End of table. 2.2 Number w IV IVa IV6 IV6 IV6 V VI Winding Name Positive feedback Rectifiers 125, 24, 18 V Rectifier 15 V Rectifier 12 V Conclusions 11 6-12 including: 6-10 10-4 4-8 8-12 14 -18 16-20 Number of turns 16 74 54 7 5 12 10 10 Wire brand PEVTL-0.355 ZZIM PEVTL-0.355 PEVTL-0.355 Winding type Ordinary in three wires Ordinary in two wires, two layers Ordinary in two wires Same -“- Ordinary in four wires The same Resistance, Ohm 0.2 1.2 0.9 0.2 0.2 0.2 0.2 0.2 Note. Transformers TPI-3, TPI 4 2, TPI-4-3, TPI-5 are made on a magnetic core M300NMS Ш12Х20Х15 with an air gap of 1.3 mm in the middle rod, transformer TPI-8-1 is made on a closed magnetic core M300NMS-2 Ш12Х20Х21 with an air gap a gap of 1.37 mm in the middle rod of any electrical alterations, but in this case the X2 connector of the MP-4-6 module must be shifted to the left by one contact (its second contact becomes like the first contact) or when connecting MP-44-3 instead of MP-3, the fourth contact of connector X2 becomes, as it were, the first contact.
In table 2 2 shows the winding data of pulse power transformers.
The general view, overall dimensions and layout of the printed circuit board for installing pulse power transformers are shown in Fig. 2.16.
Rice. 2.16. General view, overall dimensions and layout of the printed circuit board for installing pulse power transformers. A feature of the SMPS is that they cannot be turned on without a load. In other words, when repairing the MP, it must be connected to the TV or load equivalents must be connected to the MP outputs. The circuit diagram for connecting load equivalents is shown in Fig. 2 17.
The following equivalent loads must be installed in the circuit: R1-resistor with a resistance of 20 Ohms ±5%, with a power of at least 10 W; R2—resistor with a resistance of 36 Ohms ±5%, power of at least 15 W; R3 - resistor with a resistance of 82 Ohms ±5%, power of at least 15 W; R4 -RPSh 0.6 A =1000 Ohm; in amateur radio practice, instead of a rheostat, a 220 V electric lamp with a power of at least 25 W or a 127 V lamp with a power of 40 W is often used; Rice. 2.17. Schematic diagram of connecting load equivalents to the R5 power module - a resistor with a resistance of 3.6 Ohms, a power of at least 50 W; C1 - capacitor type K50-35-25 V, 470 μF; C2 - capacitor type K50-35-25 V, 1000 μF; SZ capacitor type K50-35-40 V, 470 µF.
Load currents should be: for a 12 V circuit 1„o„=0.6 A; on a circuit 15 V 1nom = 0.4 A (minimum current 0.015 A), maximum 1 A); along a 28 V circuit 1„OM=0.35 A; along the circuit 125... 135 V 1„Ohm = 0.4 A (minimum current 0.3 A, maximum 0.5 A).
A switching power supply has circuits connected directly to the mains voltage. Therefore, when repairing an MP, it must be connected to the network via an isolation transformer.
The danger zone on the MP board from the printing side is indicated by hatching with solid lines.
Replace faulty elements in the module only after turning off the TV and discharging the oxide capacitors in the filter circuits of the mains rectifier.
Repair of the MP should begin with removing its protective covers, removing dust and dirt, and visually checking for installation defects and radioelements with external damage. 2.6, Possible malfunctions and methods for their elimination The principle of construction of the basic models of 4USCT TVs is the same, the output voltages of the secondary switching power supplies are also almost the same and are designed to power the same sections of the TV circuit. Therefore, at its core, the external manifestation of malfunctions, their possible39
A schematic diagram of a homemade switching power supply with an output voltage of +14V and a current sufficient to power a screwdriver is described.
A screwdriver or cordless drill is a very convenient tool, but there is also a significant drawback: with active use, the battery discharges very quickly - in a few tens of minutes, and it takes hours to charge.
Even having a spare battery doesn't help. A good way out when working indoors with a working 220V power supply would be an external source for powering the screwdriver from the mains, which could be used instead of a battery.
