Voltage converter for radio-controlled model. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Radio control

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On-board power supplies for radio-controlled models, as a rule, have a nominal voltage of 4,5 ... 12 V. High-quality electric motors for such a voltage are quite rare and at a considerable price. At the same time, the range of available electric motors for 24 ... 27 V is quite wide, but they require a voltage converter similar to that proposed by the author of the article.

A significant advantage of using electric motors for increased voltage is the reduced current consumption, which facilitates the requirements for transistors of the output stages of servo drives of steering machines and speed controllers. The efficiency of engine control units is increased, which saves the limited energy resources available on board the model.

The developed voltage converter allows the use of electric motors with a rated voltage of 24...27 V in conjunction with radio control equipment [1]. For steering machines of models, for example, engines of the DPR series with a hollow rotor, which have low inertia when starting off and reversing, are well suited. The servo amplifiers of the travel controller and the steering machine must be built in accordance with the recommendations given in [2]. As a stand-alone device, this voltage converter can be used for other purposes.

The scheme of the device is shown in fig. 1. This is the so-called pulse-width stabilized flyback inverter with high efficiency. With an input voltage of 4,5 ... 9 V, the stabilized output voltage can be set to any within 18 ... 27 V, changing by no more than 0,1 V with an increase in load current from 1 to 500 mA. Converter efficiency with full load - 85%.

(click to enlarge)

Voltage diagrams at the characteristic points of the circuit, shown in fig. 2 were obtained on a computer model of the device using the Micro-Cap 6.22 program and completely coincide with the waveforms of signals in a real converter.

The master oscillator on the elements DD1.1 and DD1.2 generates rectangular pulses. At the inputs 8, 9 of the element DD1.3 they come differentiated circuit C3R2R3. The values ​​of the resistors R2 and R3 are chosen in such a way that the constant component of the voltage at the point of their connection slightly exceeds the threshold level Un, at which the element DD1.3 changes its state. Negative emissions, crossing the threshold, form short positive pulses at the output of element DD1.3 (pin 10). The latter charge the capacitor C5 through a small direct resistance of the base-emitter section of the transistor VT2.

At the end of the pulse, the left (according to the diagram) plate of the capacitor C5 is connected to a common wire, and the voltage to which the capacitor is charged is applied to the base of the transistor VT2 in negative polarity, closing it. Next, the recharging of the capacitor C5 by the collector current of the transistor VT1 begins. The speed of this process depends on the voltage at the base of VT1. Transistor VT2 remains closed until the voltage at its base reaches approximately 0,8 V. As a result, the duration of positive pulses at the collector VT2 and inputs 12, 13 of the DD1.4 element depends on the operating mode of the transistor VT1. Twice inverted by the element DD1.4 and the transistor VT3, the pulses open the power key - the field effect transistor VT4.

When the transistor VT4 is open, the current in the inductor L1 increases linearly. After closing the transistor, this current is not interrupted, continues to flow, falling, through the diode VD1 and charges the storage capacitor C8. The steady-state voltage on this capacitor exceeds the supply voltage as many times as the time of energy accumulation in the magnetic field of the coil L1 (the duration of positive pulses at the gate of the transistor VT4, see Fig. 2) exceeds the time it is transferred to the capacitor C8 (the duration of the pauses between pulses there same).

Part of the output voltage from the trimmer resistor R14 is fed to the inverting input of the DC amplifier on the op-amp DA2. An exemplary voltage is applied to its non-inverting input from a resistive divider R4R5. The output voltage of the op-amp, proportional to the difference between the reference and output (taking into account the divider R13R14) voltage, is fed to the base of the transistor VT1 and controls the duration of the pulses that open the transistor VT4. Thus, a closed circuit of automatic control is formed.

If the output voltage has decreased (for example, as a result of an increase in the load current), the voltage at the inverting input of the op-amp will decrease, and at its output it will increase. As a result, the emitter current of the transistor VT1 will drop, flowing through the resistor R8, and with it the collector current. Capacitor C5 will recharge more slowly. The duration of the open state of the transistor VT4 will increase, the output voltage of the converter will increase.

The supply voltage of the main components of the converter is stabilized by the integral stabilizer DA1.

The device is assembled on a single-sided printed circuit board with dimensions 70x55 mm, shown in fig. 3. Trimmer resistor R14 - SPZ-38B or RP1-63M. The remaining passive elements are of any type, suitable in terms of parameters and dimensions.

