TL431 and LM431 are relatively low-noise, stable and low-cost shunt regulators. These can be used to build all sorts of power supplies including programmable power supplies, advantages of which are enormous. You can program the output voltage with simple switches. You can also program the output voltage with digital codes coming from a microcontroller (MCU) or from the printer port of a PC. You can adjust any output voltage individually with resistors or trimmer potentiometers to the required values. This article presents a programmable power supply built around TL431 (IC1) and two bipolar transistors BD139 and TIP31 (T1 and T2). The circuit also includes an inverter 7406 (IC2), nine diodes 1N4007 (D1 through D9), a 12V regulator 7812 (IC3), a 5V regulator 7805 (IC4) and a few other components. Using this circuit you can obtain around 18V, 2A unregulated output and 3V to 15V, 1A variable regulated power supply based on digitally programmable input as shown in Table I. You can also obta...
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