In this Instructable I will show you how I built a very simple, low cost variable 1-28 V 5 A power supply with simultaneous digital voltage & current displays and variable current limit, short circuit protection etc. It's an ugly beast but I'm time-poor, so function is more important than form. Every time I see a nice bench supply I find myself tempted, until I see the price, especially as I'm not a heavy user. The power supply I describe below meets all my needs and cost me next to nothing.

Please note that this isn't a detailed design - it's more an illustration of a concept that you can modify to suit your application and tastes. Naturally if you attempt to reproduce this work you do so at your own risk. I haven't exhaustively tested it under heavy load conditions for long periods or anything like that, but I regularly use it without problems for small scale electrical / electronic development work.

I should first say that when I have electronic stuff that breaks, I never discard the power supply ('wall wart') as these can almost always be used for something else. So the key aspect of the design is the re-use of an old power supply which supplies about 35 V DC and a few amps (from a late 90s era HP Deskjet). This is completely packaged and safe to use, and requires no modification. I use a pair of cheap electronic modules on the output to provide the voltage & current control and limiting, plus the display. Everything has been secured to a piece of scrap-box plywood with cable ties or screws as appropriate.

The power supply comprises three building blocks:

Mains power supply of fixed DC output voltage
Voltage regulator to reduce the mains power supply output to 30 V or less (optional, depending on specification of item 1)
Variable output CC/CV power supply stage with displays, capable of 1-28 V variable and up to 5 A, depending on the capabilities of items 1 & 2

Note that each part of the system has the capacity to limit the overall voltage and power delivered at the output. If your voltage regulator stage is underpowered, the best mains supply in the world will not help you get a good output from the CC/CV stage. I hope it's blindingly obvious that the input of every stage must be compatible with the output of the previous stage, otherwise you will find yourself in a potentially hazardous / damaging situation.

You will need:

A reasonably powerful, double insulated encapsulated power supply / 'wall wart', producing no more than 40 V DC at a few amps.The performance of this PSU will determine the output voltage and current capability of the finished system.
If your power supply is greater than 30 V DC, a variable output voltage regulator board from ebay such as "DC-DC Buck Converter 4V-40V 8A 100W Step-Down Voltage Power Module"
A 1-28 V variable power supply / battery charger module such as "High Quality 5A Constant Current Voltage LED Driver Battery Step-down Module DM" from ebay (cost as of August 2016 just over £4)
Interconnecting wires (red and black) with a suitable current-carrying capacity
Dual 4 mm sockets (red and black)
Base-board and cable ties / screws etc.

Step 1: Getting an Internal Supply Voltage of 30 V or Less

If you supply more than 30 V to the variable CC/CV output stage specified in this system, it will blow up fairly rapidly. This first step ensures that you are supplying the correct voltage to the output stage.

Verify the output voltage of the mains power supply by using a voltmeter or more simply by reading the label. Cut off any plug attached to its output lead. Strip back the wires ready for connection to the next stage. If the specified / measured output is less than 30 V, go to the next step.

Attach the mains power supply output lead to the input terminals of the voltage regulator stage. Use a voltmeter to verify the polarity if you are in any doubt.

Attach the voltmeter to the output terminals of the voltage regulator stage and turn on the mains power supply. A red LED should light on the voltage regulator stage and a non-zero voltage should appear at its output. Twiddle the adjustment potentiometer until the output voltage reads 30 V. You won't need to adjust this stage again. Remove the voltmeter and switch off the mains power supply.

Step 2: Add the Output Stage and the 4 Mm Connectors

Simply attach the 30 V DC supply wires (made during the previous step) to the input terminals of the variable CC/CV output stage.

Attach a pair of 4 mm screw terminals (red and black) to the output terminals of the variable CC/CV output stage.

Step 3: Power Up and Test

Ensure each of the building blocks is separate from the others. Ideally at this point you would lay out each block on a board and fix it in place. This minimizes the risk of short circuits between the unenclosed modules. I used the screw-holes provided on the voltage regulator module and the screw terminal block. The other building blocks were attached using cable ties through the plywood.

With no load attached to the output screw terminals, switch on the mains power supply. You should see both LED readouts light up on the variable CC/CV output stage.

On the upper edge of the variable CC/CV output stage there are two multi-turn potentiometers. Viewed from the normal viewing direction of the two displays, the left hand pot controls the output voltage (1-28V) and the right hand pot controls the current limit. Clockwise increases the value in both cases. These pots are small and fiddly - A plastic trimmer tool is recommended to adjust them; slipping with a metal screwdriver could short out the board. An LED to the left of the pots lights GREEN to indicate when the current limit on the load has been reached. For constant voltage use, make sure that it's off.

On the bottom right of the variable CC/CV output stage are two more LEDs. These are I believe associated with battery charging functionality, but they effectively show when current is being drawn or not. The left LED lights (blue) when a load is attached and is drawing current (as shown in the illustration). When there is no load the right LED lights RED.

I built this PSU several years ago for an outlay of less that £10 and have never felt the need to make it more presentable. It does exactly what it needs to do and does it really well. I also got to exploit an otherwise useless mains power supply for an old printer. A competent electronics experimenter could put the whole thing in a nice enclosure and provide external potentiometers and LEDs. Maybe I'll get round to it one day! Anyway, I hope you save some money and get to recycle some of those unused mains supplies you might have lying around in the shed...