Theory: voltage regulators. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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With changes in the mains voltage and load current, the output voltage of the rectifier also changes, and sometimes significantly. In some cases (for example, when powering the UMZCH terminal stage), this is quite acceptable, but, say, for radio receivers, generators and other electronic devices, the voltage should be stable when the load current changes. Here you can not do without a stabilizer. At the same time, this device performs another function - it reduces the ripple of the supply voltage to a minimum.

The basis of the simplest stabilizer (Fig. 68, a) is a chain of resistor R1 and a zener diode VD1. A zener diode is a special diode connected in reverse polarity and operating in the avalanche reversible breakdown mode. If you increase the reverse voltage on the zener diode, then at first the current will be small, and when the stabilization voltage is reached (this is indicated in the reference data), it will increase sharply. To limit the increase in current through the zener diode, it is connected through the resistor R1 (this is the so-called ballast resistor). The current through the zener diode is calculated using the formula I \uXNUMXd (Uin - UCT) / R. Thus, the input voltage must always be greater than the output, stabilized.

When powering low-power devices, such a simple stabilizer is often dispensed with, removing the output voltage from the zener diode. When calculating according to this formula, the current I must include both the zener diode current (usually 5 ... 20 mA) and the load current (of the same order).

With a higher load current, an additional transistor VT1 is used, which is connected as an emitter follower (Fig. 68, b). It "repeats" the stabilized base voltage on the load. The output voltage Uct is approximately 0,7 V (voltage drop at the base-emitter junction) less than the nameplate stabilization voltage of the zener diode. At high load currents, a composite transistor is used.

A diagram of a more advanced stabilizer is shown in fig. 69.

The zener diode VD1 is selected for a voltage approximately equal to half the output stabilized Ust. The same voltage is also applied to the base of the low-power control transistor VT2 from the voltage divider R2 - R4. If for some reason the output voltage drops, this change will be completely transmitted through the zener diode to the emitter of the transistor VT2, while at its base the voltage change will be less. As a result, the transistor will open slightly and its increased collector current will open a powerful regulating transistor VT1, compensating for the drop in output voltage. When the output voltage rises, both transistors turn off. Regulation is thus due to strong negative feedback.

Since the control signal is generated from an already stabilized output voltage, the stabilizer parameters with a simple circuit are quite high. An additional advantage of the stabilizer is that it is not afraid of short circuits at the output - during a short circuit, the control voltage also disappears, as a result of which both transistors close. The protection trip current depends mainly on the current of the zener diode, which is selected by the resistor R1.

There are many designs of voltage stabilizers, but they all have a significant drawback - the input voltage must be higher than the output stabilized, at the same current, as a result, part of the rectifier power turns into heat and is dissipated on the heat sink of the regulating transistor. This disadvantage is eliminated in switching regulators with high efficiency.

Author: V.Polyakov

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