General purpose power supplies are commonly referred to as "laboratory" power supplies. They must have a set of parameters that allow them to be used for a wide variety of operations. These are, as a rule, regulated circuits capable of delivering voltages over a fairly wide range of voltages and currents. In addition, they must ensure the safety of the devices connected to them, that is, have protection against short circuit, overload, and overheating.
Previously, such devices were assembled on transistors and operational amplifiers as master and regulating elements, therefore, they had a rather complex design and were not easy to manufacture and at the construction site. Currently, there are many specialized integrated circuits (ICs) containing in one package almost ready-made power supply-stabilizer with very high characteristics and protection for all major parameters.
Therefore, even novice radio amateurs or just people who know how to use a soldering iron can easily make a good laboratory power supply unit.
It is capable of outputting from zero to 30 volts of stabilized voltage at a current of 8 amperes. And when replacing power elements with others, the maximum voltage and current may be higher. The circuit has a smooth adjustment of the output voltage in the range of 0... 30 volts and protection against short circuit and overload at the output. It can be assembled both on domestic components and on their imported counterparts.
The circuit is based on the KR142EN12A type stabilizer microcircuit, it provides all the basic quality characteristics of the entire power supply and its protective functions. It can be replaced with an imported analogue of LM317 without any changes in the circuit (but when replacing be sure to check the pinout - the location of the terminals of each specific IC according to the technical description on her!).
With a normal, typical switching circuit, these microcircuits have a lower voltage regulation limit of the order of 1.2... 1.3 volts. In the circuit shown here, the inclusion is not quite usual, the output "1" of the IC is connected to the "common" wire not directly, but through the VD1 stabilizer and the variable resistor R4.
In addition, as can be seen from the diagram, a small negative bias voltage “minus” 5 volts is applied to this pin. When the resistance R4 is small, a negative voltage is supplied to pin "1" and "closes" the microcircuit. The voltage at the output of the power supply unit (PSU) is zero.
With an increase in the resistance R1, the stabilizer microcircuit gradually opens and the voltage at the PSU output rises to the maximum possible value. For parts shown here, this value is +30 volts.
If the load is low-power and the output current is not large, only the IC works in its normal mode. If the current in the load exceeds the maximum allowable for this microcircuit of 1.5 amperes, an additional stage on the transistors comes into operation and acts as a "key", passing the current through itself. In this case, the IC acts as a control element and continues to perform its main functions - stabilization of the output voltage and protection against short circuits and overloads.
The KS113A stabilizer, in fact, is a low voltage Zener diode of 1.3 volts. It, if necessary, can be replaced with a KS133 zener diode or a similar imported one (stabilization voltage 1... 3.9 volts). The variable resistor R4 can be set with a resistance of 2.2 to 4.7 kOhm.
The microcircuit and a powerful transistor KT819 (or similar imported) must be installed on heat sinks, effective the cooling surface of which must have an area sufficient to dissipate heat at the maximum load of the unit nutrition. They can be installed on one, common heat sink, but insulating heat-conducting gaskets should be used. Resistor power: R1, R5 - 1 W, R2 - 2 W, R3, R4 - 0.5 W.