UBZR1 Power Supply

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

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

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

However, 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 600mA the noise would decrease substantially. But UBSR1 needs only about 250mA which is far below the 1.5A 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 +12.5V, +5V, -5V, -12.5V.

The voltage current requirements of UBZR1 are as follows:

Functional Description

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

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

Performance Specs

Schematics as PDF or JPG

The circuit is built as three separate assemblies



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

Last change: 2016-09-15
2002-2017 Dr.Thomas Redelberger