Economical voltage converter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Voltage converters, rectifiers, inverters
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The voltage converter that feeds the varicaps of the electronic tuning of the Leningrad-002 transistor receiver has a rather long (about 1,5 s) output voltage settling time, therefore, when the HF and VHF bands are turned on, specific interference occurs caused by the frequency tuning of the receiver. As experiments have shown, the main reason for the delay in establishing the output voltage is the use of a compensation voltage regulator that consumes a current of several milliamps, as well as a large capacitance of the filter capacitor.
Since a decrease in the capacitance of the capacitor is unacceptable due to an increase in ripple, it was decided to replace the converter with a stabilizer with a device in which the output voltage is maintained by a constant negative feedback (NFB) that controls the operation of the oscillator.
Schematic diagram of the new voltage converter is shown in the figure. The adjustable OOS circuit is formed by field-effect transistors VT3 (bias voltage regulator), VT4 (amplifier), VT5 (current generator). The device works as follows. At the time of power-up, when there is no voltage at the output of the converter, transistors VT4. VT5 are de-energized. After starting the generator on transistors VT1, VT2, a constant voltage appears at the output of the converter and a current flows through the circuit RЗVT5R4R5).
As the output voltage increases, it increases until it reaches a certain limit, depending on the resistance of the resistor R3.
A further increase in the output voltage of the converter is accompanied by an increase in the voltage at the source-gate section of the transistor VT4, and when it becomes greater than the cutoff voltage, the transistor VT4 opens. With an increase in the voltage across the resistor R2, the transistor VT3 begins to close and the bias voltage at the bases of the transistors VT1, VT2 decreases. As a result, the increase in the output voltage stops and it stabilizes.
When the battery is discharged or the load is increased, the output voltage of the converter decreases somewhat, but after this, the bias voltage of the oscillator transistors increases and the original value of the output voltage is restored. As the test showed, when the supply voltage is reduced from 4,5 to 1,5 V, the output voltage remains practically unchanged, and when it is increased to 10 V, it increases by only 0,2 V.
Since field-effect transistors operate in the microcurrent mode in the described device, and medium-frequency KT201V transistors are used in the oscillator, the current consumed by the converter was reduced from 32 to 5 mA. The output impedance of the converter is 160 Ohm (for the previous one - 5 kOhm). output voltage settling time 0.1 s.
For the manufacture of the converter, parts of the old device were partially used: a self-oscillator transformer, capacitors with a capacity of 100 and 5 microfarads, a 27 Ohm resistor and D223B diodes, as well as an aluminum screen, the self-oscillator oscillation shape is close to a meander, however, the rational arrangement of parts on the printed circuit board and shielding of the converter made it possible to almost completely eliminate interference.
Establishing the device is connected to checking the oscillator performance and setting the required output voltage, first by selecting the resistor R3 (roughly), and then the tuned resistor R4 (exactly).
This economical voltage converter for powering varicaps can be used in any other transistor receiver.
Author: V. Gridnev, city of Barvenkovo, Kharkov region; Publication: N. Bolshakov, rf.atnn.ru
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