Voltage stabilizer in the network adapter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Surge Protectors

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Most of the imported small-sized network power supplies, called adapters, do not include rectified voltage stabilizers. Therefore, they are characterized by a high level of ripple and low stability of the output voltage, which is not suitable for many household electrical and radio appliances. The way out of this situation, according to the author, may be to install a small-sized voltage stabilizer in the adapter. This will require a minimum of widely used parts.

A diagram of one of these devices is shown in Fig. 1. It is formed by two transistors and two resistors. The field-effect transistor VT1 performs the function of a current generator, and the bipolar composite transistor VT2, turned on by a follower follower, acts as a current amplifier. A relatively stable current flows through the resistor R1, therefore, by changing its resistance, the value of the output voltage (Uout) can also be adjusted from almost zero. Resistor R2 sets a small initial current necessary to avoid an increase in the output voltage when supplying a load that consumes a current of several milliamps.

The maximum voltage at the output of the stabilizer is determined by the approximate formula Uout (in volts) \u1d R1 IVT1,5 (A) - 1, where R1 is in kiloohms, and IVTXNUMX is the initial drain current of the field-effect transistor in milliamps.

For normal operation of the field effect transistor, it is necessary that the constant voltage on it be at least 3 V, approximately the same voltage should be for the normal operation of the VT2 bipolar transistor. This means that the voltage at the input of the stabilizer (UBX) must exceed the output adapter by at least 3 V. Using the same formula, you can determine the required nominal resistance of the variable resistor R1, which provides the required range of output voltage. The shaft of this resistor should be equipped with a handle - "Beak" and a graduated scale.

The stabilization coefficient of this version of the stabilizer is 50 ... 60 at an output current of 200 mA, the output resistance is approximately 0,5 Ohm.

The field-effect transistor VT1 is selected from the KP303, KP305 or KP307 series with an initial drain current of 5 ... 10 mA. The static current transfer coefficient of the base of the transistor VT2 is at least 1000, so it must be composite - KT829 with letter indices A - G, KT973A, KT973B, and if the adapter case allows, then KT827A - C. You can also use two bipolar ones by turning them on according to the scheme of a composite transistor: the first of them is a low-power one from the KT315, KT312, KT3102 series, and the second is a powerful one from the KT815, KT817 series.

More practical may be a stabilizer with switchable fixed values ​​​​of the output voltage, made, for example, according to the circuit in Fig. 2. In it, a variable resistor (R1 according to the diagram in Fig. 1) is replaced by a chain of fixed resistors R1 - R6, switched by switch SA1. By selecting each of these resistors, starting with resistor R1, the desired values ​​​​of the output voltages are set.

When using adapters with such stabilizers, it should be borne in mind that with a constant input voltage, for example 12 V, and with a low output voltage, for example 3 V, a significant part of the useful power is allocated to the transistor VT2. This leads not only to additional heating of the transistor, but to a decrease in the efficiency of the entire power supply. This can be avoided by simultaneously switching the output stabilized voltage (as in the stabilizer according to the circuit in Fig. 2) and the voltage of the secondary winding of the network transformer, as shown in the diagram shown in Fig. 3. This will require a two-section switch for several positions, for example, PD - 41, and, of course, a thorough study of the installation of the power supply. And if it turns out to be prone to self-excitation, then a ceramic capacitor with a capacity of 7 ... 0,01 μF will have to be connected in parallel with the output resistor R0,1 - it will eliminate this unpleasant phenomenon.

To reduce the ripple of the output voltage, it is useful to connect the base of the transistor VT2 (Fig. 1 and 2) with a common wire through an oxide capacitor with a capacity of at least 47 microfarads for a voltage of 16 V, and the conclusions of the VD1 bridge connected to the transformer with a ceramic capacitor with a capacity of at least 0,01, XNUMXuF.

For a voltmeter, two sub-ranges are quite enough: 0 ... 1 and 0 ... 15 V (or 0 ... 20 V), and the first of them can generally be assigned only for measuring the angle of the ZSK.

The described device provides the setting of the angle of the ZSK of the breaker of the battery ignition system with the required accuracy, which was verified by the author in practice. If the car is equipped with an electronic ignition unit, it is necessary to temporarily return to the battery system to set the angle.

Literature

1. Zatulovsky M. Device of a motorist. - Radio, 1981, No. 2, p. 21, 22.
2. Khukhtikov N. A simple device for a car enthusiast. - Radio, 1994, No. 2, p. 34, 35.

Author: I. Nechaev, Kursk

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