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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Transceiver mains power supply - do-it-yourself. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies The acquisition of an expensive imported transceiver, as a rule, is associated with significant material costs. Often there is no money left to buy a power supply. And here the happy radio amateur faces the problem of self-manufacturing a power supply device. What requirements must it meet? First of all, along with the necessary power, a homemade power supply must have good reliability so that the likelihood of damage to the connected transceiver is minimal. Reliability, as is known, depends on the total reliability of all structural elements and their functional importance. In the network power supply unit, the voltage regulator plays an important role. This article provides a description of a home-made network power supply, the main "highlight" of which is the stabilizer circuit. The unit has been working without comment for about six months together with the KENWOOD TS-570D transceiver. Recently, during the summer heat, it passed additional tests, working for about a day at a dummy load at rated current. Power supply parameters:
No less important problem than the choice of a stabilizer circuit is the calculation and manufacture of a power transformer. This task is almost always associated with a lot of difficulties - you need to get the right size iron, wires of the required cross section and, most importantly, make laborious winding. All these moments cause radio amateurs to have a deaf aversion to the independent manufacture of a transformer and a desire to get it ready. Which, in turn, pushes the moment of going on the air on a brand new transceiver to the back burner. In fact, a homemade transformer is not such a difficult thing. You never know what you can do till you try! From my experience, I prefer to use W-shaped plates as a core. Despite the fact that the required dimensions of the transformer are somewhat larger than those with a toroidal core, technological convenience prevails. First of all, it is necessary to assess the suitability of the existing core or figure out which one to look for. Then calculate the wire diameters and the number of turns of the windings and, finally, correctly evaluate the results. Looking in the old reference book, you can find the following approximate formulas there:
It should be borne in mind that the number of turns of the primary winding turns out to be somewhat smaller in practice, and the number of turns of the secondary winding is larger compared to the calculated one. However, the primary winding should be wound with a margin of 20 to 30 percent first. The margin is useful for further adjustment of the number of turns for the optimal operation of the transformer. When winding, it is desirable to count the number of turns for the subsequent correction of the calculated parameter "N". After completing the rough winding of the network winding, it is necessary to fix the seven turns, assemble the magnetic circuit and measure the current of the primary winding at idle. This measurement will give quite complete information about the quality of the work performed At this stage. The value of the measured current depends on the overall power of the transformer or, more simply, on the size of its core. For transformers with a power of 200 - 1000 W, the no-load current can have a value of the order of 100 - 150 mA. If the measured current is less than this value, this means that the efficiency of the transformer will be below the norm and it will not be possible to obtain the expected power from it. In this case, part of the turns must be unwound from the winding and the current measurement must be repeated again. To avoid unexpected troubles associated with accidental inter-turn short circuits, it is advisable to make the first measurement by turning on a mains light bulb with a power of at least 100 W in series with the winding. If you build a graph of the dependence of the no-load current on the number of turns, then on this graph you can see a rather sharp break, which shows that for a certain number of turns, even a slight decrease in them leads to a sharp increase in current. So, the number of turns can be considered optimal when the current graph a little ns reaches the fracture point upwards. The general criterion for the quality of the completed primary winding can be considered the absence of noticeable heating of the transformer core during operation without load for several hours. I want to note that trying to wind the transformer using the "coil-to-coil" method is a very laborious task. It is quite possible to wind the primary winding "in bulk". Modern winding wires with their reliable varnish insulation allow this winding method. It is only necessary to monitor the uniformity of the distribution of turns over the surface of the winding, so as not to create areas with an increased interturn potential difference. So, the primary winding is finished. The coils are fixed, flexible conclusions are made, and insulation from a non-melting material is laid over the coils, which can be used as a fluoroplastic tape taken from FT-3 capacitors. Now we need to perform the shielding of the network winding. It is best to do this with thin copper foil, wrapping it in one layer on the surface of the newly made network winding. The shield winding has only one output. which is then connected to a common (ground) power bus. In no case should the shield winding be closed, otherwise it would lead to the death of your transformer. Between the overlapping ends of the foil, it is imperative to lay reliable insulation. After isolating the shielding winding, you can proceed to no less responsible business - winding the secondary, high-current winding. Its design depends on the choice of rectifier circuit. If it is planned to use a bridge rectifier, then a simple tapless winding is wound. If there is enough free space in the transformer window, it is desirable to use a two-diode, two-diode full-wave rectifier circuit and, accordingly, a double secondary winding with a middle terminal. The losses in the winding and on the rectifier in this case will be less than in the first case. For a powerful secondary winding, a thick copper wire with a diameter of several millimeters or a copper bar is usually used. This makes manual winding difficult and may damage the insulation of the underlying turns. In my design, I used a kind of "litz wire" - a bundle of several wires folded together with a diameter of about 0,8 mm. With this method of winding, it is important to monitor the parallel arrangement of the individual wires of this bundle so as not to cause a mismatch current between the individual wires of the winding. An important question is what voltage should the secondary winding be calculated for? The answer to it depends on many factors. Such as