The Role of Ground Line
The primary function of the ground wire is that when an electrical appliance is faulty, the power supply may break down (or: destroy) some components and make the shell of the electrical appliance live. Grounding the outer shell of the electrical appliance can make the leakage maintenance device.
1. Signal "ground";
Signal "ground" is also known as reference "ground", which is the reference point of zero potential, and is also the common segment of circuit signal circuit, graphical symbol "".
1) DC ground: DC circuit "ground", zero potential reference point.
2) Communication: the zero line of communication. It should be distinguished from the ground line.
3) Power Ground: Zero Potential Reference Point of High Current Network Equipment and Power Amplifier Equipment.
4) Imitatively: Zero Potential Reference Points of Amplifiers, Sample Holders, A/D Converters and Comparators.
5) Digitally: also called logically, it is the zero potential reference point of digital circuit.
6) "hot ground": switching power supply does not need to use transformers, the "ground" of its switching circuit is related to the municipal power grid, which is called "hot ground". It is charged, and the graphic symbol is "hot ground".
7) "Cold ground": Because the high-frequency transformer of switching power supply blocks the input and output terminals, and because its reaction circuit is usually photoelectric coupling, which can transmit reaction signals and block both sides of the "ground"; so the output terminal is called "cold ground", which is not charged. The graphic symbol is.
2. Maintaining the "land";
Maintenance of "ground" is a connection method set up to maintain the safety of personnel. Maintain electrical appliances at one end of the "ground" line and reliable connection with the earth at the other end.
3. The "ground" in the sound.
1) Shielding wire grounding: In order to prevent disturbance, the metal housing of the sound system is connected with the signal "ground" by wires, which is called shielding grounding.
2) Audio special "ground": In order to prevent disturbance, professional audio needs to be connected with audio special "ground" besides shielding "ground". This grounding device should be specially buried, and connected with the corresponding grounding terminal of the barrier transformer and the shielded voltage stabilized power supply as the special audio grounding address in the sound control room.
Treatment of different ground lines
1. Digital and imitative should be separated;
In high-demand circuits, it is necessary to separate digitally from imitatively. Even for A/D and D/A converters, it is better to separate the two "ground" on the same chip, and only connect the two "ground" on the system point.
2. Floating and grounding;
System floatation is to float the ground wires of each part of the system circuit and not connect with the earth. This connection must be anti-disturbance ability. However, the insulation resistance between the system and the ground should not be less than 50M. Once the insulation performance decreases, it will cause disturbance. Generally, the system floats and the chassis is grounded, so that the disturbance resistance can be enhanced and the system is safe and reliable.
3. One point grounding;
In low-frequency circuits, wiring and components will not have much impact. The general frequency of the circuit is less than 1 MHz. One point grounding is selected.
4. Multi-point grounding.
In high frequency circuit, parasitic capacitance and inductance have great influence. In general, the frequency of the circuit is more than 10MHz. Choosing multi-point grounding is not only the correct grounding planning and device, but also the correct grounding processing of various signals. In the control system, there are roughly the following lines:
(1) Digital: also known as logic, is the zero potential of various switching (digital) signals.
(2) Imitatively: the zero potential of various analog signals.
(3) Signal ground: generally sensor ground.
(4) Communication: Communication power supply lines, such fields are generally noise-producing areas.
(5) DC ground: DC power supply ground.
(6) Shielding ground: also known as shell ground, to prevent electrostatic induction and magnetic induction.
The above ground wire processing is an important issue in system planning, installation and commissioning. Here are some views on grounding:
(1) The control system should be grounded at one point. Generally speaking, the high frequency circuit should be grounded near multiple points, and the low frequency circuit should be grounded at one point. In low frequency circuit, the inductance between wiring and components is not a big problem, but the disturbance of the loop formed by grounding has a great influence, so a point is often used as the grounding address; but a point grounding is not suitable for high frequency, because the inductance on the ground increases the grounding impedance at high frequency, and at the same time, it occurs between different lines. Inductive coupling. Generally speaking, when the frequency is below 1MHz, one-point grounding can be used; when the frequency is above 10MHz, multi-point grounding can be used; when the frequency is between 1 and 10MHz, one-point grounding can be used, or multi-point grounding can be used.
(2) Communications and signals cannot be shared. Because there will be rare mV or even several V voltage between two points of a power supply ground wire, this is a very important disturbance for low-level signal circuit, so it is necessary to block and prevent it.
(3) The comparison between floating ground and grounding. This method is simple, but the insulation resistance between the whole system and the earth can not be less than 50M. This method has a certain anti-disturbance ability, but once the insulation decreases, it will cause disturbance. Another way is to ground the chassis and float the rest. This method can resist disturbance and is safe and reliable, but it is complicated to realize.
(4) Imitatively. It is very important to imitate the connection of the ground. In order to improve the ability of anti-common-mode disturbance, the shielded floating technology can be used for simulating signals. The grounding treatment of specific analog signals should be strictly planned in accordance with the requirements of the operation manual.
(5) Shielding ground. In order to reduce the capacitive coupling noise, detect and control accurately in the control system, it is necessary to select the shielding method for the signal. According to the purpose of shielding, the connection of shielding ground is also different. The shielding of electric field deals with the problem of distributed capacitance, which generally connects with the earth. The shielding of electromagnetic field mainly prevents the radiation disturbance of high frequency electromagnetic fields such as radar and radio. Made of low resistance metal material with high conductivity, it can connect with the earth. Magnetic shielding is used to prevent magnetic induction such as magnets, motors, transformers, coils, etc. The shielding method is to close the magnetic circuit with high magnetic conductivity materials, and it is generally better to connect the earth. When the signal circuit is grounded at one point, the shielding layer of low frequency cable should also be grounded at one point. If there is more than one shielding layer address of the cable, noise current will occur, which constitutes the source of noise disturbance. When a circuit has an ungrounded signal source connected to the grounded amplifier in the system, the shielding of the input end should be connected to the common end of the amplifier; on the contrary, when the grounded signal source is connected to the ungrounded amplifier in the system, the input end of the amplifier should also be connected to the common end of the signal source.
The grounding of electrical system should be categorized according to the requirements and purposes of grounding. Instead of simply and arbitrarily connecting different grounding systems, it should be divided into several independent grounding systems. Each sub-system has its own joint address or main grounding line, which can be connected eventually to implement total grounding.
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