But, unfortunately, specialized sources for powering screwdrivers from the mains are not commercially produced (only chargers for batteries, which cannot be used as a mains source due to insufficient output current, but only as a charger).
In the literature and on the Internet there are proposals to use car chargers based on a power transformer, as well as power supplies from personal computers and for halogen lighting lamps, as a power source for a screwdriver with a rated voltage of 13V.
All of these are probably good options, but without pretending to be original, I suggest making a special power supply yourself. Moreover, based on the circuit I have given, you can make a power supply for another purpose.
Schematic diagram
The circuit is partially borrowed from L.1, or rather, the idea itself is to make an unstabilized switching power supply using a blocking generator circuit based on a TV power supply transformer.
Rice. 1. The circuit of a simple switching power supply for a screwdriver is made using a KT872 transistor.
The voltage from the network is supplied to the bridge using diodes VD1-VD4. A constant voltage of about 300V is released at capacitor C1. This voltage powers a pulse generator on transistor VT1 with transformer T1 at the output.
The circuit on VT1 is a typical blocking oscillator. In the collector circuit of the transistor, the primary winding of transformer T1 (1-19) is connected. It receives a voltage of 300V from the output of the rectifier using diodes VD1-VD4.
To start the blocking generator and ensure its stable operation, a bias voltage from the circuit R1-R2-R3-VD6 is supplied to the base of transistor VT1. The positive feedback necessary for the operation of the blocking generator is provided by one of the secondary coils of the pulse transformer T1 (7-11).
The alternating voltage from it through capacitor C4 enters the base circuit of the transistor. Diodes VD6 and VD9 are used to generate pulses based on the transistor.
Diode VD5, together with circuit C3-R6, limits surges of positive voltage at the collector of the transistor by the value of the supply voltage. Diode VD8, together with the circuit R5-R4-C2, limits the surge of negative voltage on the collector of transistor VT1. Secondary voltage 14V (at idle 15V, under full load 11V) is taken from winding 14-18.
It is rectified by diode VD7 and smoothed by capacitor C5. The operating mode is set by trimming resistor R3. By adjusting it, you can not only achieve reliable operation of the power supply, but also adjust the output voltage within certain limits.
Details and design
Transistor VT1 must be installed on the radiator. You can use a radiator from the MP-403 power supply or any other similar one.
Pulse transformer T1 is a ready-made TPI-8-1 from the power supply module MP-403 of a domestic color TV of type 3-USTST or 4-USTST. Some time ago these TVs were either dismantled or thrown away altogether. Yes, and TPI-8-1 transformers are available for sale.
In the diagram, the terminal numbers of the transformer windings are shown according to the markings on it and on the circuit diagram of the MP-403 power module.
The TPI-8-1 transformer has other secondary windings, so you can get another 14V using winding 16-20 (or 28V by connecting 16-20 and 14-18 in series), 18V from winding 12-8, 29V from winding 12- 10 and 125V from winding 12-6.
Thus, it is possible to obtain a power source for powering any electronic device, for example, an ULF with a preliminary stage.
The second figure shows how rectifiers can be made on the secondary windings of the TPI-8-1 transformer. These windings can be used for individual rectifiers or connected in series to produce higher voltage. In addition, within certain limits it is possible to regulate the secondary voltages by changing the number of turns of the primary winding 1-19 using its taps for this.
Rice. 2. Diagram of rectifiers on the secondary windings of the TPI-8-1 transformer.
However, the matter is limited to this, because rewinding the TPI-8-1 transformer is a rather thankless job. Its core is tightly glued, and when you try to separate it, it breaks not where you expect.
So, in general, you won’t be able to get any voltage from this unit, except perhaps with the help of a secondary step-down stabilizer.
The KD202 diode can be replaced with any more modern rectifier diode with a direct current of at least 10A. As a radiator for transistor VT1, you can use the key transistor radiator available on the MP-403 module board, modifying it slightly.
Shcheglov V. N. RK-02-18.
