Update: as a follow-up to this project, I have upgraded my power supply to a new digital design.
Read more here. You can still find the previous, analogue design below, which is simpler and easier to build:
Variable regulated power supply
Having a variable regulated power supply that can output precise voltages in the 0 .. 30Volts interval is a great add-on for any electronics lab. Especially when it's a high power supply that can handle as much as 20Amps of current.
For this article, I'm going to show you my variable regulated power supply, built from scratch, the circuit diagram I've used and a few safety tips. It's based on the LM317 that controls a few high power bipolar transistors connected in parallel, to achieve a 0..30V voltage interval and a maximum current of 20Amps.
Step by step guide
First thing we need is a high power transformer. I've ordered a custom toroidal unit, with a primary for 220V mains, and two secondaries one of 24Volts, 10Amps max and another one of 12V, 0.5Amps max. It's very heavy and it was quite expensive. I also needed a rectifier bridge, and got one capable of handling 400V at 35A max:
For the project enclosure, I opted for a computer power source metal casing, scrapped from a defective unit: little space but eventually there was enough to fit everything inside and still keep it well organised.
An aluminum heat-sink was added to the case, fixed tightly with several screws so the case itself would also absorb some of the extra heat, for better efficiency. Part of the case was cut so that I could fit a voltmeter and some power transistors directly on the aluminum heatsink:
The low power secondary would be used to obtain 12V and 5V with the L7812/L7805, to power the digital voltmeter applied on the high power secondary and also various microcontroller projects that don't require too much current.
I've soldered everything on a test board, using tick copper wire were needed (eg. for the connections to the transistors). A few holes in the front panel and I was able to place the 5k pot for coarse voltage adjustments (and a second 1K pot for fine adjustments, connected in series with the first), the LEDs, the fuse, and the black array of wire connectors for the output (The variable regulated output, the max regulated output, the non rectified alternative current output, the low power 5V and 12V output, etc).
My transformer gives 10A max at 24V, but the regulator block can handle up to 20A max, because of the 6 transistors. I preferred adding a few extra transistors, to distribute the load and help dissipate heat better. If you need more current or less, simply change the number of transistors to suit your needs.
This supply is extremely reliable. I've used it for more then a year now, the initial design had 4x 2N3055, but they failed quite frequently because of my high voltage experiments that produced spikes transmitted back to the supply circuit. The TIP3055 seems to be almost indestructible, so I highly recommend it.
The neon bulb works as a snubber, to protect the supply against high voltage kick backs. I'm not sure you need that if you're only doing low voltage stuff.
Here are some variants created by my readers
As a common effort of several people, this circuit diagram comes with several improvements. You can read the comments below for more details. Instead of the 2n3055 I recommend the TIP3055.
John also provided a regulated power supply that has a nice current control feature, but it uses the LM723 instead:
Here is the ready-built supply, using LM723:
Phil provided several helpful advises (see comments section) for others willing to build a power supply. He also experimented with multiple circuits, seeking for the better alternative.
poparamiro built this supply to power a set of fans, to use them in overclocking:
nachtfalke's variant also uses a toroidal transformer and 6xTIP3055 power transistors. Again with applications in overclocking, here is yet another nice, clean build:
As a follow-up to this project, I have upgraded my power supply to a new digital design. Read more here.
For switched power supplies, see this article.