Definition of floating land-EEWORLD

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1. Interference requires a certain amount of energy. When the controller is completely isolated from the ground (floating), the impedance of the power frequency interference loop is extremely large, and the interference current flowing through the controller and its internal parts is extremely small, which is not enough to interfere with the controller . 2. When the controller housing is well connected to the ground, since the controller and the ground are at the same potential, the power frequency interference current is bypassed by the grounding point of the controller housing and cannot enter the controller, and thus cannot interfere. 3. When the controller housing and the ground are between the above two, there will be power frequency interference . 4. If there may be safety issues in the use of the controller, the housing must be well grounded. 5. The ideal grounding of multiple devices is to connect to the ground at one point as much as possible to avoid ground interference between devices. 6. Sometimes specific problems require specific analysis.

If several devices are interconnected and cannot be well grounded, then they had better all float. In fact, this is not very realistic. In practical applications, power supply and drive are likely to use industrial frequency power grids, and industrial frequency leakage is inevitable (assuming insulation resistance of 100M ohms and 380VAC voltage, there is a peak leakage current of 5.373uA flowing through the controller. In the controller of MOS devices, some devices have an operating current of only 0.1uA ).

Therefore, in general, the controller casing is best grounded well.

If you can really isolate all devices from the power frequency (floating ground), if there is no large current between your devices (here it can be called signal ground current) or the signal ground impedance between your devices is very small, then your multiple device signal grounds can be directly interconnected. Otherwise, the signal transmission between your devices needs to be isolated (such as photoelectric, transformer, mechanical, etc.).

Do the electrical or electronic parts inside the controller need to be grounded at one point with the casing?

When the outer shell and the internal circuit are completely floating, there will still be power frequency leakage current between the outer shell and the internal circuit due to the capacitive coupling effect between them. At this time: 1. When your circuit requirements are not very high, you can ignore it. 2. When your circuit requirements are very high, you must ground the internal circuit and the outer shell at one point. At the same time, be careful and make sure that the outer shell is well connected to the ground. 3. In order to prevent the internal output from touching the outer shell and causing a short circuit (such as power supply equipment) when the internal circuit and the outer shell are connected at one point, the internal circuit and the outer shell are connected with a capacitor with a large enough capacity. In this way, for power frequency interference, the internal and outer shell are at the same potential, and for DC output, they are isolated. ============================= Floating means " floating to the ground " . Floating grounding is a measure to overcome common mode interference (CMR) .

Floating ground is used in many places , such as the input end of DVM ( including A/D conversion part ) . That is, the potential of this point is the same as the ground , which is 0 potential . However, this point is not directly connected to the ground . It is a point with zero potential to the ground created by the circuit . The " ground " here can also be considered as the common end . There is impedance between this " ground " and the actual ground , and the impedance is infinite , in order to overcome common mode interference .

For example, a very common example: when the two ends of the coaxial cable shield are not grounded at the same point, there is a voltage difference between them because the two grounding points are not in the same position , which is the common mode interference voltage . This is equivalent to forming an interference source in series with the signal source , that is, the common mode interference is converted into the actual series mode interference . The common mode rejection ratio CMRR is defined as : CMRR=20log Vcm/Vsm , Vcm and Vsm are the common mode interference voltage and its conversion into the corresponding series mode interference voltage . And this series mode interference voltage is related to the shielding layer resistance to ground at the output end of the cable ( that is, the end opposite to the signal source ) . If this impedance --> infinite , the CMRR can be very large , so we can add a grounded shielding layer outside the cable , and the shielding layer of the cable itself is connected to the floating ground (0 potential point ). At the signal source end , the two shielding layers are connected together . That is , the shielding layer resistance to ground at the output end of the cable = the resistance between the 0 potential point and the ground .