Antenna amplifier power supply. Encyclopedia of radio electronics and electrical engineering

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Often, to power an antenna amplifier, a stabilized voltage source of 9 ... 12 V with a maximum load current of 20 mA is required. You can, of course, use the power supply of the TV amplifier, but this is not always convenient. Therefore, you may need an independent power supply. And since it must provide reliable galvanic isolation from the network, it is unacceptable to use a simple transformerless unit with a quenching capacitor or resistor. It is sometimes difficult to manufacture or select the necessary step-down transformer. The way out of this situation can be the use of a pulsed low-power power supply with an isolation transformer on a ferrite ring magnetic circuit.

A diagram of such an antenna amplifier power supply is shown in fig. 1. It contains a pulse signal generator on a powerful DA1 operational amplifier, which is powered by a VD1-VD4 rectifier. Capacitor C1 dampens the excess voltage of the network, and capacitors C3 and C4 smooth out the ripple of the rectified voltage.

The output current of the K157UD1 operational amplifier reaches 300 mA, so the generator assembled on it according to the multivibrator circuit is loaded directly on the primary winding of the T1 transformer. Generation frequency - 25...30 kHz. The impulse voltage that occurs in the secondary winding of the transformer is rectified by diodes VD6, VD7, and the rectified voltage is smoothed by the C5L1C6 filter. Zener diode VD8 stabilizes the output voltage of the power supply.

Fig. 1

The VD5 zener diode is not directly involved in the operation of the device - it only protects the operational amplifier and other elements from unacceptably high voltage in the event of a generation failure or other malfunction.

The block is connected to the cable for reducing the television antenna through decoupling chokes L2, L3.

Most of the mains voltage - about 90% - is extinguished by capacitor C1. So it turns out that the generator is powered by a current source with an internal resistance of Rc1? 6,2 kOhm, the current value through which cannot exceed 30 ... 33 mA. That is why the VD8 zener diode in the secondary winding circuit of the transformer T1 is connected to the rectifier directly, without a quenching resistor (the active resistance of the inductor L1 is not taken into account - it is small), which will not cause an overload of the generator. This is explained by the fact that with an increase in the current in the secondary winding, the current consumed by the generator also increases. And since this current is limited by the capacitance of the capacitor C1, the supply voltage of the generator decreases accordingly, the output voltage decreases, and hence the consumed current. Therefore, the VD8 zener diode at the output of the rectifier will be fed with a relatively stable current.

This achieves a sufficiently high stabilization coefficient:

Kst~(0,7…0,8)RC1/Rg,

where Rg, - dynamic, resistance of the zener diode VD8. At Rg = 15 Ohm Kst? 300, which is quite enough to power an amplifier for this purpose. Without an amplifier, the current flowing through the VD8 zener diode does not exceed 25 mA, and with an amplifier it decreases by the value of the current consumed by the amplifier.

All parts of the block, except for capacitor C1 and chokes L2, L3, are placed and mounted on a printed circuit board (Fig. 2). Inductors L2 and L3 are connected in a hinged way between the board and the connector for connecting the amplifier, and the capacitor C1 is mounted on a separate board.

Diodes VD1-VD4 can be KD105B-KD105G, D226B or rectifier block KTs402A-KTs402G, KTs404A-KTs404G, and VD6-VD7 - D219A, D310, D311, D312, KD510A, KD521A - KD521G. The VD5 zener diode can be made up of several series-connected zener diodes with a total stabilization voltage of 30 ... 35 V. The VD8 zener diode - for a stabilization voltage of 9 ... 10,5 V and with a maximum stabilization current of up to 25 mA.

It is desirable that the capacitor C1 (with a capacity of 0,47 ... 0,56 μF) is specially designed for operation on alternating current, for example, MBGO, K42-19, K78-4, K70-7, MPGO for a rated voltage of at least 300 V. It can be made up of two MBM capacitors connected in parallel with a capacity of 0,25 microfarads for a nominal voltage of 500 V or connected in series with a capacity of 1 microfarad for a voltage of 160 V. Capacitor C2 - KLS, KM, KD, and C3-C6 -K50- 6, K50-24.

Transformer T1 and inductor L1 are wound on ring magnetic cores of size K20x12x6 mm made of ferrite 2000NM. Winding I of the transformer contains 35 turns, winding II - 40x2 turns of wire PEV-2 0,2, and choke L1 - 145 ... 150 turns of the same wire. Inductors L2 and L3 type DM with inductance 100...500 μH.

The electrical safety of the unit depends on the careful manufacture of the transformer, therefore, despite its simplicity, it requires special attention. First of all, with a needle file it is necessary to round the sharp edges of the ring and wrap it with two layers of varnished cloth or insulating tape. The wire is wound in such a way that the windings are located on opposite sides of the ring and the distance between them is at least 5 mm. The windings are wrapped with insulating tape on top.

The K157UD1 microcircuit can be replaced with a medium-speed operational amplifier, for example, K153UD2, with an additional transistor output stage, as shown in the diagram in Fig. 3.

In a pulse generator, you can also use the K174UN7 chip by turning it on according to the circuit shown in fig. 4. But then it will be necessary to halve the number of turns of the primary winding of the transformer, double the capacitance of the capacitor C1 and apply the VD5 zener diode to a stabilization voltage of 15 ... 17 V.

Fig. 2

Fig. 3

Fig. 4

Author: I. Nechaev, Kursk; Publication: cxem.net

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