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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Power supply on a unified transformer TH46-220-50. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies The most labor-intensive component of a classic network power supply is, as you know, a step-down transformer. To facilitate the repetition of the power supply offered to the attention of readers, it uses a ready-made unified transformer TN46-220-50 (incandescent transformer of the 46th standard size for a mains voltage of 220 V, 50 Hz). The presence of four secondary windings made it possible to obtain the same number of fixed values of alternating and direct voltage at the output of the unit. The unit is well protected from overloads both from the network and from the load side; there is an indication of connection to the network, presence of load, and the state of the self-resetting fuse. For repair and adjustment of various structures, laboratory power supplies (PSUs) with an adjustable output stabilized DC voltage are usually used. But such power supplies create “greenhouse” operating conditions for the device connected to their output, while after installation or repair it may be operated with a power supply whose output voltage is not stabilized. In order to bring the test results closer to real operating conditions, for example, of a manufactured UMZCH, voltage stabilizer, charger, it is desirable to be able to check their performance from a power source with an unstabilized output voltage.
The schematic diagram of a single-channel power supply for several fixed output voltages of direct and alternating current is shown in Fig. 1. The basis of the device is a unified step-down “incandescent” transformer TN46-220-50 (T1), which has four secondary windings, each of which is designed for an output voltage of 6,3 V at a load current of 2,3 A. These windings can be connected in series and in parallel, in this case they are connected in series. The mains voltage is supplied to the primary winding (pins 1, 5) of transformer T1 through the closed contacts of switch SB1, fuse link FU1, automatic thermal fuse FU2 and two-winding inductor L1. The LC filter C1L1C2 and varistor RU1 reduce the negative impact of impulse noise, both coming from the network and created by this power supply unit when it is turned on/off. A load presence indicator unit is connected in series with the primary winding, made on elements VD1-VD8, R1, R2, HL1. The HL1 LED shines brightly when a load consuming more than 25 W is connected to the power supply output. The output voltage of the power supply is selected by switch SA2: 6,3; 7,6; 12,6; 18,9 and 25,2 V are the alternating voltage values at a load current of about 2,3 A and a mains voltage of 220 V. Switch SA1 can lower them by about 1,3 V, which is convenient if due to underload or increased network voltage, there is increased voltage on the secondary windings of the transformer. Through the polymer self-resetting fuse FU3, AC voltage is supplied to the XS1 socket, to which a load designed for AC power can be connected. Sockets XS2, XS3 and plug XP2 are supplied with DC voltage from the output of the rectifier bridge VD9. Capacitors C7, C8 smooth out the ripples of the rectified voltage, C3-C6, the shunt diodes of the bridge, suppress the so-called multiplicative background. The HL3 LED is a power-on indicator, powered by a relatively stable current of about 12...15 mA, which forms a node on transistors VT1, VT2 and resistors R4-R6. Capacitor C9 prevents self-excitation of the transistors. The brightness of the HL4 LED depends on the set output voltage. In addition to display functions, these nodes are needed to quickly discharge capacitors C7, C8 after switching SA2 to a lower output voltage. The HL2 LED lights up when the self-resetting fuse FU3 trips.
The network filter and load presence indication assembly is assembled on a mounting plate measuring 66x42 mm (Fig. 2). The 74x59 mm board (Fig. 3) contains HL2-HL4 indication units and a self-resetting fuse. The cross-section of copper wires through which the load current flows must be at least 1,2 mm2. All parts of the device are housed in a metal case with dimensions of 107x128x128 mm; a view of the layout of the units is shown in Fig. 4.