Literature:
1. Kompanenko L. - A simple pulse voltage converter for a TV’s power supply. R-2008-03.
The power supply contains a small number of components. A standard step-down transformer from a computer power supply is used as a pulse transformer.
At the input there is an NTC thermistor (Negative Temperature Coefficient) - a semiconductor resistor with a positive temperature coefficient, which sharply increases its resistance when a certain characteristic temperature TRef is exceeded. Protects power switches at the moment of switching on while capacitors are charging.
Diode bridge at the input for rectifying the mains voltage to a current of 10A.
A pair of capacitors at the input is taken at the rate of 1 microfarad per 1 W. In our case, the capacitors will “pull” a load of 220W.
Driver IR2151 - for controlling the gates of field-effect transistors operating under voltages up to 600V. Possible replacement for IR2152, IR2153. If the name contains the index “D”, for example IR2153D, then the FR107 diode in the driver harness is not needed. The driver alternately opens the gates of the field-effect transistors with a frequency set by the elements on the legs Rt and Ct.
Field effect transistors are preferably used from IR (International Rectifier). Select a voltage of at least 400V and with minimal open resistance. The lower the resistance, the lower the heating and the higher the efficiency. We can recommend IRF740, IRF840, etc. Attention! Do not short-circuit the flanges of field-effect transistors; When installing on a radiator, use insulating gaskets and bushing washers.
A standard step-down transformer from a computer power supply. As a rule, the pinout corresponds to that shown in the diagram. Homemade transformers wound on ferrite tori also work in this circuit. Homemade transformers are calculated for a conversion frequency of 100 kHz and half the rectified voltage (310/2 = 155V). The secondary windings can be designed for a different voltage.
Output diodes with a recovery time of no more than 100 ns. These requirements are met by diodes from the HER (High Efficiency Rectifier) family. Not to be confused with Schottky diodes.
The output capacity is a buffer capacity. Do not abuse and install a capacitance of more than 10,000 microfarads.
Like any device, this power supply requires careful and careful assembly, correct installation of polar elements and caution when working with mains voltage.
A properly assembled power supply does not require configuration or adjustment. The power supply should not be turned on without load.
Power supply option with an output transformer on a ring core.
I decided to assemble this switching power supply with an output transformer on a ring core. As it turned out, the conversion frequency with R2 10 kOhm and C5 1000 pF is not 100 kHz but 70 kHz. It is determined by the formula:
As a core, I used the available, domestic magnetic core M2000NM 45x28x12. The calculation was carried out using the ExcellentIT program
During setup, I turned on a 60W incandescent lamp instead of a fuse, so that in case of installation errors I would not “burn” the power supply. If the lamp lights up during the setup process, it means there is a short circuit somewhere; if it blinks, the output transformer is most likely incorrectly designed. The power supply worked immediately, the calculations turned out to be correct. The only thing was that the quenching resistor R1 was heating up. I had to increase its power to 5 W. It is also advisable to install more powerful diodes with a short recovery time.
Before disassembling it, it is not harmful to measure the inductance and quality factor of the windings, and it is even better to take this data from a live sample, so that you have something to compare with after repair.
According to the posting, a hair dryer does not always help in the case of large cores. For gluing, I first used a small laboratory tile, then a flat heating element from
electric kettle (there is even a thermal switch set to 150 degrees, but to be on the safe side, you can turn it on via LATR and select the temperature). I made sure to press it tightly with the free part of the ferrite (if it was the gluing side, then after sanding off the glue flow) to the cold surface of the heater and only then turned it on.
When disassembling, the main thing is patience - I pulled harder and that’s another problem.
Regarding the cores, there were almost no problems with disassembly and reassembly except for GRUNDIGs and PANASONICs. In khryundels (filled with TPI compound in old TVs) the main problems are precisely related to the cores, more precisely with their cracking. It is not possible to install another core of suitable size there due to the fact that the operating frequency of these TPIs is 3-5 times higher and low-frequency cores do not live in them. In this case, the use of cores saves from large FBT. For a full recreation, a live sample from the same product is required to compare characteristics. (if you really want to restore it, you can find it)
(Please do not ask questions about the cost and feasibility of this work, but the fact remains that such hybrids work.)
