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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Low-voltage voltage stabilizers on the chip KR142EN19
Encyclopedia of radio electronics and electrical engineering / Surge Protectors Despite the fact that microcircuits of low-voltage (3 ... 5 V) voltage regulators with a low drop (low drop) have now appeared, for example, the LP29xx series from National Semiconductor, they are still not very common, especially among radio amateurs. But low-voltage stabilizers are now becoming of particular relevance. Almost all audio players are powered by 3V, many modern radio chips also require this voltage, not to mention microprocessors. The devices brought to the attention of readers are an attempt to make such low-voltage stabilizers using affordable and inexpensive elements. The circuitry of voltage stabilizers for powering devices with low-voltage power supply has its own peculiarities. For example, the simplest protection of stabilizers is most effective by limiting the maximum load current at a low output voltage. The voltage drop across the regulating transistor of the stabilizer when the output is closed differs little from the working one, and the transistor overheats slightly. It is very important for low-voltage stabilizers to reduce the minimum voltage between the input and output, since this increases not only the efficiency of the equipment, but also its reliability. For example, if a microcircuit is used in a three-volt stabilizer with a voltage drop across it also of three volts, then the rectifier supplying this device should supply voltage, taking into account ripples of about 9 V. If this voltage, due to a breakdown of the microcircuit, hits the load, it is very likely that it will come out out of service. For a stabilizer with a voltage drop of less than 0,4 V, an input voltage of about 5 V is enough. A three-volt load is likely to withstand such a voltage. Until recently, there was a problem - to choose a source of exemplary voltage for a low-voltage stabilizer - a zener diode. Usually low-voltage zener diodes have very low parameters. The development of relatively simple low-voltage stabilizers, taking into account all of the above, allows the KR142EN19 microcircuit - an integral analogue of a low-voltage zener diode (Yanushenko E. KR142EN19 microcircuit. - Radio, 1994, No. 4, p. 45,46). This microcircuit is produced in a plastic case with three terminals: an anode (3), a cathode (2) and a control electrode (1). When the voltage on its control electrode relative to the cathode is less than +2,5 V, the anode current of the microcircuit does not exceed 1,2 mA, and it depends little on the voltage between the anode and cathode of the microcircuit. As soon as the voltage at the control electrode exceeds the threshold of +2,5 V, the anode current of the microcircuit increases sharply until the voltage at the anode drops to 2,5 V. The resistor connected to the anode must limit this current to no more than 100 mA. The control electrode current is very small - a few microamperes, and this current should also be limited, since if it is increased too much, the voltage at the anode of the microcircuit may increase. The circuit of a low-voltage voltage regulator on a KR142EN19 microcircuit with a regulating transistor in the positive conductor is shown in fig. 1. The voltage drop on it does not exceed 0,4 V, and the stabilization coefficient is more than 600.
When the voltage on the output voltage regulator engine (resistor R7) rises to 2,5 V, the DA1 chip opens, which causes the transistor VT1 to open, the transistor VT2 to close, and then the regulating transistor VT3 to close. With the voltage regulator R7, you can set the output voltage less than the 3 V indicated in the circuit up to about 2,6 V, however, during the process of turning on the stabilizer, especially without load, a short-term increase in the output voltage to 3 V is possible. This stabilizer can also be adjusted to a voltage greater than 5 V, but then it will overheat greatly when shorted to the load, since it is protected only by limiting the output current, which depends on the resistance of the resistor R2. The maximum operating current increases with a decrease in its rating. If you need to significantly increase the output current of the stabilizer, you can try to reduce the values of the resistors R1 and R2 by the same number of times and use more powerful transistors. In place of VT1, it is permissible to use a transistor of the KT626 series, and VT2 - KT630. We can replace the KT814A (VT3) transistor with any of the KT816, KT837 series with the maximum base current transfer coefficient. The stabilizer should not use emitter followers to increase the output current. This increases the time it takes for the signal to travel through the feedback loop and may result in oscillating. If, nevertheless, generation occurs, it should be tried to eliminate it by increasing the capacitance of capacitors C1 and C2, as well as connecting a capacitor with a capacity of several hundred picofarads between the anode and the control electrode of the microcircuit. A variant of a stabilizer with a regulating transistor in the negative conductor is shown in fig. 2.
When the voltage at the control electrode rises to +2,5 V relative to the cathode, the microcircuit opens and closes the transistors VT1 and VT2. The maximum operating current is set by selecting the resistor R2. In the described devices, somewhat unusual output voltage dividers are used, in contrast to the traditional one, when a variable resistor is included in the upper arm according to the circuit. In this case, if the contact in the circuit of the variable resistor engine is broken, the voltage at the output of the stabilizers can only decrease, while when using a traditional divider, the output voltage reaches its maximum level, which can damage the load. In both stabilizers described above, in order to reduce the dependence of the maximum operating current on temperature, it is useful to provide thermal contact between the diodes VD1, VD2 and the heat sink of the regulating transistor. If such stabilizers are used as adjustable, it is useful to include constants in series with variable resistors (to each extreme terminal). Their resistances should be selected so that the limits for adjusting the output voltage correspond to those indicated in the diagrams. In the absence of such resistors, the stabilizers can exit the stabilization mode in the extreme positions of the engines. Author: S.Kanygin, Kharkov, Ukraine
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