding ground
Shielding and grounding should be used to play the good shielding effect. Mainly to take into account the electromagnetic compatibility, a typical two shield is static and alternating electric field shielding, the following describes:
Electrostatic shield: when you use the full metal shield will charged conductor surrounded, the inner side of the shield will induction and electric conductor as dissimilar to the outside of the charge, appear with equal amounts of live conductors of the same charge, thus there are electric field on the outside. If the metal shield to ground, and the outside of the charge will flow into the Earth, metal shell outside will not exist electric field, the equivalent of a shell internal electric field charged body are masked.
Alternating electric field shielding: to reduce the alternating electric field on sensitive circuits (such as multistage amplifier, RAM, ROM circuit) coupling interference voltage, can interfere with sources and sensitive circuits between the electrical conductivity of metal shield, or the interference source, sensitive circuits were shielding and grounding metal shield. As long as the metal shield good grounding, can greatly decrease the alternating electric field on sensitive circuits for coupling interference voltage, this circuit will work properly.
Shielded grounded in the production of life has a broad application, let's go over:
Circuit of shielding enclosures grounding
Various signal sources and amplifiers and other vulnerable to electromagnetic interference shielding cover circuit should be set. Because the signal circuit and shielding enclosures between parasitic capacitance, so to signal circuit ground terminal and shielding enclosures attached to eliminate parasitic capacitance, and shielding enclosures grounded to eliminate common mode interference.
Cable's shield to ground
In some communications equipment in weak signal transmission cables, in order to ensure the signaling process security and stability, use outside cable with shielding network to enable signal transmission stability, prevent interference with other devices and prevent their being disturbed. For example CCTV cameras using a coaxial cable, outside of metal mesh is used to shield the signal. Again such as a network cable with 8 thin metal wire wound, four of the root action of the play screen, ensure that the signal of digital signal correctly.
Low-frequency circuit cable shield ground low-frequency circuit "1MHz > cable shield ground should be a single point of grounding, bonding shields should be the venue of the circuit, usually the power of the anode. For multiple-layer shielded cable, each shield should point grounding, but the shield should mutually insulated.
High-frequency circuit cable's shield to ground
High-frequency circuit cable shield ground should be more grounded. High-frequency circuit signals in delivery will have severe electromagnetic radiation, the transmission of digital signals will be severely attenuation, if there is no good shielding, makes digital signal an error.Generally use a principle: when the cable is longer than the work signal wavelength 0.15 times, the adopted work signal wavelength 0.15 times interval grounding type. If not, then at least will shield at both ends of the Earth.
System Shield ground
When the whole system needs to be resistant to external electromagnetic interference, or the need to prevent system to external electromagnetic interference should be screened for the entire system, and to shield from the system Earth. For example, the computer's chassis, sensitive electronic equipment, certain instruments.
Equipment in the electronic equipment, you did an outstanding job in a specific work, often containing a variety of circuits, such as low level signal circuits (such as high-frequency circuit, digital circuit, small signal analog circuits, etc.), high-voltage power circuit (such as power supply circuits, relay circuit, etc.). In order to install circuit board and other components, in order to resist external electromagnetic interference and needed equipment has a certain mechanical strength and shielding effectiveness of shell.
These more complex equipment grounding is generally follows the following principles: 50Hz power line safety grounding bolts should be received, for a stand-alone equipment, safety grounding bolts located in the equipment on metal shell, and a good electrical connection; to prevent case charged, endangering the security of the person, absolute power is not allowed for grounding wire instead of a zero line ground wire casing; to prevent high voltage, high current and high power circuit (such as power supply circuits, relay circuit) for low voltage circuits (such as high-frequency circuit, digital circuit, analog circuit, etc.), be sure to include their separate them and then ground locations. The former is power to the (strong), the latter as a signal to the (weak), signal into a digital and analog, digital and analog to separate earthing, best use of separate power supply and grounding, signal ground and power ground and chassis ground phase insulation; signal ground to separate a and device Shell gas insulated signal ground grounding bolts, the signal to bolt and safety grounding bolts of connections there are three methods, the choice of the method depends on the grounding effect: a is not connected, a floating ground formula, as floating, and recommendations are not adopted; the second is a direct connection to be the single point of grounding-type, that is in the low-frequency circuit with single-point grounding; three is a 3 1 μ F capacitor connected to a DC floating ground type, AC ground type. All other Earth final gather in safety grounding bolts, the point should be on AC power into line, and then through the Earth wire is grounding reliable buried in soil.
7, systematically
Industrial power transmission line, the device stops running a busy, so prevalent in the electric and magnetic interference. And industrial control systems and have dozens or even hundreds of input and output channel distribution in which the mutual coupling between the wires is channel interference of one of the main reasons. They are mainly manifested as capacitive coupling, inductive coupling, electromagnetic field. System grounding is to make the system set up the connection with Earth, is the electronic power devices to prevent such interference and normal.
System earthing should note the following: reference equipment grounding considerations; device shell device shell ground and Cabinet enclosures; Cabinet enclosures with Cabinet enclosures attached to the ground and the system shell; for systems, safety grounding bolts located in the system of metal casing, and maintain a good electrical connection;
— — When the system cabinets, equipment too much time, will lead the digital ground, analog ground, power groundAnd Cabinet shell ground too much, to do this, you can consider laying two parallel to each other and with system shell insulation of half-circular ground straps, one to signal to the bus, another for shielding and Cabinet enclosures and bus; system signal to the nearest received signal to the bus, System Shield, and Cabinet enclosures to the nearest received shielding and Cabinet enclosures and bus; two semicircular ground straps in the central part of the ground near the safety shield and bolts, and Cabinet enclosures and bus safety grounding bolts on the received signal to the bus to bolt on the received signal; bus electrical properties to good resistance to small as possible.
— When the system uses three-phase power supply, because the load power consumption and electricity are not at the same time, if not grounded, will inevitably lead to unbalanced, resulting in a three-phase power supply center point potential offset, as this will power line safety grounding bolts received, forced the three-phase power center point potential remains zero potential, thereby preventing three-phase power center point potential offset the resulting interference;
— Grounding line should be used copper conductor insulated conductors or cables, not to use galvanized flat iron or metal hose; work grounding and protection earthing connection, you must separate the protection of the grounding conductor must not use the metal vlexible hose; laying the ground electrode into the surface must be poured into the water, the depth of ground around the best use of charcoal powder is buried, used to enhance conductivity; general system grounding resistance less than 4 Ω, total point grounding resistance less than 1 Ω, mobile equipment, grounding resistance not more than 10 Ω.