With some Panas, the trick is to have very small gaps, and this is where a preliminary inductance measurement helps.
I don’t recommend gluing with superglue because I had several repetitions due to cracking of the adhesive seam. Kneading a drop of epoxy is of course fussy, but more reliable, and after gluing it is good to compress the joint (for example, applying a constant voltage to the winding - it will tighten itself and even warm it up slightly).
About the pan with boiling water - I confirm for the case with FBT (it was necessary to tear out the cores from 30 dead flies) it works perfectly, I did not mock the TPI in this way, which had to be rewound.
At the moment, everything that was rewound (by me, and in especially severe cases by the mentioned specialist N. Novopashin) is working. There were even successful results in rewinding line transformers (with an external multiplier) from quite ancient industrial monitors, but the secret of success is in vacuum impregnation of the windings (by the way, Nikolai impregnates almost all rewound trances except outright consumer goods) and unfortunately this cannot be cured on the knee.
The mentioned Rematik device was recently used to check the high voltage trans of the backlight from the dashboard of a Mercedes - it showed everything OK on a obviously broken trance, although the DIEMEN device also deceived us on it - the trance was broken only at a fairly high voltage, which in fact allowed us to measure it at a low voltage.
A screwdriver or cordless drill is a very convenient tool, but there is also a significant drawback - with active use, the battery discharges very quickly - in a few tens of minutes, and it takes hours to charge. Even having a spare battery doesn't help. A good way out when working indoors with a working 220V power supply would be an external source for powering the screwdriver from the mains, which could be used instead of a battery. But, unfortunately, specialized sources for powering screwdrivers from the mains are not commercially produced (only chargers for batteries, which cannot be used as a mains source due to insufficient output current, but only as a charger).
In the literature and on the Internet there are proposals to use car chargers based on a power transformer as a power source for a screwdriver with a rated voltage of 13V, as well as power supplies from personal computers and for halogen lighting lamps. All of these are probably good options, but without pretending to be original, I suggest making a special power supply yourself. Moreover, based on the circuit I have given, you can make a power supply for another purpose.
And so, the source diagram is shown in the figure in the text of the article.
This is a classic flyback AC-DC converter based on the UC3842 PWM generator.
The voltage from the network is supplied to the bridge using diodes VD1-VD4. A constant voltage of about 300V is released at capacitor C1. This voltage powers a pulse generator with transformer T1 at the output. Initially, the triggering voltage is supplied to power pin 7 of IC A1 through resistor R1. The pulse generator of the microcircuit is turned on and produces pulses at pin 6. They are fed to the gate of the powerful field-effect transistor VT1 in the drain circuit of which the primary winding of the pulse transformer T1 is connected. The transformer begins to operate and secondary voltages appear on the secondary windings. The voltage from winding 7-11 is rectified by diode VD6 and used
to power microcircuit A1, which, having switched to constant generation mode, begins to consume current that the starting power supply on resistor R1 is not capable of supporting. Therefore, if the diode VD6 malfunctions, the source pulsates - through R1, capacitor C4 is charged to the voltage required to start the microcircuit generator, and when the generator starts, the increased current C4 discharges, and generation stops. Then the process is repeated. If VD6 is working properly, immediately after startup the circuit switches to power from winding 11 -7 of transformer T1.
Secondary voltage 14V (at idle 15V, under full load 11V) is taken from winding 14-18. It is rectified by diode VD7 and smoothed by capacitor C7.
Unlike the standard circuit, a protection circuit for the output switching transistor VT1 from increased drain-source current is not used here. And the protection input, pin 3 of the microcircuit, is simply connected to the common negative of the power supply. The reason for this decision is that the author does not have the necessary low-resistance resistor (after all, you have to make one from what is available). So the transistor here is not protected from overcurrent, which of course is not very good. However, the scheme has been working for a long time without this protection. However, if desired, you can easily make protection by following the typical connection diagram of the UC3842 IC.
