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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Power supply for any design. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur [an error occurred while processing this directive] Beginning radio amateurs collect a wide variety of electronic devices in radio circles, which require a direct current of 1,5; 3; 4,5; 6; 9 and 12 V. To check almost any of the designs, you can use a commercially available imported power supply, say, of the "ELECA" type, with a set of the indicated voltages, or assemble the proposed unit, which, firstly, will be cheaper and, secondly , has the best parameters. The simplest power supplies, called adapters, usually consist of a step-down mains transformer, a rectifier, and a smoothing capacitor. Most imported adapters are designed for a fixed output voltage, but there are models, for example "ELECA", in which the output voltage can be set in steps from 1,5 to 12 V at a load current of up to 1 A. However, as practice has shown, this voltage is idling and under load it varies somewhat, but it is still permissible to use such a power supply. Nevertheless, a stabilized power supply with adjustable output voltage should be considered the most convenient for collective use in a radio club. When adjusting and testing structures, errors are almost inevitable, leading to short circuits in the power circuit. It hardly makes sense to waste time, effort and attention, hoping to avoid such mistakes. It is more expedient to make a power supply with protection against short circuits at the output. It is this block (Fig. 1) that is proposed for repetition. It allows you to get an output voltage in the range from 1,5 to 15 V, which is divided into four subranges. Within each of the subranges, the output voltage can be smoothly adjusted by two variable resistors. The permissible load current is 0,2 A, but if necessary, it is easy to increase it. The voltage stabilizer is protected against short circuits of the output voltage circuit, and trigger-type protection is applied - in case of a short circuit, the stabilizer is turned off and restarted by pressing the "Start" button.
Consider the device and the operation of the power supply. Alternating voltage from the secondary windings of the step-down transformer T1 is supplied through the SA2.1 section of the subrange switch to the rectifier, assembled on diodes VD1 - VD4. The rectified voltage is smoothed out by an oxide capacitor C1 and fed through the fuse FU2 to a voltage regulator made on transistors VT1 - VT3. Moreover, transistors VT1, VT2 of different structures form a composite transistor that acts as a regulatory element, and a comparison node is assembled on transistor VT3, which generates a control current for the composite transistor. Capacitor C2 ensures the stability of the stabilizer. The output voltage can be set by the SA2 subrange switch and variable resistors R4 (rough) and R5 (fine). As reference voltage sources on sub-range "1" (limits of output voltage change under load with a consumption current of 0,2 A - from 1,18 ... 2,94 V), series-connected diodes VD6, VD7 are used, on sub-range "2" ( 1,8 ... 3,62 V) - LED HL1, on sub-range "3" (4,04 ... 9,25 V) - LEDs HL1, HL2 connected in series, on sub-range "4" (6,25, 15,08 ... 1 V) - LEDs HL3 - HL1,18. As you can see, by choosing one or another subrange, you can get any voltage you need to power the structure in the range from 15,08 to XNUMX V. In the event of a short circuit between the sockets of the X2 connector, to which the load is connected, the stabilizer is turned off, i.e., the composite transistor is practically closed. The stabilizer is restarted by pressing the SB1 button. Its contacts SB1.1 connect the resistor R1 to the composite transistor, and SB1.2 disconnect the load for this time. But first you need to check the power circuit and eliminate the short circuit. If, after releasing the button, the voltage does not appear at the output sockets (the needle of the PV1 voltmeter does not deviate), you will have to repeat the search for a short circuit. In order to simplify the design of the power supply, it does not include overload protection for the current consumed by the load, at which the VT2 transistor can overheat and fail. For such a critical situation, a FU2 fuse has been introduced, which "triggers" at a current exceeding 0,5 A. The power supply unit uses a unified TNZO incandescent transformer with several secondary windings designed to supply the load with a current of up to 0,58 A. By switching the windings (section SA2.1), the voltage supplied to the rectifier is changed. In turn, switching the voltage is necessary in order to reduce the power dissipated on the transistor VT2 - after all, it depends on the voltage drop between the collector and emitter of the transistor and the current consumed by the load. Any other step-down transformer with a power of 10 ... 15 W with a voltage on the windings of 12,6 V (between pins 7, 10), 5 V (11, 12 and 14, 15), 1,3 V (15, 16) is suitable. In addition to those indicated in the diagram, in place of VT1 it is permissible to use any transistors of the KT501, KT502, KT3107 series, in place of VT2 - KT815, KT817, KT805M (in a plastic case), in place of VT3 - KT - 315. transistors, the greater the output impedance of the stabilizer. In addition, for the VT2 transistor, it is necessary to make a U-shaped heat sink from aluminum sheet 1,5 ... 3 mm thick (it is installed vertically), the width and height of which is 30 mm, and the width of the bends is 10 mm. The transistor is mounted on it so that it is convenient to solder its leads to the conductors of the printed circuit board. Diodes VD1-VD4 - any of the series KD105, KD209, KD258 or others with a permissible forward current of at least 300 mA, VD5 - VD7 - any low-power silicon. LEDs HL1 - HL3 - any of the AL307 series, it is important that HL1 is red and the rest are green. Suitable LEDs and other series of the corresponding glow color and with a maximum operating current of up to 20 mA. Capacitors C1, C3 - K50 - 16, K50-35 or similar oxide, C2 - ceramic of any type. Fixed resistors - MLT - 0,25 (R2), MLT - 0,125 (others), variables - of any type, possibly smaller, necessarily group A (with a linear dependence of resistance on the angle of rotation). You can generally do without the resistor R5, but then you have to reduce the resistance of the resistor R3 to 510 ohms. Switch SA2 - biscuit (it is more reliable than push-button P2K), button SB1 - KM1 - 2 or similar with two groups of contacts. The PV1 voltmeter can be made up of any microammeter (and even a milliammeter) and an additional resistor. The resistance of the additional resistor in kiloohms is determined by dividing the maximum voltage measured by the voltmeter by the current limit of the used dial indicator in milliamps. Part of the parts (mainly the stabilizer) is mounted on a board (Fig. 2) made of one-sided foil fiberglass.
The board is placed inside the instrument housing (Fig. 3), where a transformer is also installed.
A voltmeter, variable resistors, and a "Start" button are fixed on the front wall of the housing (Fig. 4). Through the hole in the front wall, insulated stranded conductors are released, soldered to the X2 connector.
The operation of the assembled power supply is checked under a load that provides a current consumption of up to 0,2 A at a given output voltage in all subranges. The voltage regulation limits can be changed by selecting the resistor R3, and the reliability of starting the stabilizer by selecting the resistor R1 (you may have to simulate a short circuit of the output wires of the unit for this mode). In addition, the resistor R1 must be of such resistance that when the SB1 button is pressed (with the stabilizer running), the output voltage increases slightly. Author: D.Turchinsky, Moscow
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