|
Arabic
Bengali
Chinese
English
French
German
Hebrew
Hindi
Italian
Japanese
Korean
Malay
Polish
Portuguese
Spanish
Turkish
Ukrainian
Vietnamese|
ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Why did the computer burn out? Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Computers Due to non-observance of some simple rules, a personal computer sometimes fails. Especially dangerous is the moment of connecting a cable connected to another computer, printer and other peripheral devices to its interface connector. Therefore, it is necessary to follow the rules for grounding instrument cases and laying connecting cables. On fig. 1 shows a diagram of the computer power circuit from the AC mains. It necessarily has a filter that protects both the computer from interference coming from the network, and the network from interference generated by the computer. It can be quite complex, consisting of several capacitors and inductors, but usually consists of two capacitors (C1 and C2) of the same capacity, the connection point of which is connected to a common wire and the computer case. Capacitors form a capacitive voltage divider, therefore, between an ungrounded computer case and any of the supply wires, a voltage of approximately 90 ... 130 V operates (for a 220 V network, taking into account the possible spread of capacitor capacities). Even if there is no filter, as such, in the computer, the said divider forms parasitic capacitances between the windings and the magnetic circuit of the power transformer. Depending on their value, the voltage on the case can, generally speaking, turn out to be anything in the range from 0 to full network voltage.
Today, most consumers (both domestic and industrial) are supplied with electricity through a three-phase 220/380 V network. For example, a four-wire cable is brought to a residential building containing three phase wires ("phases", the voltage between any of them is 380 V) and neutral ("zero"), grounded at the transformer substation. Single-phase consumers (and this is the vast majority of household appliances) are supplied with a voltage of 220 V, connecting them between "phase" and "zero". Since energy losses are minimal with an equal load on the "phases", the wiring is carried out in such a way that the voltage of each of them is supplied to a third of the total number of apartments. It follows from the foregoing that one of the sockets of any power outlet is grounded through the "neutral" wire. The voltage between the metal case of a fully functional computer (or any other electrical appliance) connected to it and the ground is 90 ... 130 V mentioned above. A current of approximately 1 mA flows through the conductor. This is quite enough to feel a fairly strong electric shock when touching the case. The voltage between the ungrounded housings of two appliances powered by the same "phase" can reach about 40 V. If they are connected to different "phases", the situation is much worse. Let us turn to the vector diagram shown in fig. 2. Here U1 and U2 are the voltages on the computer cases, powered respectively from "phases" A and C. As you can see, due to the phase shift, the difference between these voltages (U1 - U2) will be at least 190 V, even without taking into account the spread of capacitor capacities in filters.
The voltage between the housings of devices connected to different electrical networks can reach the largest value. This happens, for example, at industrial enterprises that have two separate networks - lighting and power, energy which comes not only from different transformers, but sometimes even from different power plants. The phase relations here are completely arbitrary. The neutral wires of such networks are grounded at a considerable distance from one another. Due to the stray currents flowing in the ground, the potential difference between them, and therefore between computer cases, can be very significant - up to several thousand volts, if a high-voltage line passes near the ground electrode, an electric transport route or during a thunderstorm. At the moment when two devices are connected by an interface cable, the pins of the multi-pin connectors do not touch the corresponding sockets at the same time. This is inevitable due to the always present small deviations in dimensions and distortions of the articulating parts. All "inter-shell" voltage is applied to the first of the connected circuits. Well, if it turns out to be GND (common wire) - the potentials of the cases will equalize and the rest of the circuits will connect safely. Unfortunately, in most cases, the design of the connectors does not guarantee the connection of the "ground" contacts first. So a voltage of several tens or even hundreds of volts affects (albeit not for long) the inputs and outputs of interface microcircuits calculated for units of volts. Sooner or later this will damage them. Many troubles can be avoided by first disconnecting the connected devices from the network. But even in this case, the potential difference of the cases can be quite significant due to the accumulation of static charges, especially if the surface of the table on which they stand is covered with plastic or other good dielectric. For complete safety, it is necessary to securely connect the instrument cases to each other beforehand. It's time to talk about an additional wire in the computer's power cable connecting its case to the third pin of the "Europlug". The reciprocal contact is available in the "Euro socket", and it is guaranteed that they will connect first. If all the components of your "computer complex" are powered from the same multi-socket network block, when the plugs are inserted into it, the cases are connected, even if it is connected to a regular network socket without a special grounding contact. Before connecting the interface cables, it is enough to turn off the network from all devices at the same time with a switch, usually provided on the block. However, when purchasing a block, make sure that all its sockets really have a third contact and these contacts are interconnected. Many "left" manufacturers save on this. The above recommendations are good if the components of the complex are on the same table or at least in the same room. Everything becomes more complicated if they are installed in different rooms - there is a high probability of power supply from different "phases" of the network, which significantly increases the risk of damage to interface chips. Grounding of instrument cases in such a situation can guarantee the equality of their potentials only if both rooms have a common ground loop. But even in this case, long interface wires, especially those laid next to power wires, can be subject to dangerous interference. In such situations, the cases (or the third contacts of the power outlets) are interconnected with a separate wire. It is laid along the same route as the interface cable, and possibly closer to it. Connected devices need to be grounded in only one place. I would like to warn against trying to do this by connecting to the neutral wire of the network. Firstly, domestic standards do not stipulate which particular outlet socket it is connected to, and the design of sockets and plugs (including "euro") allows two articulation options. In this situation, mistakes are inevitable. Secondly, a malfunction (for example, a break in the power supply cable) can cause the voltage between the "zero" socket and the ground to reach the full value of the mains, flowing through consumer devices connected to this and other sockets. Author: N. Kurilovich, Moscow
Artificial leather for touch emulation
15.04.2024 Petgugu Global cat litter
15.04.2024 The attractiveness of caring men
14.04.2024
▪ 168-core processor-analogue of a neural network ▪ Two-storey garage in the country ▪ Smartphone as thick as a credit card ▪ High Speed Digital Signal Processors TMS320C6414/15/16
▪ section of the site Encyclopedia of radio electronics and electrical engineering. Article selection ▪ article Old but golden. The art of audio ▪ article How many aggregate states does a substance have? Detailed answer ▪ article Head of the postal department. Job description ▪ article Small metal detector. Encyclopedia of radio electronics and electrical engineering ▪ article Active loop antenna. Encyclopedia of radio electronics and electrical engineering
Home page | Library | Articles | Website map | Site Reviews
www.diagram.com.ua |
See other articles Section 
Latest news of science and technology, new electronics:
All languages of this page