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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Economical power supply for antenna amplifier. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Antenna amplifiers Over the past 20 years, a huge number of regional commercial television companies have appeared, broadcasting through very frail transmitters of dubious quality. In order to receive their signal tolerably, complex antennas were needed with the obligatory presence of an antenna amplifier and a good coaxial drop cable. For this reason, it is now much more difficult to find a personal television antenna without an amplifier. The Chinese and, to some extent, the domestic industry quickly responded to the needs of the population, and you can buy a good quality antenna amplifier without any problems at a symbolic price, sometimes cheaper than one microwave transistor for such an amplifier. Unfortunately, the power supplies that are equipped with television antennas with amplifiers are often made according to national Chinese traditions: minimum costs, but reliability as it turns out. Therefore, such power supplies often overheat and fail even at rated AC voltage. The constantly hot power supply of the antenna amplifier not only consumes an unnecessarily large current from the network, but can also cause a fire, for example, if the mains voltage is increased. Taking into account the fact that the antenna power supply usually works around the clock and is often left unattended, a home-made power supply was made, which has both high reliability and safety, and low power consumption.
The device is a modernization of the industrial antenna amplifier power supply. The modernization was carried out in order to increase the reliability, efficiency and safety of the device. As a step-down transformer T1, an imported industrial transformer with a low no-load current was used. The +12 V rectifier and voltage regulator are made on the basis of a module from an old antenna amplifier power supply unit, in which a step-down transformer burned out. The missing parts that the "Chinese" usually consider superfluous were installed on a miniature printed circuit board of the power supply: capacitors C1-C4 and a safety resistor R2. In addition, capacitor C5 was installed with a margin for operating voltage, and the capacitance of capacitor C6 was increased from 0,01 μF to 1 μF. Resistor R3 is set to 4,7 kΩ instead of 1,5 kΩ. Voltage stabilizer microcircuits of the 78L12 type, made in a miniature TO-92 package, often fail when feeding antenna amplifiers. To eliminate this phenomenon, a small heat sink measuring 15x10 mm is glued to the microcircuit case with thermally conductive glue. For the same purpose, a resistor R2 is installed, which reduces the power dissipated by the microcircuit. Installation of chokes L1-L3 is optional, but the author, when using this power supply in conjunction with an internal computer TV tuner and an individual external antenna, managed to eliminate a small moire when receiving signals on meter TV channels. The inductor L1 is mounted on the printed circuit board of the stabilizer, and the miniature inductors L2, L3 and capacitors C7, C8 are in the antenna plug housing. The breaking resistor R1 reduces the voltage on the primary winding of the step-down transformer, and also acts as a fuse. Details and construction As a T1 transformer, the author used a ready-made EASTAR 430-035 transformer from a faulty uninterruptible power supply unit. A distinctive feature of this transformer is its low no-load current consumption, which does not exceed 1,3 mA at an AC voltage of 220 V, which corresponds to a power consumption of less than 0,3 W. The transformer, without overheating, withstands a long-term increase in mains voltage up to 300 V and a short-term increase up to 380 V. With such a transformer, the current consumed by the power supply unit with the load off is 1,8 mA, with a load of 21 ... 38 mA, which means that the power supply unit consumes from network power is not more than 1 W with the connected load. For comparison, the domestic industrial power supply IPS-5 for an antenna amplifier consumes a current of about 13 mA from the network when operating with the same load, similar to the "Chinese" ones - 20 ... 40 mA. If you do not have such economical transformers, then you can wind the required transformer with a low no-load current yourself. A transformer made on a W-shaped magnetic core with a central core area of 1,3 cm2 contains: a primary winding of 12000 turns with PEL-1 wire with a diameter of 0,05 mm, a secondary winding of 1000 turns with a winding wire with a diameter of 0,16 mm. If a larger magnetic circuit with a cross-sectional area of 2,25 cm2 is used, then the primary winding should contain 7100 turns of wire with a diameter of 0,05 ... 0,07 mm, and the secondary - 700 turns of wire with a diameter of 0,15 ... 0,23 mm . Both versions of transformers are designed for continuous operation at mains voltage up to 320 V. As long-term practice shows, supplying electricity consumers with mains voltage of 280 ... 320 V instead of 220 V can last for many hours, while the voltage of 380 ... 420 V is usually present on the AC mains for no more than a few minutes. Resistor R1 used imported discontinuous, you can use domestic non-flammable R1-7-2. The remaining resistors are types MLT, C1-4, C2-23. Capacitor C5 is an imported analogue of K50-35, the rest are ceramic K10-17, K10-50 or imported analogues. Rectifier diodes with a load current of up to 50 mA can use any of the 1N4148, KD521, KD522, and with a higher load current, any of the 1N4000-1N4007, KD209, KD243 series. The 78L12 low-power regulator chip is mounted on a small heat sink for increased reliability. You can also use more powerful microcircuits KR142EN5A, KR142EN5V, xxx-7805-x. The reliability of the stabilizer in this case will increase, but the efficiency will decrease. The inductor L1 consists of 7 turns of a double-folded mounting wire wound on a cylinder of ferrite 400NN-1000NN from the IF circuit of an old domestic transistor radio. Inductors L1, L2 can be used with small-sized industrial inductances of 3...20 μH. You can also use SMD chokes for surface mounting. As already mentioned, L2, L3, C7, C8 are located in the antenna plug. The presence of these chokes, in addition to protection against conventional interference, also has a positive effect on the noise immunity of the antenna system from the powerful radiation of cell phones. Some time ago, the author actively practiced feeding antenna amplifiers directly from television and radio receivers. As it turned out later, this method is not without drawbacks, since it was necessary to either modify each device connected to the antennas and / or use special adapters, so using a separate power supply for the antenna amplifier turned out to be more practical. Author: A.L. Butov, p. Kurba, Yaroslavl region; Publication: cxem.net
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