Details. Pulse transformer T1 is a ready-made TPI-8-1 from the power supply module MP-403 of a domestic color TV of type 3-USTST or 4-USTST. These TVs are now often dismantled or thrown away altogether. Yes, and TPI-8-1 transformers are available for sale. In the diagram, the terminal numbers of the transformer windings are shown according to the markings on it and on the circuit diagram of the MP-403 power module.
The TPI-8-1 transformer has other secondary windings, so you can get another 14V using winding 16-20 (or 28V by connecting 16-20 and 14-18 in series), 18V from winding 12-8, 29V from winding 12- 10 and 125V from winding 12-6. In this way, you can obtain a power source to power any electronic device, for example, an ULF with a preliminary stage.
However, the matter is limited to this, because rewinding the TPI-8-1 transformer is a rather thankless job. Its core is tightly glued and when you try to separate it, it breaks not where you expect. So, in general, you won’t be able to get any voltage from this unit, except perhaps with the help of a secondary step-down stabilizer.
The IRF840 transistor can be replaced with an IRFBC40 (which is basically the same), or with a BUZ90, KP707V2.
The KD202 diode can be replaced with any more modern rectifier diode with a direct current of at least 10A.
As a radiator for transistor VT1, you can use the key transistor radiator available on the MP-403 module board, modifying it slightly.
The use of pulse transformers ensures increased reliability and durability, reduced overall dimensions and weight of power supply units and modules. But it should also be noted that switching stabilizers used in TV power supplies have the following disadvantages: a more complex control device, increased noise levels, radio interference and output voltage ripple, and at the same time worse dynamic characteristics.
In master oscillators of horizontal or vertical scanning, operating according to the blocking oscillator circuit.
Pulse transformers and autotransformers are used. These transformers (autotransformers) are elements with strong inductive feedback. In the technical literature, pulse transformers and autotransformers for horizontal scanning are abbreviated as BTS and BATS; for personnel scanning - VTK and TBK. Pulse transformers VTK and TBK are practically no different in design from other transformers. Transformers are manufactured for both volumetric and printed circuit mounting.
Pulse transformers of types TPI-2, TPI-3, TPI-4-2, TPI-5, etc. are used in power supplies and modules.
Winding data for transformers operating in pulse mode, used in stationary and portable television receivers, are given in Table. 7.13.
Table 7.13. Wet data of pulse transformers used in televisions
Designation | Brand and diameter | ||||||
typenomshala | transformer windings | wires, mm | permanent |
||||
transformer | |||||||
Magnetizing | PEVTL-2 0.45 | ||||||
PEVTL-2 0.45 | |||||||
Stabilization | PEVTL-2 0.45 | ||||||
Positive about- | Private in | PEVTL-2 0.45 | |||||
military communications | |||||||
Rectifiers with on- | Private in | ||||||
yarns, V: | two wires | ||||||
PEVTL-2 0.45 | |||||||
PEVTL-2 0.45 | |||||||
PEVTL-2 0.45 | |||||||
PEVTL-2 0.45 | |||||||
PEVTL-2 0.45 | |||||||
Magnetization Same | Private in two wires | PEVTL-2 0.45 | |||||
PEVTL-2 0.45 | |||||||
Stabilization | PEVTL-2 0.45 | ||||||
Rectifiers with on- | |||||||
yarns, V: | |||||||
PEVTL-2 0.45 | |||||||
Private in two wires | PEVTL-2 0.45 | ||||||
PEVTL-2 0.45 | |||||||
PEVTL-2 0.45 | |||||||
Foil one layer | |||||||
Positive about- | PEVTL-2 0.45 | ||||||
military communications | |||||||
or Ш (УШ) | Magnetization | Private in two wires | PEVTL-2 0.45 | ||||
Magnetization | PEVTL-2 0.45 | ||||||
Stabilization | Private, pitch 2.5 mm | PEVTL-2 0.45 | |||||
Rectifiers with on- | |||||||
yarn, V: | |||||||
PEVTL-2 0.45 | |||||||
Private in two wires | PEVTL-2 0.45 | ||||||
PEVTL-2 0.45 | |||||||
PEVTL-2 0.45 |
Continuation of the table. 7.13
Designation | Name | Brand and diameter | Resistance |
||||
typonokmnala | wires, mm | permanent |
|||||
transformer | |||||||
Positive about- | PEVTL-2 0.45 | ||||||
military communications | |||||||
Magnetization | Private in | PEVTL-2 0.45 | |||||
two wires | |||||||
PEVTL-2 0.45 | |||||||
Stabilization | PEVTL-2 0.25 | ||||||
Weekend rectifier | |||||||
voltage | |||||||
PEVTL-2 0.45 | |||||||
Private in | PEVTL-2 0.45 | ||||||
two wires | |||||||
Private in | PEVTL-2 0.45 | ||||||
two wires | PEVTL-2 0.45 | ||||||
Positive about- | PEVTL-2 0.45 | ||||||
military communications | |||||||
Primary | |||||||
Secondary | |||||||
12 plates | |||||||
Primary | Universal | ||||||
Secondary | |||||||
Primary | |||||||
Secondary | |||||||
Primary | |||||||
Recuperative | |||||||
Primary | |||||||
Feedback | |||||||
Day off | |||||||
Primary network |
Rice. 1. Network filter board diagram.
