Using a transformer from the TV. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Using a TV transformer

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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You can make a mains power supply for various home-made designs and test them on breadboards yourself. This is not difficult, and at the same time extremely useful for improving one's skills, expanding knowledge and gaining experience, which, in fact, is what all amateur radio activities are aimed at.

Radio amateurs most often require two power sources: one is low-power, for a voltage of 3 to 12 V and with a load current of tens, hundreds of milliamps at most; the other is powerful, for a voltage of 13,8 V with a maximum current of 5 ... 10 A. The first is needed to test various devices on breadboards and in other cases when the current consumption is low and it simply does not make sense to "drive" a powerful source for a long time. The second is necessary to power powerful amplifiers, CB equipment, amateur radio stations, car radios, etc. It can also successfully serve to charge car batteries if it has a maximum current limiting unit. The voltage of 13,8 V, which has already become standard, just corresponds to the voltage in the car's on-board network when the generator is running and the battery is charging.

In any end-of-life tube or tube-solid-state TV, you will find transformers, and other parts for both low-power and high-power power supplies. A low-power 12 V unit can, for example, be assembled using a ready-made vertical scan output transformer (TVK) from a tube TV. In some cases, the output transformer of a tube audio frequency amplifier (TVZ) is also suitable, but the effective (effective) voltage on its secondary winding will be about 6 V, while the rectified one will not exceed 9 V.

How to assemble a power supply has been repeatedly described in amateur radio literature, and it is not worth repeating here. Let us dwell only on some little-known, but important points. They apply to any homemade device.

First of all, it is necessary to determine the suitability of the transformer for the power supply, and for this it is necessary to measure the no-load current of the primary winding and the voltage on the secondary. You will need an avometer, a 220 V table lamp with a power of 25 ... 40 W and a 12 V car lamp with a power of 1 ... 5 W to check the output voltage under load.

On a clean desktop with a good dielectric coating (dry plywood, getinaks, plastic), a chain of series-connected table lamp, an avometer set to an alternating current measurement limit of at least 0,5 A and the primary winding of the tested transformer is assembled. The terminals of the secondary winding (or windings) of the transformer remain free. The lamp here performs a protective function: if you make a gross mistake by connecting a low-voltage secondary winding instead of the primary, if there is a short circuit in the transformer winding (or windings), etc., nothing bad will happen - when turned on, the lamp will glow, and the avometer will only show the current it consumes. Instead of a lamp, you can use a powerful (for example, wire) resistor with a resistance of 1 ... 1,5 kOhm. If the no-load current is normal, the next time you turn on the lamp or resistor, you no longer need to use it.

When working, you must strictly follow the safety rules: make all connections without connecting the circuit to the network, insulate them with PVC pipes, equip the circuit with a power cord with a plug, and only then, with your left hand behind your back or in your pocket and holding the plug in your right hand, connect it to the outlet, look at the reading of the avometer and disconnect the circuit.

The no-load current should be no more than 20 ... 30 mA for a low-power transformer (you may have to switch the avometer to a lower limit by disconnecting the circuit under test from the network) and no more than 100 ... 150 mA for a powerful one. A larger current indicates that the number of turns of the primary winding is small and, therefore, the magnetic induction in the magnetic circuit is too high. Such transformers "hum", heat up and have a strong stray field that creates electromagnetic pickups on other equipment (see, for example, V. Polyakov's article "Reducing the stray field of a transformer" in Radio, 1983, No. 7, pp. 28, 29 ). In some cases, if there is a free secondary winding of one and a half to two dozen volts, you can turn it on in series with the primary and get quite decent from an unusable transformer - it turns out that the number of turns must be increased quite a bit in order to significantly reduce the no-load current.

The no-load current also depends on the assembly of the magnetic circuit - the denser its parts or plates are adjacent to each other, the better. In one of the experiments, the no-load current of the TVZ-1-9 transformer was 40 mA. Its W-shaped magnetic core is assembled end-to-end with a small gap (in the audio frequency amplifier of the TV, a constant biasing anode current of the lamp passes through the primary winding, so the gap is necessary so that the magnetic circuit does not become magnetized to saturation). In transformers operating without magnetization, a gap is not needed, so the magnetic circuit had to be disassembled and reassembled "overlapping" when the W-shaped plate contactors are located on one side or the other. As a result, the no-load current decreased to 25 mA, and the "buzz" of the transformer became almost inaudible. After reworking, this transformer was perfect for a low-power 6 V power supply.

Consider now the issues of manufacturing powerful power supplies. For them, network transformers of lamp and lamp-semiconductor TVs, for example, TS-270 or TS-180, are suitable. Type decoding is simple: network transformer, the number indicates power. Its design is very convenient and easy to repeat: two coils are put on the sides of an O-shaped magnetic core, made up of two parts and fastened with ties. The primary (network) winding has two identical parts on two coils with three leads from each. The section between pins 1-2 is designed for 110 V, and between pins 2-3 - for 17 V. A mains switch is probably not needed, because there are practically few networks with a voltage of 127 V, but the presence of 127-volt windings is very useful. By connecting them in series (Fig. 1), we get a transformer operating in light mode, without saturation of the magnetic circuit and with an open circuit current of only about 50 mA. Such a transformer can work for days. If you need to force it for a while, turn off pins 3 and 3' and connect pins 2 and 3' (3 and 2') or even 2 and 2' because this mode is considered normal on a TV! The rectifier output voltage or charging current will increase.

