Power Supplies

Symmetrical power supply

A symmetrical power supply may be designed from two standard optional amplifiers if only a small (of the order of a few mA) is required.

In the diagram, the upper half provides the positive voltage. A 3.3 V Zener diode, connected to the non-inverting (+) input of opamp IC I, serves as a reference. To ensure correct starting of the circuit, the diode is initially powered via R1, and then, when the output voltage is sufficiently high, via D1. Part of the output voltage is fed back to the inverting (-) input of IC1, viap1. The lower the feedback voltage, the higher the output. The supply voltage for the TL071 may be 36 V maximum so that the output voltage can go up to about 30 V. It is, however. safer to make the supply to the opamp rather lower and accept a slightly lower output voltage.

The lower part of the diagram is a mirror of the upper part: it provides the negative output. Opamp IC2 needs an additional (negative) supply voltage.

The output voltages may be made more stable by connecting a 10 pF electrolytic capacitor across each of the outputs.





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