Compact power supply, 9 volts 0,6 amps. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Compact power supply, 9 volts 0,6 amps. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The inscription on the unnamed network adapter meant that it provides an unstabilized output voltage of 9 V at a load current of 850 mA, that is, it delivers 7,65 W of power to the load. After disassembly, a rectifier bridge and a carelessly assembled step-down network transformer with an overall power of no more than 3 VA were found in the device. Inspection showed that the frame of his coil was partially melted. Using the case of the device, it was decided to assemble a power supply with much better characteristics in it.

The scheme of the new power supply is shown in fig. 1. The AC mains voltage is supplied to the primary winding of the step-down transformer T1 through the protective resistor R1. From the secondary winding, an alternating voltage of about 12 V through a polymer self-healing fuse FU1 is supplied to a bridge rectifier made on Schottky diodes VD1-VD4.

Rice. 1 (click to enlarge)

The voltage regulator is based on the PQ09RE11 chip, which is a linear integrated voltage regulator of positive polarity with an output voltage of 9 V at a load current of up to 1 A. The maximum input voltage of the microcircuit is 35 V, the maximum power dissipation is 15 W. Unlike conventional stabilizers, for example, the 7809 series, KR142EN, the PQxxRx11 series stabilizers have a low minimum voltage between input and output, which does not exceed 1 V at a load current of 0,5 A. Conventional integrated linear voltage regulators require that the input voltage be as at least 2 ... 3 V more than the output. A small voltage drop across the stabilizer can significantly increase the efficiency of the device, since a step-down transformer with a lower voltage on the secondary winding can be used.

In addition, unlike three-pin stabilizers, PQxxRx11 series microcircuits have a fourth pin, which allows you to turn the output voltage on and off with an external signal. In the described power supply, the control output is used to block the operation of the stabilizer when the voltage on the filter capacitor C5 drops below 9 V. The stabilizer turns on when the voltage on the capacitor is more than 9,6 V. Capacitor C7 provides a turn-on delay of about one second.

Thus, the power supply has three types of overload protection. The first is the fast trigger protection built into DA1, which operates at a load current of more than 1,3 A (short circuit and holding current of about 0,8 A). The second is slow protection with a self-healing fuse FU1 (it is important, first of all, to protect the step-down transformer from overload in case of malfunctions of the rectifier and voltage stabilizer elements, as well as thermal protection against overheating of the PSU). Finally, the third is a node on resistors R2-R4 and capacitor C7. When the device is overloaded, the voltage at the terminals C5 decreases, which leads to the shutdown of the stabilizer and an attempt to turn it on again after 0,3 ... 1 s. The HL1 LED lights up when there is an output voltage.

The body of the power supply with dimensions of 65x50x37 mm is made by the manufacturer in the form of a mains plug. Since the mass of the new unit had increased to 300 g, it was considered impractical to plug it directly into the mains socket, so the contact pins were removed, and a flexible wire with a plug at the end was used to connect to the 220 V network.

As a step-down in the described unit, you can use a unified transformer TP112-5. The author used a converted T10-220-50 V transformer from a domestic accounting calculator. Before disassembling the transformer, parts of its magnetic circuit are marked, and after disassembling, the mating surfaces are cleaned of glue. Both secondary windings are removed, a new one (150 turns of PEV-2 wire 0,43) is wound tightly, turn to turn. The parts of the magnetic circuit are glued in the same order with BF-2 glue, the assembled transformer is clamped through thick paper pads into a vise, where it must be kept for at least a day. After that, the primary winding is connected to the network, and the secondary is loaded with a 12 V, 0,5 A incandescent lamp and left in this state for another day. The no-load current of the converted transformer is 18 mA at a mains voltage of 230 V (for comparison: this parameter of the rejected adapter transformer was 30 mA).

Since most of the space in the case was occupied by a transformer, the rest of the parts had to make room. Polymer self-resetting fuse MF-R110 (possible replacement - MF-R090, MF-R090-0-9, LP30-090, LP30-110) is fixed on the paper frame of the transformer. Resistor R1 (non-flammable or bursting) is placed in an insulating silicone tube located next to the input of the power cord into the housing. The remaining resistors are any small-sized ones. Diodes VD1-VD4 and capacitors C1 - C4 are soldered in accordance with the diagram and glued to the case with Quintol glue in a free place under the paper frame of the transformer. The same glue fixed in the case and oxide imported capacitors C5 and C9.

A multilayer ceramic capacitor C6 (for surface mounting, with a rated voltage of at least 25 V) is soldered to the terminals of capacitor C5, and C8 and C10 (also for surface mounting) are soldered directly to terminals 2 and 3 of the DA1 chip (first) and the contacts of the power plug ( second). The remaining non-polar capacitors are ceramic small-sized ones. Resistors R2-R5 and capacitor C7 are mounted on a circuit board with dimensions of 24x15 mm (Fig. 2), and the DA1 microcircuit is mounted on a duralumin plate heat sink with dimensions of 87x31x1 mm. Possible replacement for the PQ09RE11 - PQ09RF11, PQ09RD11 chip, rectifier diodes SB140 - SB150, SB160, 1 N5819, MBRS140T3. L-934MBC blue LED (lens diameter 3 mm) replace RL30-CB744D or similar.

Small size power supply, 9 volts 0,6 amps
Fig. 2

A flexible cord with a length of about 1200 mm is used to connect the load. The copper cross section of each core must be at least 0,5 mm² (do not trust the marking on the wire, it happens that it does not correspond to the actual cross section of the wires!). A view of the installation of parts in the device case is shown in fig. 3. For better cooling, 36 ventilation holes with a diameter of 3,3 mm are drilled in its walls.

Small size power supply, 9 volts 0,6 amps
Fig. 3

Unmistakably made from serviceable parts, the device starts working immediately. If necessary, by selecting the resistor R2, you can set a different voltage threshold for turning on / off the stabilizer DA1. If a large case is used for the new unit, it is advisable to increase the capacitance of the filter capacitor C5 to 3300-4700 uF.

When using a step-down transformer with a larger overall power and a secondary winding with a high voltage (for example, TP114-5, TP114-7), the DA1 chip is installed on a large heat sink.

Author: A. Butov `

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