Power supply with smooth polarity change, +12/-12 volts. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies
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The peculiarity of this power supply is that by turning the knob, you can not only change the output voltage, but also its polarity. In practice, the voltage is regulated from + 12 to -12 V. This is achieved due to the slightly unusual inclusion of bipolar power supply stabilizers, so that both regulators are regulated using a single variable resistor. The schematic diagram of the source is shown in fig. 2.25.
The rectifier is bipolar, made according to the standard scheme on a transformer T1 with a secondary winding tapped from the middle, a diode bridge VD1 and capacitors C1 and C2. As a result, a bipolar voltage is obtained at its output. This voltage is supplied to two stabilizers on transistors VT1 and VT3 (positive voltage regulation) and on transistors VT2 and VT4 (negative voltage regulation).
The difference from the standard bipolar circuit is that the outputs of the stabilizers are connected together, and that one common variable resistor R5 is used to regulate the voltage. Thus, if the engine of this resistor is installed exactly in the middle, and the voltage on it relative to the common wire is zero, then both stabilizers are closed, and the voltage at the output of the circuit is also zero. If the engine began to move towards positive voltages (up in the circuit), the positive voltage regulator on transistors VT1 and VT3 begins to open, and the negative voltage regulator VT4 and VT2 still remains closed.
The design uses a ready-made transformer with a power of 10 W, which produces two alternating voltages of 12 V each on the secondary winding. The capacitances of capacitors C1 and C2 should not be less than 1000 uF, it must be borne in mind that the level of ripple at the output depends on them.
Zener diodes can be any low-power voltage of 12 V. The KT817 transistor can be replaced with KT815, KT807, KT819. Transistor KT816 - on KT814. Transistors KT502 and KT503 can be replaced, respectively, with KT361 and KT315. You can use another rectifier bridge, for example, KTs402, or assemble it from diodes like D226 or KD105. Transistors VT1 and VT2 must be placed on small heat sinks.
Author: Semyan A.P.
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