Instead of the unified transformer TH46-220-50, you can use TH46-220-50K, TN-46-127/220-50. Switch SA1 is a toggle switch TP-1 or similar, both groups of contacts are connected in parallel, SA2 is a five-position biscuit switch, free groups of contacts are also connected in parallel to those used. Mains voltage switch SB1 - KV3, can be replaced with any one designed for switching mains voltage 250 V (ESB99902S, ESB76937S, KDC-A04, JPW-2104, PKN-41-1-2, etc.). The polymer self-restoring fuse LP60-300 (FU3) can be replaced with MF-R300, LP30-300. The copy used by the author triggered in about two minutes at a load current of 2,8 A. The holding current is about 200 mA at a voltage of 12,6 V (at higher voltages it is less). A self-resetting fuse with a maximum operating voltage of less than 30 V should not be used. In the absence of a suitable self-restoring fuse, instead of the one indicated in the diagram, install fuse-link FU1 with an operating current of 0,5 or 0,63 A. Thermal fuse (thermal relay) DY-03G (FU2) - from a faulty vacuum cleaner, where it was included in the circuit protecting the electric motor from overheating (with manual activation after activation). It is attached to the magnetic core of the transformer in such a way that the bimetallic plate is as close to it as possible (during installation, make sure that nothing restricts its free movement). A possible replacement for this unit is TM-XD-3CQC, ECH-009, SW03175, T23A090ASR2-20, SW03183, T23B090ASR2-20 and other similar ones, triggered at a temperature of about +80 °C. The KBU8K diode bridge is equipped with a duralumin heat sink with dimensions of 62x50x4 mm, which is attached to a steel U-shaped perforated housing cover (Fig. 4). It can be replaced by any other with an average rectified current of 8 A (KBU8A-KBU8M, RS801-RS807, BR81-BR88, BR101-BR108, etc.). The choice of a relatively powerful bridge is determined by the need to withstand overload until the self-resetting fuse FU3 trips. Possible replacement of diodes 1N4007 - any of 1N4001 - 1 N4006, UF4001 - UF4007, EGP20A, 1N4933GP-1N4937GP, as well as domestic series KD208, KD209, KD243, KD247, diodes 1N4148 - 1 N914, 1 SS244, KD510, KD521, KD103. Instead of the KT646B transistor, any of the KT646, KT645, KT3102, KT315, SS9014, 2SC9014, BC547 series will do. The KT815B transistor can be replaced by any of the KT815, KT817, KT961, KT683, 2SC2331, 2SC2383 series. Instead of the dual-crystal red-green LED L-57EGW, you can use any of the L-937, L-117 series, and instead of the LEDs L-1503CB/ID (red color) and L-1503CB/YD (yellow), any general use of continuous light is suitable , for example, series KIPD36, KIPD66. Fixed resistors - C2-23, C2-33, C1-4, C1-14, RPM or analogues with appropriate power dissipation, varistor RU1 - INR14D471 or any other with a classification constant voltage of 470 V (for example, FNR-20K471, FNR-14K471 , TVR20-471). During installation, a heat-shrinkable tube is put on it. Capacitor C1 - high-voltage ceramic with a rated alternating voltage of at least 250 V or constant 1000 V, C2 - film with rated values of the same types of voltage, respectively, not lower than 250 and 630 V, C3-C6 - small-sized film (soldered to the terminals of the VD9 diode bridge) , C9 - small-sized ceramic. Capacitors C7, C8 are imported oxide capacitors with a rated voltage of at least 50 V. If the total capacitance of them and the blocking capacitors at the load power input is about 10000 μF or more, this may lead to increased wear on the contacts of switches SA1 and SA2, so try not to change output voltage with connected load. Double-winding inductor L1 - industrial production. Any similar one with an inductance of 100 μH and a total winding resistance of up to 6 Ohms will do. Since during assembly of the device this choke ended up near the terminals of the secondary winding of transformer T1, a heat-shrinkable tube was put on it. The appearance of the assembled device is shown in Fig. 5. The front, rear and bottom walls of the housing connecting them are made of polystyrene sheets 3 mm thick and additionally reinforced with stiffeners. When gluing, keep in mind that polystyrene dissolved in acetone or dichloroethane can take several months to harden. The top and side walls of the case are formed by a U-shaped bracket bent from sheet steel (a part with ventilation holes from a “school” overhead projector was used). The weight of the power supply is about 1,7 kg.
The device, assembled flawlessly from serviceable parts, starts working immediately after connecting to the network. Resistor R2 is selected so that when there is no load and the mains voltage is 240 V, the HL1 LED glows barely noticeably. Without load at a mains voltage of 240 V, the power supply consumes only about 30 mA from the mains, which is very good for transformers of this design. During testing of the device, it turned out that when switch SA2 is set to the “25,2 V” position and a load current of 2,3 A (power delivered to the load is about 58 W), after 2...3 hours of continuous operation the transformer heats up so much that thermal fuse FU2 trips. It follows from this that the actual long-term power of the transformer is less, so it is desirable that during long-term operation the load current does not exceed 2 A. For a short time (a few seconds in total every 5 minutes) the load current can reach 4 A. Together with the described power supply, you can operate a switching voltage stabilizer , described in the author’s article “Pulse voltage stabilizer on the MC34165P chip” (Radio, 2014, No. 4, pp. 28-30). Author: A. Butov
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