adjustable transformer. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The proposed device is a network transformer with a large number of switchable secondary windings. Its output voltage can be adjusted with three switches from 0 to 399 V in 1 V steps. The position of the switches clearly indicates the nominal value of the output voltage.

A number of designs of adjustable transformers and autotransformers are described in amateur radio literature. The article [1] proposes an autotransformer based on a network transformer from TS-180 tube TVs, in which the output voltage is regulated by switching the windings.

In the book [2] us. 173, 174 tells about a laboratory autotransformer, which can also work as a transformer, when the output voltage is removed from the secondary windings that are not galvanically connected to the primary (mains). Its output voltage can be adjusted by switches within 1 ... 347 V in 1 V increments. In the 1 ... 127 V subrange, there is a galvanic isolation of the output from the mains. But in other subranges it is absent, which is a disadvantage of this device.

A very simple design of an adjustable network transformer with a large number of switched secondary windings is described in the article [3]. The advantage of this device is the galvanic isolation of the output from the network in the entire interval of regulation of the output voltage of 1.465 V with the same step. The output voltage is controlled by toggle switches, each of which corresponds to a certain voltage value. The output voltage is equal to the sum of the values ​​​​of all the included toggle switches.

A common disadvantage of these devices is the need to perform arithmetic operations to set the desired output voltage.

When developing the proposed device, the task was to simplify the setting of the output voltage so that its value was clearly determined by the position of three switches: one for hundreds of volts, the second for tens, and the third for units. It is this design that is offered to the attention of readers.

Main Specifications

• Rated input voltage, V .......220
• Output voltage, V ......0...399
• Output voltage adjustment step, V ....... 1
• Maximum output current, A, at an output voltage of 1 ... 99 V ....... 0,68
• 100...199 V.......0,34
• 200...399 V.......0,17
• Dimensions, mm.......120x170x105

Fig. 1

The scheme of the proposed device is shown in fig. 1. Its basis is a network transformer T1 with one primary (I) and twelve secondary windings (II-XIIl). Each secondary winding can be included or not included in the output circuit by the contacts of the corresponding relay, as in the device [3]. The relay windings K1-K12 are powered by one of the secondary windings (VII) of the transformer T1 through the diode bridge VD1-VD4. Diodes VD5-VD16 are connected to the relay windings in order to suppress self-induction EMF pulses when the current is interrupted through the windings. But the difference between the proposed device is that a number of secondary winding voltage values ​​​​are chosen differently, and SA2-SA4 switches and VD17-VD34 diode decoders are used to control the relay, which made it possible to obtain a clear indication of the output voltage value by the position of the switches.

The device works like this. When all switches SA2-SA4 are set to the "0" position, the windings of all relays are de-energized, all the secondary windings of the transformer T1 are disconnected from the output by their contacts. When switching SA4 to the "100" position, power is supplied to the coil of the relay K11, which, with its contacts K11.1, connects the XII coil with a voltage of 100 V to the output. Switching SA4 to position "200" similarly using relay K12 connects winding XIII with a voltage of 200 V to the output. In position "300" through diodes VD17 and VD18, power is supplied to the windings of both relays K11 and K12, the contacts of which include both windings in the output circuit XII and XIII. Since they are connected in this case in-phase-series, the output voltage is 300 V.

Switch SA3 sets the output voltage in increments of 10 V. In position "10", winding VII is connected to the output circuit through contacts K6.1. In positions "20"..."50" - one of the windings VIII-XI. In position "60" through the diodes VD23 and VD28, power is supplied to the windings of two relays K6 and K10, the contacts of which include the in-phase-series connected windings VII and XI with a total voltage of 60 V. In positions "70" ... 90 "The output circuit also includes two windings of the transformer T1 with the corresponding total voltage.

Switch SA2 similarly sets the output voltage in steps of 1 V. One or two of the windings I-VI of the transformer T1 are connected to the output circuit using the relay K1-K5. In the case when such a small step of regulating the output voltage is not required, the device can be significantly simplified without winding these windings and without installing SA2, relays K1-K5 and diodes VD5-VD9, VD19-VD22, VD27, VD29, VD31, VD33.

Fig. 2

Network transformer T1 - converted 080-481 55-01 GP0651 CLASS B VIKING B-2 from Mustek 600-USB uninterruptible power supply (Fig. 2). First, at the nominal mains voltage on its primary winding, the voltage values ​​​​on its secondary windings were measured. Then the magnetic circuit was disassembled, and the secondary windings were wound with counting the number of turns. As a result of dividing the number of turns by the voltage, the result is: 2,8 turns per volt. To compensate for the voltage drop under load, the number of turns of the secondary windings has been increased to three turns per volt. It is with this coefficient that new secondary windings are wound with PEV-2 wire. When winding them, it is desirable to mark the beginnings and ends to ensure correct phasing when connected.

The overall power of the applied transformer is about 84 VA. The number of turns and the diameter of the wire of the secondary windings are given in the table. The interlayer insulation of the windings is made with FUM fluoroplastic tape.

All 833H-1C-FC 12V relays can be replaced by others whose contacts are designed for switching voltages of at least 400 V and currents of at least 0,7 A. Supply voltage ripples do not affect the operation of the applied relays due to their inertia, but can cause " buzz". In the case of using other relays, as well as to suppress the "hum", it may be necessary to connect a smoothing capacitor to the output of the diode bridge VD1 - VD4, the capacitance of which is selected experimentally. No more than six relays are turned on at the same time, so the rectifier bridge diodes VD1-VD4 must withstand the current of the windings of six relays. The remaining diodes are one relay. Capacitor C1 - film from the K73-17 series.

Fig. 3

The appearance of the proposed device is shown in Fig. 3. It can be seen that with the switch positions of 392 V, the voltmeter shows 369 V. This happened due to the fact that at the time of photographing there was a low voltage in the network. And only at a nominal mains voltage of 220 V, the readings of the multimeter and switches will match. This feature is also characteristic of the structures mentioned above [1–3].

A properly installed device does not require adjustment. However, it is necessary to make sure that the secondary windings (II-XIII) of the T1 transformer are connected correctly. To do this, check the change in the output voltage when moving each switch from position "0" to the maximum with fixed positions of the remaining switches. The output voltage should increase by the voltage of the connected winding. Otherwise, the winding is connected incorrectly, its conclusions must be reversed.

The author used an adjustable transformer to establish mains voltage stabilizers (including setting the operation thresholds of their comparators), to determine the input voltage change interval of small-sized switching power supplies, and to select fluorescent lamps that operate at the lowest supply voltage. The device has been working flawlessly since the beginning of 2008.

Literature

1. Solonenko V. Autotransformer based on TS-180. - Radio, 2006, No. 5, p. 36.
2. Evseev A.N. Useful schemes for radio amateurs. - M.: SOLON-R, 1999.
3. Moroz K. Adjustable transformer based on LATR. - Radio, 2008, No. 8, p. 25, 26.

Author: S. Butrimenko

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