Why is the device shell electrified?

The power supply of the equipment used in the market today is powered by a switching power supply. In all 3C power supplies that have passed the national compulsory certification, the 220V AC input line uses three wires The power supply is live (L), neutral (N) and ground (E). As the 3C standard stipulates, the power supply passing through the 3C must be added with an EMI filter circuit to filter out the interference of high-frequency pulses in the grid to the power supply, and also to filter out the electromagnetic interference generated by the power supply itself to the outside world. The filter circuit has two filtering methods, X and Y, X is between the live wire (L) and the neutral wire (N), and Y is between the live wire (L), the neutral wire (N) and the earth (E). To use Y-type to filter interference, you must set a Y capacitor between the live wire (L), the neutral wire (N) and the ground wire (E), that is, the internal “ground wire” of the power supply and the “live wire” and ” Two small capacitors are connected at the same time between the neutral line to discharge the generated static electricity. The voltage sometimes reaches 110V , but because the capacity of the Y capacitor is mostly between 100-47000PF, the AC flux is not large, so the current is very small. The following is the circuit diagram of the EMI circuit:?   It can be seen from Figure 1 that 220V alternating current is input to the EMI filter circuit of the power supply through the live and neutral wires. Pay attention to the point A in Figure 1, the connection point of the capacitor C3 and C4 and The equipment shell is connected together, and the shell and the grounding wire are connected together. When there is no grounding wire (when there is only two-wire power supply), the voltage of point A to the ground is 110V due to the series partial voltage of capacitors C3 and C4 ( The theoretical value is half of the input voltage 220V). Such a high voltage will of course cause the neon lamp of the electric pen to light up. Because the ground wire of the equipment chassis and the ground wire of the power supply are connected together, the chassis and other equipment will “Live”, but because the capacitance of C3 and C4 is very small (200W power supply usually uses 3300pF, 300W power supply usually uses 4700pF), so the “leakage current” through point A is very small, and it will not cause “electric shock to the human body” “Dangerous”, but there will be a feeling of “electric shock”. The intensity of the “electric shock” feels different depending on the power of the power supply. Generally, the power supply of 300W, because the capacity of C3 and C4 is 4700pF, the “leakage current” is more than 200W. The 3300pF used will be larger, and the “electric shock” will feel stronger. This is why users will feel that some power supplies will have stronger “leakage” and some power supplies will have weaker “leakage”. In addition, the sensed intensity is also related to each person’s tolerance and the degree of dryness of the skin. So how big is this “leakage current”, we can use the following simple formula to estimate: ?  I=1000C/14.5?  The above formula I is “leakage current”, the unit is (mA), C is the capacitance, the unit is (UF), the unit can not be wrong when calculating. Let’s use this formula to estimate the “leakage current” of 200W 3300pF and 300W 4700pF respectively: ?  200W 3300pF=1000*0.0033/14.5=0.23mA?  300W 4700pF=1000*0.0047/14.5=0.32mA?  Estimate from above It can be seen that the “leakage current” generated by the EMI filter capacitor is in the order of several milliamps. This current is very small and will not be dangerous to the human body.  Note: C3 and C4 are also called Y capacitors. The voltage withstand voltage of these two capacitors is above 400V, and ordinary ceramic capacitors cannot be used instead. Special safety Y capacitors should be used.  2. The method to solve the leakage current? The power supply that has passed the 3C certification must have an EMI (anti-interference) system. The Y circuit is indispensable, and only the power supply that has not passed the 3C certification
It is possible that there is no Y circuit. Therefore, 3C power supplies will have “leakage” phenomenon. When accidentally touching the metal part of the chassis, there will be a feeling of “electric shock”. Many users are still worried. The following two methods can effectively eliminate this “leakage” phenomenon.  1. The best way is to use three-wire power supply, so that point A has a good grounding, the case will not be “electric”. Therefore, the equipment is strictly required to be grounded during use, which can effectively eliminate the “leakage” phenomenon of using 3C power supply. At the same time, it can also ensure the safety of equipment and personnel when the equipment is really leaking.  2. For users who cannot use the three-wire power supply due to various reasons, users can make a ground wire by themselves, insert a one-foot-long iron wire into the wet ground, and use the wire to connect the iron wire to the metal steel plate of the computer case connect them. ?

Post time: Sep-22-2020