Miniature power supply, 220/5-12 volts 100 milliamps. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The proposed block is intended for power supply from a network of small-sized radio-electronic devices (pocket radios, voice recorders, watches, etc.). The output voltage can be selected in the range from 5 to 12 V. One of the advantages of the unit is its small dimensions: all its parts are placed in the mains plug housing.

The schematic diagram of the block is shown in Figure 5.37.

Main technical characteristics:

• mains voltage with a frequency of 50 ... 500 Hz, V ..... 100 ... 250;
• output voltage (depends on the integral stabilizer used), V.....5...12;
• rated load current (with an output voltage of 5 V), mA.....20;
• maximum (at the same voltage), mA.....100;
• ripple level (at rated current), no more than, %.....1.

It works as follows: the mains voltage rectified by the diode bridge VD1 through the divider R1, R3, R4 is supplied to the base of the transistor VT2, and through the resistor R2 - to the base of the composite transistor VT4, VT5 During each half-cycle, until the voltage at the connection point of the collectors VT1, VT3 relative to the emitter VT2 does not exceed 100 V, it is closed, VT4, VT5 are open and the capacitor C1 is charged through resistors R1, R10 and the emitter-collector section of the transistor VT5. When the voltage at the indicated point is above 100 V, VT2 opens and shunts the emitter junction of the composite transistor . Capacitor C1 is discharged, feeding the oscillator on transistors VT1, VT3. The oscillation frequency of the oscillator is approximately 60 kHz. A voltage of about 1 V is removed from the secondary winding of the transformer T7. It is rectified by the diodes VD2, VD3, smoothed by the capacitor C2 and stabilized by the integral stabilizer DA1. Capacitor C3 reduces the level of high-frequency ripples.

The maximum voltages of the collector-emitter of transistors VT1, VT3 in steady state do not exceed 200 V, VT4 and VT5 - 210 V. The maximum current of the transistor VT5 with the values ​​\u2b\u4bof the elements indicated on the diagram and the static current transfer coefficient of the base h5b of transistors VT25, VT300, equal to 4, does not exceeds 5 mA. At the moment of switching on, the collector-emitter voltage of transistors VT300 and VT5 can exceed 0,5 V, and the collector current VT5 is 10 A, which will lead to their failure. To limit the collector current VT4 at this moment, resistor RXNUMX and a zener diode VDXNUMX are used.

To limit the collector-emitter voltage of the composite transistor, it is advisable to turn on a varistor for a voltage of about 5 V between the collector and emitter VT250. When using the unit to power a low-power load (with a current consumption of not more than 5 ... to 10 Ohm, and reduce the capacitance of capacitor C6 to 7 ... 470 uF (in this case, the unit will heat up less and its reliability will increase).

In addition to KT3130A (VT2), any transistor of this series, as well as the KT3102 series or foreign-made ones with similar characteristics (for example, BCW60D) can be used in the device. Transistors KT940A are interchangeable with KT969A, BF469 / PLP (VT1, VT3) or KT969A, BF459 (VT4, VT5). Capacitors C1, C2 - imported, it is possible to use K50-35, C3 - K10-17. Diodes VD2, VD3 - any small-sized silicon with a permissible forward current of at least 100 mA, a reverse voltage of at least 20 V and an operating frequency of at least 150 kHz. Resistors R1 ... R3 - C1-4 or others with an operating voltage of at least 350 V, the rest - C2-33, C2-23, MLT, OMLT or the like.

Transformer T1 is wound on two ferrite (2000NM) rings folded together, size K10x8x1. Windings 2-4 and 5-8 contain 1 turns of wire PEV-0,1-2, windings 3-3 and 4-200 - 6 turns of the same wire, windings 7-7 and 8-14 - 22, 28 , or 1 turns of PEV-0,17-5 wire (respectively, for output voltages of 9, 12 or 40 V). For interwinding and external insulation, it is recommended to use fluoroplastic film or PET film. In the author's version, the power supply is mounted in a standard power plug with a diameter of 27 and a height of XNUMX mm.

The printed circuit board (Fig. 5.38) is made of double-sided foil fiberglass 0,5 mm thick.

The distance between the centers of the holes in the board for the pins of the power plug is 19 mm. All resistors, except R2 and R3, are installed perpendicular to the board. The zener diode VD4 is soldered to the printed conductors from the mounting side of the transistor VT2. To the pads marked with the letters "a" and "b", solder the wires coming from the pins of the mains plug, and to the pads with the numbers 1.8 - the conclusions of the windings of the transformer T1. It is placed above the capacitor C3 in the free space between the transistors VT1, VT3 and the capacitor C2. Assembled from serviceable parts and without installation errors, the unit does not require adjustment.

Author: Semyan A.P.

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