Soviet TVs Horizont Ts-257 used power supply with intermediate conversion of mains voltage with a frequency of 50 Hz into rectangular pulses with a repetition frequency of 20...30 kHz and their subsequent rectification. The output voltages are stabilized by changing the duration and repetition rate of the pulses.
The source is made in the form of two functionally complete units: a power module and a network filter board. The module provides isolation of the TV chassis from the network, and elements galvanically connected to the network are covered with screens that restrict access to them.
Main technical characteristics of a switching power supply
- Maximum output power, W........100
- Efficiency..........0,8
- Limits for changes in network voltage, V......... 176...242
- Instability of output voltages, %, no more..........1
- Rated values of load current, mA, voltage sources, V:
135 ....................500
28 ....................340
15 ..........700
12 ..........600 - Weight, kg...................1
Rice. 2 Schematic diagram of the power module.
It contains a mains voltage rectifier (VD4-VD7), a starting stage (VT3), stabilization units (VT1) and blocking 4VT2), a converter (VT4, VS1, T1), four half-wave output voltage rectifiers (VD12-VD15) and a compensation voltage stabilizer 12 V (VT5-VT7).
When the TV is turned on, the mains voltage is supplied to the rectifier bridge VD4-VD7 through a limiting resistor and noise suppression circuits located on the power filter board. The voltage rectified by it passes through the magnetization winding I of pulse transformer T1 to the collector of transistor VT4. The presence of this voltage on capacitors C16, C19, C20 is indicated by LED HL1.
Positive mains voltage pulses through capacitors C10, C11 and resistor R11 charge capacitor C7 of the trigger stage. As soon as the voltage between the emitter and base 1 of unijunction transistor VT3 reaches 3 V, it opens and capacitor C7 is quickly discharged through its emitter-base 1 junction, the emitter junction of transistor VT4 and resistors R14, R16. As a result, transistor VT4 opens for 10...14 μs. During this time, the current in the magnetization winding I increases to 3...4 A, and then, when transistor VT4 is closed, it decreases. The pulse voltages arising on windings II and V are rectified by diodes VD2, VD8, VD9, VD11 and charge capacitors C2, C6, C14: the first of them is charged from winding II, the other two are charged from winding V. With each subsequent switching on and off of the transistor VT4 recharges the capacitors.
As for the secondary circuits, at the initial moment after turning on the TV, the capacitors C27-SZO are discharged, and the power module operates in a mode close to a short circuit. In this case, all the energy accumulated in transformer T1 enters the secondary circuits, and there is no self-oscillatory process in the module.
Upon completion of charging of the capacitors, oscillations of the residual energy of the magnetic field in transformer T1 create such a positive feedback voltage in the winding V, which leads to the occurrence of a self-oscillatory process.
In this mode, transistor VT4 opens with positive feedback voltage, and closes with voltage on capacitor C14 supplied through thyristor VS1. It happens like this. The linearly increasing current of the opened transistor VT4 creates a voltage drop across resistors R14 and R16, which in positive polarity through cell R10C3 is supplied to the control electrode of thyristor VS1. At the moment determined by the operating threshold, the thyristor opens, the voltage on capacitor C14 is applied in reverse polarity to the emitter junction of transistor VT4, and it closes.