(click to enlarge)

As a DD1 microcircuit, except for K561LA7, you can use K561TL1, other microcircuits of the K561 series at a supply voltage of 3 V are unstable. For the same reason, you should not replace the K140UD608 (DA2) chip with other op-amps. Transistors VT2, VT3 can be any series KT315 or KT3102, aVT1 - series KT361, KT3107.

The efficiency of the converter significantly depends on the voltage drops across the diode VD1 and on the open transistor VT4. The latter is proportional to the open transistor channel resistance given in the reference books. Therefore, when choosing replacements for the indicated transistor and diode, one should pay special attention to these parameters, choosing devices for which they are minimal. The cutoff voltage of the field-effect transistor should be no more than 4 V. The amplitude value of the current switched by it in this case is much greater than the load current, so the transistor should be selected with a permissible drain current of at least 6 A. If the VT4 transistor heats up noticeably under load, it must be equipped with a heat sink, a place for which the board provides. Diode VD1 must be designed for a direct current of at least 10 A. The KD2998V indicated in the diagram can be replaced with KD213A.

Coil L1 with an inductance of 18 ... 20 μH should have a small leakage magnetic flux, therefore, an armored B-26 magnetic circuit made of M1500NM ferrite was chosen for it. A winding of five turns of rigid insulated wire with a diameter of 1,5 ... 2 mm is wound on a mandrel of a suitable diameter, removed from the mandrel, protected with a layer of insulating tape and placed in a magnetic circuit. A non-magnetic gap of 0,2 mm is required between its cups. An insulating gasket of appropriate thickness is placed between the central cores. This prevents the cups from breaking when the magnetic circuit is tightened with a screw. To reduce the board area, the L1 coil is attached to it lying on its side. The winding leads are inserted into the corresponding holes and soldered to the pads.

Capacitors C7 and C9 are shown in the diagram (see Fig. 1) and the board drawing (Fig. 3) with dashed lines. Usually they are not necessary, but if the VT4 transistor gets very hot, and “spurious” positive pulses are visible on the voltage waveform at its gate in the intervals between the main ones, installing these capacitors can help. Their capacity is selected empirically.

When starting to check the assembled converter, it should be borne in mind that with an output voltage of 27 V and a load current of 0,5 A, the primary power supply with a voltage of 6 V must be rated for a current of at least 2,5 A. Before turning on the converter for the first time, the engine of the tuning resistor R14 should be in the middle position, then with its help set the required output voltage.

If the converter does not work, you should temporarily unsolder the L1 coil and, by applying a voltage of +27 V from an external source to the output circuit, ensure that the shape of the signals at the points indicated in Fig. 2 corresponded to that shown in this figure.

If necessary, the converter can be converted to another input and output voltage according to the method described in [3]. Initial data: minimum voltage of the primary source - Umin; output voltage - Uout; maximum load current - In.

The calculation is carried out in the following order:

1. Power delivered to the load,

2. The power consumed by the precision,

(It is assumed that the efficiency of the converter is at least 80%).

3. The average value of the current consumed from the source,

4. Coil current L1 (peak value),

5. We select a field-effect transistor VT4 with a permissible drain current of at least lm and a minimum open channel resistance rok.

6. We select the VD1 diode with a permissible forward current of at least lm and a minimum voltage drop Upr at this current.

7. Voltage drop across the open transistor VT4

8. The duration of the open state of the transistor VT4

(if the coil design is not changed, L1=20 µH).

9. The duration of the closed state of the transistor VT4

10. The pulse repetition period of the master oscillator

The calculated value of Tn is achieved by selecting the value of the resistor R1. Further, without installing coil L1 in the converter and leaving its circuit broken, the base of the transistor VT1 is temporarily disconnected from the output of the op-amp and connected to the engine of a variable resistor with a nominal value of 47 kOhm, one of the extreme terminals of which is connected to the output of the integral stabilizer DA1, and the other to a common wire . The newly introduced variable resistor sets the duration of positive pulses at the gate VT4 equal to t1. The voltage is measured at the base of the transistor VT1 and the same is set at the input 3 of the op-amp DA1, choosing the value of the resistor R5. Having restored all connections, the trimming resistor R14 achieves the desired voltage at the output of the converter.

Literature

1. Dnischenko V. Proportional radio control equipment. - Radio. 2001, no. 11, p. 24-26; No. 12, p. 31-33.
2. Dnischenko V. Proportional radio control equipment (returning to printed material). - Radio, 2002, No. 6, p. 31.
3. Shcherbina A. et al. Application of microcircuit stabilizers of series 142, K142.KR142. - Radio. 1991, no. 5, p. 68-70.

Author: V.Dnishchenko, Samara

See other articles Section Radio control.

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