30V, 5A Adjustable Lab Power Supply
30v 5A lab power supply
    I bought this power supply in China in early 2009 for 30 euros and although I have not used it much it was working fine until some days ago when I shorted the output by chance and it stopped working even though it was supposed to be protected.
    It is a linear type with two 2N3055 transistors mounted on a radiator on the back.  It has a very substantial transformer with three different voltage outputs for the main power supply which are switched with relays depending on the output voltage required.  As you increase the output voltage you can hear the relays clicking as higher transformer voltages are selected.  This keeps wasted heat dissipation lower.  Then it has a separate ac output to power the control electronics.
    After the short the output voltage reading would not rise above about 10 volts and the relays seemed to act erratically.  A first visual inspection of the control circuit did not reveal any obvious damage.  I started to reverse engineer the schematic diagram in order to get a general idea of how it worked but as I advanced with the diagram I realized that there was something I really did not understand at all.  The control circuit is powered separately and seems to be floating with respect to the power circuit.  I still do not understand how this works.
    Deducing the schematic diagram took quite a few hours in spite of it not having a large number of components.  Rather than work directly with the board, which is cumbersome and may lead to damage of the many cables soldered to it, I took a photo of each side of the board and later used software to superimpose the components side on the reversed copper tracks side.  I then paint the copper tracks in blue and the components in red.  At the same time I am doing this I am drawing the schematic diagram.  This is not the quickest way but it provides some double checking and cross-checking and helps with the process of better understanding the circuit.  Here is a reduced and compressed image.  The original is in much larger size and much clearer but, obviously, much larger file size.

    And here is the resulting schematic diagram.  In the left we see the transformer and separate windings for the power and control circuits.  At the top is the series regulator formed by the two 2N3055 transistors.  In the lower part we have the control circuit which can be divided into two separate parts: on the left we have an LM723 which provides the voltage and current control and on the right we have a couple op-amps working as comparators which switch the two relays which select one of three transformer windings depending on the voltage needed.
Click to enlarge
    At very low voltages the lowest AC output is selected because RL2 is active.  At about 10 V output RL2 switches off and the second transformer output is connected.  At about 20 V RL1 is activated and the highest transformer output is connected.
    In the following schematics I have isolated the current and voltage control circuits and explain how they work.
    September 2011: Added watermarks to graphics.  Sorry but assholes who pirate other people's pages have made this necessary.