Thus, turning on the thyristor sets the duration of the sawtooth pulse of the collector current of transistor VT4 and, accordingly, the amount of energy given to the secondary circuits.
When the output voltages of the module reach the nominal values, capacitor C2 is charged so much that the voltage removed from the divider R1R2R3 becomes greater than the voltage on the zener diode VD1 and the transistor VT1 of the stabilization unit opens. Part of its collector current is summed in the circuit of the thyristor control electrode with the initial bias current created by the voltage on capacitor C6 and the current generated by the voltage on resistors R14 and R16. As a result, the thyristor opens earlier and the collector current of transistor VT4 decreases to 2...2.5 A.
When the network voltage increases or the load current decreases, the voltages on all windings of the transformer increase, and therefore the voltage on capacitor C2 increases. This leads to an increase in the collector current of transistor VT1, earlier opening of thyristor VS1 and closing of transistor VT4, and, consequently, to a decrease in the power supplied to the load. Conversely, when the network voltage decreases or the load current increases, the power transferred to the load increases. Thus, all output voltages are stabilized at once. Trimmer resistor R2 sets their initial values.
In the event of a short circuit of one of the module outputs, self-oscillations are disrupted. As a result, transistor VT4 is opened only by the triggering cascade on transistor VT3 and closed by thyristor VS1 when the collector current of transistor VT4 reaches a value of 3.5...4 A. Packets of pulses appear on the windings of the transformer, following at the frequency of the supply network and a filling frequency of about 1 kHz. In this mode, the module can operate for a long time, since the collector current of transistor VT4 is limited to a permissible value of 4 A, and the currents in the output circuits are limited to safe values.
In order to prevent large current surges through the transistor VT4 at an excessively low network voltage (140... 160 V) and, therefore, in case of unstable operation of the thyristor VS1, a blocking unit is provided, which in this case turns off the module. The base of the transistor VT2 of this node receives a direct voltage proportional to the rectified mains voltage from the divider R18R4, and the emitter receives a pulse voltage with a frequency of 50 Hz and an amplitude determined by the zener diode VD3. Their ratio is chosen such that at the specified network voltage, transistor VT2 opens and thyristor VS1 opens with collector current pulses. The self-oscillatory process stops. As the network voltage increases, the transistor closes and does not affect the operation of the converter. To reduce the instability of the 12 V output voltage, a compensation voltage stabilizer on transistors (VT5-VT7) with continuous regulation is used. Its feature is current limitation during a short circuit in the load.
In order to reduce the influence on other circuits, the output stage of the audio channel is powered from a separate winding III.
IN pulse transformer TPI-3 (T1) uses magnetic core M3000NMS Ш12Х20Х15 with an air gap of 1.3 mm on the middle rod.
Rice. 3. Layout of the windings of the TPI-3 pulse transformer.
Winding data of the TPI-3 transformer switching power supply are given:
All windings are made with PEVTL 0.45 wire. In order to uniformly distribute the magnetic field over the secondary windings of the pulse transformer and increase the coupling coefficient, winding I is divided into two parts, located in the first and last layers and connected in series. Stabilization winding II is made with a pitch of 1.1 mm in one layer. Winding III and sections 1 - 11 (I), 12-18 (IV) are wound in two wires. To reduce the level of radiated interference, four electrostatic screens were introduced between the windings and a short-circuited screen on top of the magnetic conductor.
The power filter board (Fig. 1) contains elements of the L1C1-SZ barrier filter, a current-limiting resistor R1 and a device for automatic demagnetization of the kinescope mask on the thermistor R2 with a positive TKS. The latter provides a maximum amplitude of the demagnetization current of up to 6 A with a smooth decline within 2...3 s.
Attention!!! When working with the power module and TV, you must remember that the elements of the power filter board and some of the module parts are under mains voltage. Therefore, it is possible to repair and check the power module and filter board under voltage only when they are connected to the network through an isolation transformer.