In my opinion, the supply units (PSUs) are perhaps the weakest link almost any modern radio equipment. One of the main reasons for this sad phenomenon - the instability of the mains voltage. This particularly applies to rural areas. In low power equipment most commonly used blocks food network containing low frequency transformer, which is the the expensive part. From here it becomes clear desire of producers to reduce the cost P, reducing as far as possible the number of turns and the diameter the wires of the windings of the transformer. Under normal line voltage such the transformer works (with high heating), but with a small overvoltage often "ordered to live long."
At the end of may 2007 during a thunderstorm out of order power adapter Ethernet switch D-Link DES-1005D" in our local school. The cause of the fault - the circuit in the primary winding of the transformer. The switch completely was not injured. The new block at hand was not, and the performance of school computer network was required to recover rapidly. In the case of BP, it was stated its output voltage (7.5 V). But when testing a similar unit it turned out what its real value is about 12.5 V. When analyzing a circuit, the switch it became clear that inside it is a switching regulator on the output voltage 3.3 V. as a result I conducted a little experiment it is established that this stabilizer works fine when the input voltage from 6 to 20V Greater voltage at its input I just don't submit dare to inadvertently burn the device.
A suitable transformer at hand was not, and had to do", what was". The basis for a new PSU was quite "ancient" output transformer sound TVZ-W from domestic unified tube-semiconductor TVs. Instead transformer TVZ-W can take TVZ-9 with similar parameters and dimensions. Since this transformer is noticeable more "native", had the whole scheme to collect in another body. As the case is very well suited steel screen from the device for correction of distortions raster unit scans BR-2 unified domestic the lamp is a solid-state color TVs (ULETT-61-P or ULPC-T-61-I). To this corps had only to produce the missing wall thickness of steel about 1 mm, for which the transformer and the payment details. All the details soldered on a small glass fibre laminate Board size 20x30 mm. Their findings connected in accordance with the scheme of thin tinned copper wire 00,6 mm. Any specific requirements for installation, apart from the quality of the soldering, no.
Feature of PD is the use of rare nowadays rectifier voltage doubler. In the network are pins 1 and 2 of the transformer. Diodes VD1, VD2 optionally substituted any modern or legacy, to be calculated on the average a direct current of at least 1 A and the reverse voltage is not lower Their frequency 35 V. the properties of this scheme do not matter much. Electrolytic capacitors C1 and C2 may be of virtually any type, of suitable size and the tension. The capacity of each of them should be increased to 1000 µf. The Capacitor C3 Is ceramic, on the operating voltage of not lower than 25 V.
Led VD3 may be of any type and color, when you install super-vivid resistance device R1 is desirable to increase up to 2...3 kω. DA1 chip using any thermal paste, for example, KPT-8, set on a small copper or aluminum radiator with the cooling surface area 8...10 cm2. You can apply any foreign equivalent to the output voltage of 12 V. Output connector x2 taken from an old burned-out block. The output "+" connects to his inner contact.
To reduce noise penetrating from the network, it is desirable shall be shorted primary winding of the transformer some ceramic capacitor 0.01...0,047 UF operating voltage not lower than 1600 In With the same purpose metal enclosure should be grounded.
For long-term reliable operation of the PSU I recommend slightly to upgrade the power transformer. Its core is carefully disassembled, and then the plates of the core are going to overlap. This simple step can reduce the idle current of several times. After Assembly, the core is impregnated napolcom. Before the alteration in continuous operation the transformer is slightly heated, and could hear a slight buzz. After upgrading heating brand was not felt, the buzz is completely gone. Therefore, the power supply in this embodiment it is safe to use for full-time work
In the manufacture of the described power supply no calculations were made. The parameters used parts were chosen tentatively ("by eye"). After manufacturing during testing, thermal and electrical parameters of all the details of the device were measured and proved to be quite "light".
The unit can be used for almost any semiconductor equipment, designed for a voltage of 12 V with maximum the current consumption of no more than 0.5. .0,6 A. Parallel to the output terminals in case it is useful to install electrolytic capacitor 470...1000 UF on the operating voltage of not lower than 16 V.
If necessary, the output voltage is easily reduced to 9 V. this you will need to place DA1 to install the chip CRRA or any foreign analogue. The size of the radiator for it should increase by 1.5 times due to increased the voltage drop on it. This stabilizer will have significantly lower performance index but, owing to the relatively small power output, this can easily to live with.
Literature
Author: M. Ryjenkov, village of Suvo, Rep. Buryatia