the properties of the magnetic circuit, the capacitance of the rectifier filter capacitor, the limits of possible fluctuations in the mains voltage, the properties of the voltage stabilizer. Many of these questions are easier to answer by experimenting than by trying to calculate theoretically. In any case, it is necessary to focus on the magnitude of the rectified voltage of the order of 20 volts. Increasing this figure is useful for increasing the stability of the output voltage due to a larger voltage margin for stabilization. However, this, in turn, leads to a tougher thermal regime of the transformer and stabilizer, to the need to use electrolytic filter capacitors for a higher voltage, that is, more expensive and larger. In a word, here it is necessary to adhere to the rule of the "golden mean" and not allow forcing the modes of the power supply units to achieve unreasonably high load parameters. After the test winding of the secondary winding, one must not forget to check the no-load current of the mains winding again. It should not increase by more than 5 - 10 mA. Further, it is desirable to check the quality of execution of each stage of assembling the power device by loading it on an equivalent, which can be a garland of suitably connected incandescent lamps. I used old 12 volt high beam car bulbs, connecting both strands in parallel. One lamp in this inclusion "eats" about 6A. Having assembled the rectifier circuit together with the filter capacitor, we measure the load capacity, average voltage and ripple voltage at the rated load current. Of greatest interest is the voltage value at the minimum of the pulsation period. Measured by an oscilloscope, it should be less than three volts (min. stabilization margin) more than the output voltage of the stabilizer and, in our case, will be 13,8 + 3 = 16,8 V. It is important to choose the correct capacitance of the filter capacitor. Usually it is chosen on the order of 100000 microfarads. I experienced difficulties in acquiring such a capacitor and gained the necessary capacity by connecting the existing capacitors in parallel. I managed to place them in all the nooks and crannies of the block body by gluing the capacitors with "hot melt" glue. The conclusions of the same poles must be connected by wires at one point, in the immediate vicinity of the output connector. You can use a smaller capacitor, but it is necessary to slightly increase the voltage of the secondary windings, controlling the ripple voltage under load, as described above. When the assembly of the transformer and rectifier was finally completed, I faced the modern difficult question of choosing a voltage stabilizer circuit. On the one hand, there are a lot of circuits with transistors as a regulating element, on the other hand, it would be tempting to use a fully integrated stabilizer. The latter option would be preferable both for its manufacturability and quality parameters guaranteed by the microcircuit, if not for the price. Previously and now, I widely use KR142EN12 microcircuits in my designs. They are good for everyone - price, availability and their parameters, they are not afraid of a short circuit. Only here the current is small. Only about two and a half amperes. Imported analogues of our LM317T microcircuits are cheaper, more stable and more powerful, hold three amperes, but still this is far from what is needed. Even earlier, to increase the power of stabilizers, I connected the conclusions of two such microcircuits in parallel. The maximum current also increased exactly twice. In this case, I went on an experiment and connected as many as nine microcircuits in parallel, evenly placing them on a common heatsink. According to the standard scheme, I connected two resistors to a common control output and turned on a simple circuit. The load test results fully justified my assumptions - the excellent stabilizing properties of the circuit remained the same as those of a separate microcircuit, and the maximum current increased in proportion to their number.
The microcircuits used in the stabilizer should be tested separately before installation. The output voltages of each chip may differ by a small amount. But I deliberately did not try to choose instances with the same parameters, arguing as follows - let, at a current, suppose two amperes, only one of the nine microcircuits works. But when the current increases to more than three amperes, the loaded chip will feel an overload. The internal short circuit protection circuit will start to operate in it, that is, its internal resistance will gradually increase, and the flowing current will be redistributed to the next microcircuit. This will continue until all microcircuits are included in the voltage stabilization process. With a further increase in current above the nominal, a rapid decrease in the output voltage will be observed - the overload protection function will finally work. Such a scheme, in addition to extreme simplicity and a minimum of elements used, has another advantage - better heat dissipation of microcircuits distributed over the radiator. In my design, three needle-shaped radiators from the horizontal scanning of Elektronika 401 TVs were used, mounted on a common aluminum base. Just in case, a cooling fan is mounted under the radiators, however, you don’t have to turn it on - the heat sink temperature is low even with intensive work on transmission. The output voltage of such a circuit can be adjusted in a very wide range - from two to several tens of volts. Table 1 shows the average values of the resistance of the regulating resistor (3,3 kΩ variable resistor), depending on the required output voltage. Table 1
I note that the radiator with microcircuits must necessarily be isolated from the power supply housing. It is better not to connect the case itself galvanically to the stabilizer circuit, but to connect it to a protective ground. It is desirable to install a simple LC filter at the mains voltage input. It will protect the transceiver from network interference. The operation of the power supply is indicated by two lamps HL1 - any neon, HL2 - an incandescent lamp. It also acts as a discharge resistor. By the duration of its glow after turning off the unit from the network, one can judge the quality of the capacitor C5, and by brightness - the stability of the output voltage. In conclusion, I will say that the cost of one LM317 chip in Moscow is a little more than 3 rubles - almost two times cheaper than our domestic KR142EN12, but superior in reliability. Author: S.Makarkin, RX3AKT; Publication: N. Bolshakov, rf.atnn.ru
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Comments on the article: Nicholas Thanks, the idea works [;)] Alexander where does the LM 317 hold 3 A. according to the characteristics of 1,5A Alexander Good afternoon Nikolay. How many wires with a diameter of 0.8 did you use in the secondary winding?
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