John L Errington MSc

John Errington's tutorial on Power Supply Design

How to design and build a linear DC Power supply


In the 1990s I was asked to design and build a power supply to provide a variable voltage DC power supply that would provide variable voltages at currents of up to 5 amps, for experiments in a Solid State Physics lab at Northumbria University. The voltage was required to be very stable and repeatable, with no significant temperature dependence, ripple or noise.

Following this successful project (and other similar designs, each with their own challenges) I wrote an article for Practical Electronics (now defunct) describing the design process for this unit, but with more general applications. The following pages are based on this article, and it is hoped they will be of use to anyone who is designing a supply for their own use.

The design starts with a discussion of transformer, rectification, and smoothing. Further pages examine the choice of components and design ideas for voltage regulation circuits. Throughout these pages I will aim to provide a thourough and detailed explanation, with "rules of thumb"; and more detailed calculations for those who wish to use them. Each stage is accompanied by Example Designs.

"Linear"? For higher wattge power supplies its more efficient to use a switching technique. These pages examine only the traditional linear techniques, but much of the explanation is applicable to both.

A mains driven DC power supply will usually have the following parts:

Parts of a DC power supply:
Component or module Purpose
Transformer To change mains input voltage to provide voltage required
Rectifier To convert AC to DC
Smoothing capacitor(s) To reduce the amount of AC ripple on the DC voltage
Voltage regulation To provide the required output voltage
Current limiting To provide protection for both internal and external parts
Feedback To eliminate the effect of line resistances


Next: choosing a transformer and rectifier