Improvement of the parametric voltage stabilizer. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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Parametric voltage regulators on zener diodes are widely used by radio amateurs, especially beginners, in the development and construction of various electronic devices, although integrated voltage regulators are gradually replacing them. The diagram of a parametric voltage regulator is shown in fig. 1. It consists of a ballast resistor Rb and a zener diode VD1, in parallel with which the load Rn is connected.

The load and zener diode currents flow through the resistor Rb and its value must be such as to provide both the load current and the zener diode current within the working area. Moreover, this condition must be satisfied for all values ​​of the supply voltage. For this reason, the efficiency and stabilization factor of such a stabilizer is usually low. On fig. 2 graphically shows the dependence of the load voltage on the supply voltage (input) with the details indicated in the diagram and a change in the supply voltage from 6 to 9 V. Curve 1 was taken for the resistance Rb = 100 Ohm, and curve 2 - for Rb = 300 Ohm. In the first case, the current consumed by the stabilizer varies from 6 to 30 mA, but when the supply voltage is less than 7 V, the load voltage begins to decrease sharply. In the second case, the current varies from 5 to 11 mA, and the stabilization effect is very weak.

You can improve the parameters of the stabilizer if, instead of a resistor, you install a device that has the property of a current stabilizer, for example, a suitably connected field effect transistor (Fig. 3).

On fig. Curve 2 in Figure 3 shows the dependence of the load voltage on the input voltage when using a field-effect transistor with an initial drain current of 10 mA. In this case, the consumed current also varies from 6 to 10 mA, but the voltage stability at the load is noticeably higher. However, when the supply voltage is less than 7 V, the voltage at the load also decreases markedly. This is due to the fact that at a drain-source voltage of less than 1 ... 1,5 V, the stabilizing properties of the field-effect transistor deteriorate sharply. In addition, it is not very easy to select a field effect transistor with the required parameters.

If you replace the field-effect transistor with a bipolar one (Fig. 4), then you can improve the parameters of the stabilizer and simplify the setting of the required current by selecting the resistor R1.

The dependence of the voltage on the load for the resistance of the resistor R1 = 75 kOhm is shown in fig. 2 (curve 4). In this case, the consumed current, as in the case of a field-effect transistor, varies from 6 to 10 mA, but with a supply voltage of less than 7 V, the voltage at the load changes less. Reducing the resistance of the resistor R1 by about 1,5 times (up to 51 kOhm) will lead to the fact that the current consumption will vary from 9 to 16 mA, but the voltage stability at the load will be noticeably higher (curve 5).

Thus, the advantages of the stabilizer, made according to the scheme shown in Fig. 4, is, firstly, that the stabilizer starts working when the difference between the supply voltage and the stabilization voltage of the zener diode is from 0,2 ... 0,3 V, and secondly, that by selecting the resistor R1, you can set the required current through a stabilizer. In addition, by installing a capacitor with a capacity of 100.. .200 uF between the base and emitter of the transistor, you can significantly reduce the voltage ripple on the load. This is very important in mains power supplies. The disadvantages of the stabilizer include a noticeable effect of ambient temperature and supply voltage on the collector current.

If the base current of the transistor is made stable using a current stabilizer on a field-effect transistor (Fig. 5), then the voltage stability at the load will be even higher. The required collector current is set by the trimming resistor R2. However, the device in this case becomes noticeably more complicated, although it will be more economical, since the current consumed changes little when the supply voltage changes.

The described modifications of the parametric voltage regulator can be made to an already manufactured device by replacing (in accordance with Fig. 4) the ballast resistor with a transistor and installing the resistor R1 using the surface mounting method. For a stabilizer with a low-power zener diode, transistors of the KT208, KT209, KT361 series with any letter indices or similar ones, with a low collector-emitter saturation voltage, can be used. Fixed resistors are recommended to use MLH S2-33, trimmer - SP3. The adjustment comes down to setting the required current through the zener diode by the resistor R1.

Author: I. Nechaev, Kursk

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