A simple switching voltage converter for a TV power supply. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies
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For many years, TVs have been using power supplies, which are switching sources that convert a high direct voltage (about 300 V) into a series of also constant output voltages of other (usually lower) values, for example, 125, 60, 25, 15 V, etc.
High voltage is obtained from alternating (50 Hz) mains 220 V using a full-wave rectifier, and output voltages are obtained from pulsed, relatively high repetition rates (tens of kilohertz), formed mainly on the secondary windings of the power transformer.
Between these nodes, a rather complex pulse converter is connected, the driving part of which is assembled on a microcircuit or transistors, which often fail. And it is not always possible to immediately find the necessary new parts to replace the faulty ones.
Fig. 1
In order to make the TV work, you can temporarily assemble a very simple switching converter according to the circuit shown in fig. 1. The output powerful transistor VT1 is left in the device (of the PSU itself or replaced by its serviceable or analogue), the collector and emitter terminals of which are connected to the existing TV circuits. The connection is shown for the Yunost 42TTs408D TV, and all nodes of the driving part of the power supply converter must be disconnected from the base of the transistor. Of course, in another TV, the collector circuits of the transistor may look different, but they are left unchanged.
The base circuit of the transistor VT1 is assembled according to the presented scheme. It uses elements R2 (R28 by numbering on the TV), VD1 (VD10), VD2 (VD11) from the TV PSU with a change in inclusion and new additional parts R1, R3, C1. Capacitor C1 - K73-9 for a voltage of 100 V.
Fig. 2
As a result, such a converter is a blocking oscillator, the voltage oscillograms at characteristic points of which can be seen in Fig. 2 when using the device in the above TV. In other devices that use different transformers, the waveforms may differ slightly.
Transistor VT1 is installed on the heat sink available in the PSU. In addition, an additional metal plate is screwed to the heat sink, which approximately doubles its scattering area.
To set the converter to the nominal mode, it is necessary to rotate the trimmer resistor R2 to obtain the required line voltage - in our case, 125 V at the output of the rectifier in the secondary circuit.
It should be pointed out again that the use of such a converter is a temporary measure, since the stability of the resulting voltages will be low. As a result, when changing the brightness of the image, its size also changes slightly.
Author: L. Kompanenko, Moscow; Publication: radioradar.net
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Comments on the article:
Telemaster
The beginning is good, but the most important thing was forgotten - TRANSFORMER !!! I don't have any data. What section, what kind of ferrite or iron, how much to wind, what kind of wire, etc. ????? In the absence of information, this scheme does not make sense. Why did you write it at all?
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