Chelys Lyra

UBZR1 Power Supply

When I started UBZR1 I re-used a power supply I already got - a compact switching power supply of ±12 V at 1.5 A each. It can generate additional voltages by extender modules.

I added two 5 V extender modules. One to generate +5 V for digital circuitry and another one for −5 V for some analog stuff.

It turned out, I needed to trim the 12 V up to 12.5 V because I had OpAmps going up to 10 V output only if power was +12.5 V.

But this switching power supply's output voltages are quite noisy, especially if you draw only a small fraction of the maximum current. I measured that only from about 600 mA the noise would decrease substantially. But UBSR1 needs only about 250 mA, which is far below the 1.5 A the power supply can do and also far below the "minimum" load this switching power supply needs.

Hence I decided to design and build my own power supply. To keep it quiet, I went for a classical analog non-switching design.


This module feeds all the UBZR1 modules with stabilised DC voltages of +12.5 V, +5 V, −5 V, −12.5 V.

The voltage current requirements of UBZR1 are as follows:

Functional Description

I am using a standard toroidal transformer to go from mains 230 V AC voltage to 2 times 12 V AC. I wired this as 24 V with centre tap. I put three rectifying circuits with corresponding electrolytic capacitors:

As the currents drawn from ±5 V are relatively low, I decided to spare a separate transformer and accept the additional power loss by getting it out of the 12 V AC.

Performance Specs

Schematics as PDF

The circuit is built as three separate assemblies:



I took the idea to generate auxiliary voltages by separate rectifiers from an old Elektor July/August 1978 article, about improving the efficiency of an LM723 based circuit.

Last change: 2019-09-19
© 2002-2020 Dr. Thomas Redelberger