More about power supplies with a quenching capacitor. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The use of capacitors to step down the voltage supplied to a load from a lighting network has a long history. In the 50s, radio amateurs widely used capacitors in transformerless power supplies for radio receivers, which were connected in series to the filament circuit of radio tubes. This made it possible to eliminate the quenching resistor, which is the source of heating for the entire structure. Recently, there has been a return of interest in power supplies with a quenching capacitor; in recent publications [1, 2], options for such structures and their calculation are considered in detail.

The disadvantage inherent in all such devices without exception - increased danger due to the galvanic connection of the output with the electrical network - is clearly recognized, but allowed based on the literacy and accuracy of the user. However, these deterrents are not sufficient to keep out of trouble, which is why transformerless devices can have only very limited use.

The author tried to approach the issue from a slightly different perspective. Let's ask ourselves a question: will a radio amateur take risks by building a source according to one of the mentioned schemes, if it is possible to use a ready-made, especially a small-sized transformer? Hardly. He will most likely make such a decision, not having such a product and giving in to independent production. It is not difficult to understand this: after all, for winding 5 ... 6 thousand turns of ultra-thin (0,05 mm) wire, one cannot do without a winding machine with a counter and appropriate skills.

Here, a compromise source that provides electrical safety, with a quenching capacitor and a simple transformer accessible to a beginner radio amateur, may be of interest. Such a transformer will turn out if the voltage on its primary winding is limited to a value of about 30 V. For this, 600 ... 650 turns of a relatively thick wire, convenient for winding, are enough; for the sake of simplicity, you can use the same wire for both windings. The excess voltage here will be taken over by a capacitor connected in series with the primary winding (the capacitor must be designed for a rated voltage of at least 400 V). According to this principle, it is advisable to organize the power supply of low-voltage loads with a current in the primary circuit (taking into account a small transformation ratio) up to 0,5 A.

On fig. 1 shows a diagram of such a device, suitable for working with a LED garland of a desktop mini-Christmas tree or for an audio player. The inclusion of LEDs (8-10 pieces) is carried out in parallel (Fig. 2); this eliminates the usual confusion of wires, it is easier to make them invisible in the "needles" of the trunk and twigs. The transformer can be assembled on a magnetic circuit W 12x15. For winding, a PEV-1 wire with a diameter of 0,16 mm is suitable; the number of turns of the primary and secondary windings is 600 and 120 ... 140, respectively. It will be possible to make such a transformer, as they say, "on the knee". An electrical strength of at least 2 kV will be provided by an insulating gasket between the windings made of 0,1 mm thick lavsan film or capacitor paper.

In order for the device not to fail when the load is disconnected [1], a D1G zener diode should be connected to the output of the VD4-VD815 bridge. In normal mode, it does not work, because it has a minimum stabilization voltage higher than the working one at the output of the bridge. Fuse FU1 protects the transformer and stabilizer in the event of a breakdown of capacitor C1.

From the editor. To limit the current when the power supply is connected to the network in series with capacitor C1, it is necessary to turn on a resistor with a resistance of several hundred ohms, and to discharge the capacitor after disconnection, a resistor with a resistance of several hundred kOhms is connected in parallel with it.

In a circuit of series-connected capacitive resistance (capacitor C 1) and inductive (transformer T1), voltage resonance may occur. This should be remembered when designing and establishing such power supplies.

Literature:

1. Biryukov S. Calculation of a network power supply with a quenching capacitor. - Radio. 1997, No. 5, p. 48-50.
2. Khovaiko O. Power sources with capacitor voltage dividers. - Radio, 1997, No. 11, pp. 56,57.

Author: Yu. Prokoptsev, Moscow; Publication: N. Bolshakov, rf.atnn.ru

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