UBZR1 Power Supply

Chelys Lyra

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.

Summary/Usage

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:

+12.5 V at 230 - 250 mA
−12.5 V at 220 - 240 mA
+5 V at 100 mA
−5 V at 10 mA

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:

The first rectifier circuit feeds the +12.5 V and −12.5 V output stages
The second generates two auxiliary DC voltages, to feed the circuits to stabilise the +12.5 V and −12.5 V
The third feeds the +5 V and −5 V stabilising circuitry, which are built from industry standard ICs.

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

+11.5 V to +13.5 V adjustable, up to 500 mA output current, adjustable current limit
−11.5 V to −13.5 V adjustable, up to 500 mA output current, adjustable current limit
+5 V, up to 100 mA
−5 V, up to 100 mA

Schematics as SVG Files

Power Block Schematics

Power Positive Regulator Schematics

Power Negative Regulator Schematics

The circuit is built as three separate assemblies:

Input AC jack with input filter and input toroidal transformer
Printed circuit board that takes in the transformers low voltage AC output and has the output jacks for the 4 output voltages including 4 ground connections
A heat sink assembly which contains the two bipolar power transistors. This assembly is mounted physically at the back of the UBZR1 case and electrically connected to the printed circuit board with stranded cable and terminal pins.

Interface

Industry standard mains jack for 230 V AC
Clamping terminal blocks, raster 5 mm

Acknowledgements

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.