Power supply for Ethernet switch D-Link DES-1005D. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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In my opinion, power supplies (PSU) are perhaps the weakest link in almost any modern radio equipment. One of the main reasons for this sad phenomenon is the instability of the mains voltage. This is especially true for rural areas. In low-power equipment, power supplies are most often used that contain a low-frequency network transformer, which is their most expensive part. From this it becomes clear the desire of manufacturers to reduce the cost of 6P, reducing as far as possible the number of turns and the diameter of the wires of all transformer windings. With a normal voltage in the network, such a transformer works (with increased heating), but with a slight overvoltage it often "orders to live long".

At the end of May 2007, during a thunderstorm, the power supply of the "D-Link DES-1005D" Ethernet switch at our local school failed. The cause of the malfunction is a short circuit in the primary winding of the transformer. The switch itself was not damaged at all. The new unit was not at hand, and the school computer network needed to be quickly restored. Its output voltage (7,5 V) was indicated on the PSU case. But when testing a similar unit, it turned out that its real value is about 12,5 V. When analyzing the circuit of the switch itself, it became clear that a switching regulator for an output voltage of 3,3 V was located inside it. As a result of a small experiment I conducted, it was found that this the stabilizer works normally when the input voltage changes from 6 to 20V. I simply did not dare to apply more voltage to its input, so as not to inadvertently burn the device.

There was no suitable transformer at hand, and I had to make "from what was." The basis of the new power supply was a rather "ancient" TVZ-Sh output sound transformer from domestic unified tube-semiconductor TVs. Instead of a TVZ-Sh transformer, you can take TVZ-9 with similar parameters and dimensions. Since this transformer is noticeably larger than the "native" one, I had to assemble the entire circuit in another case. The steel screen from the device for correcting raster distortions of the BR-2 scanner of domestic unified tube-semiconductor color TVs (ULPITST-61-P or ULPTs-TI-61-I) was very well suited as a case. For this case, it was only necessary to make the missing wall of steel with a thickness of about 1 mm, on which a transformer and a board with parts were installed. All parts are soldered on a small fiberglass board measuring 20x30 mm. Their conclusions are connected in accordance with the diagram with a thin 00,6 mm tinned copper wire. There are no specific requirements for installation, except for high-quality soldering.

A feature of the PSU is the use of a voltage doubling rectifier, which is rarely found at present. The network includes conclusions 1 and 2 of the transformer. Diodes VD1, VD2, if necessary, are replaced by any modern or obsolete ones, which must be designed for an average forward current of at least 1 A and a reverse voltage of at least 35 V. Their frequency properties do not matter much for this circuit. Electrolytic capacitors C1 and C2 can be of almost any type suitable for size and voltage. It is better to increase the capacitance of each of them to 1000 microfarads. Capacitor C3 - ceramic, for an operating voltage of at least 25 V.

LED VD3 - any type and color, when installing a super-bright device, it is desirable to increase the resistance R1 to 2 ... 3 kOhm. Chip DA1 with the use of any thermal paste, for example, KPT-8, is installed on a small copper or aluminum radiator with a cooling surface area of ​​8...10 cm2. You can use any foreign analogue for an output voltage of 12 V. The X2 output connector is taken from an old burned-out block. The "+" output is connected to its internal contact.

To reduce interference from the network, it is desirable to shunt the primary winding of the transformer with some ceramic capacitor with a capacity of 0.01 ... 0,047 μF for an operating voltage of at least 1600 V. For the same purpose, the metal case of the device should be grounded.

For long-term trouble-free operation of the power supply, I recommend slightly upgrading the power transformer. Its core is carefully disassembled, and then the core plates are assembled with overlap. Such a simple measure reduces the no-load current by several times. After assembly, the core is impregnated with zaponlak. Before reworking, during prolonged operation, the transformer heated up slightly, and a slight buzz was heard. After the upgrade, the heating was absolutely not felt, the buzzing completely disappeared. Therefore, the power supply in this embodiment can be safely used for round-the-clock operation.

In the manufacture of the described power supply, no calculations were made. The parameters of the parts used were selected tentatively ("by eye"). After manufacturing during testing, the thermal and electrical parameters of all parts of the apparatus were measured and turned out to be quite "light".

The block can be successfully used to power almost any semiconductor equipment designed for a voltage of 12 V with a maximum current consumption of not more than 0,5. .0,6 A. In this case, it is useful to install an electrolytic capacitor with a capacity of 470 ... 1000 μF in parallel with the output terminals for an operating voltage of at least 16 V.

If necessary, the output voltage can be easily reduced to 9 V. To do this, you need to install the KR1EN142A chip or any of its foreign analogues in place of DA8. The size of the radiator for it should be increased by 1,5 times due to the increased voltage drop across it. Such a stabilizer will have a noticeably lower efficiency, but, due to the relatively small output power, this can be easily put up with.

Literature

1. G.P. Veresov. Power supply of household radio-electronic equipment. - M.: Radio and communications. 1983.

Author: M.Ryndenkov, Suvo village, Resp. Buryatia

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