Using a TV transformer

Among the secondary windings of these transformers there are several, designed for a voltage of 40 ... 60 V and a relatively small current. They are useless for a charger, but filament windings for a voltage of 6,3 V and a current of 4,7 A will do. If the transformer has three such windings, they must be connected in series and connected to a bridge rectifier on powerful (ten-ampere) semiconductor diodes (Fig. 1). A 12 V car lamp with a power of 50 to 150 watts can successfully serve as a charging current limiter.

To obtain the desired power, several lamps are connected in parallel. With a normal charging current, the lamps barely glow, you can judge the charging current by their glow, and the voltage drop across them is small. The same limiter protects the device from shorting the output or from connecting the battery in reverse polarity - while the lamps glow brightly (and with reverse polarity, the batteries most often burn out). If you put the lamps on 26 V and even more power, the "fool protection" will be complete - the lamps will not fail even when the battery is connected back to the device plugged into the network.

The situation will turn out to be somewhat worse when there will be only two filament windings for a voltage of 6,3 V and a current of 4,7 A, as, for example, in the TS-180-2 transformer. When they are connected in series, we get only 13 V. There is no time for a charging current limiter - it is barely enough even with a direct connection of the battery to the output of the rectifier bridge. It is advisable to assemble the bridge not on silicon, but on germanium diodes, for example, D305. They have less forward voltage drop (0,3 V instead of 0,7 V), so the charging current will be greater. It can be brought up to 5 A by forcing the primary winding mode as the battery charges. But nevertheless, the power of the transformer in this case is used only by a third. In order to make a charger with a current of 10 ... 15 A on this transformer (and such a current is quite acceptable at the beginning of charging batteries with a capacity of 40 ... 50 Ah), it is necessary to wind a new secondary winding. It is not so difficult.

Many are stopped by the lack of a large diameter wire for the secondary winding. Indeed, a thick wire is needed for a large current (see table).

Using a TV transformer

But you can successfully do with what you have, using winding in several wires. If you wind a push-pull winding for a rectifier according to the scheme of Fig. 2 into three wires and connect two such windings placed on two transformer coils in parallel, the required wire diameter for a 15-amp device will be only 0,8 mm. To speed up the work, both halves of the winding on each coil must be wound in six wires. The number of turns of the secondary winding is 2x46.

Using a TV transformer

The technology here is as follows: having freed the coils from all windings, except for the primary one with its external insulation, they wind 46 test turns to find out the length of the wire, and measure six pieces of the required length. Having soldered the conclusions of three wires to the petals of the frame, they wind the winding, making sure that the wires do not overlap. At the transition to the second layer, cable paper insulation is laid. The ends of the wires, again three each, are soldered to the other two petals of the frame, then they check with an ohmmeter if the wires are mixed up. If everything is done correctly, then there will be little resistance only between pins 6 and 8, as well as 5 and 7.

Having assembled the transformer and connected the middle terminals of the windings on two coils to a common wire, it is necessary to determine which extreme terminals to connect together. For this, a transformer is connected to the network and an alternating current voltmeter (avometer) measures the voltage between the extreme terminals of the windings on different coils. Connect together those between which the voltage is zero, after which they are connected to the anodes of the diodes. An incorrect connection will result in a short circuit. Be guided by the numbering of conclusions in fig. 2 is necessary with caution, because it is not known in which direction you wound the turns, and the phase of the voltage depends on this.

In conclusion, a few words about the fight against interference coming from the network. When the transformer is made only for the charger used in the garage, the problem of interference does not bother you, and the thin foil shields located between the primary and secondary windings can be removed. If, however, radio receiving equipment is connected to a working device, it is better to leave the screens, and connect their outputs (4 and 4 ') to a common wire. Capacitor C1 filters high-frequency noise induced from the network. For additional protection, capacitors C2 and C3 are used, shunting the secondary winding at high frequency. Their capacitance can be in the range from 0,01 to 0,5 microfarads. Paper capacitors are not suitable here due to the noticeable inductance of the leads, it is better to use ceramic ones.

The described charger is even suitable for powering a shortwave radio station with a power of 100 W, consuming up to 20 A at a voltage of 13,6 V. In this case, the car battery is not disconnected, it acts as a buffer battery. The connection diagram is shown in fig. 3.

Using a TV transformer

In no case should the radio station and the battery (GB1) be connected to the charger rectifier with separate wires, since the ripple of the supply voltage will increase due to the finite resistance of the wires. With the recommended inclusion, a smoothing oxide capacitor is not even required. If you still want to install it, you need to turn it on as close as possible to the power connector of the radio station.

Author: V.Polyakov, Moscow

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