Sorry I couldn't Resist.
It all started when I ordered a couple of these kits from Bangood in China:
At $6.87 including postage I could not resist. The kit came with all the components needed except the heat sink and transformer. There were even pin jacks and wire pigtails so that the Voltage and Current control potentiometers could be extended off the board. The instructions which were downloadable were better than average for a kit from China and it even included the semblance of a schematic.
After assembling the circuit board I ran tests on it to see if it was stable and worked properly. I found that it could be used either in a constant current mode from 2 mA to 3 Amps and also in a constant voltage mode from zero to 30 volts. The circuit even had an offset adjustment trimmer on the voltage control op amp so that the potentiometer could be zeroed. The circuit requested a 24 volt transformer to work this full range. After checking my salvage stock the closest I could find to the required transformer were a couple of 21 volt toroidals. This would ultimately provide a 0 to 24 volt output which was acceptable. Unlike my previous switching power supply build this would be a linear supply. Dissipating heat would be a serious consideration. A check of the salvage showed I had an old CPU heat sink with a mounted fan that I got from the Radio Shack 20 years ago. Unfortunately there was only one. A check of EBay turned up a second heat sink for the second channel. Here is a picture of the build when I was still waiting for the second heat sink.
While it is not on the schematic the board provides a separate 7824 voltage regulator to drive a fan. It is visible in the upper left corner of the circuit board. Since the CPU fan was 12 volt I initially replaced the 7824 with a 7812.
I am using an identical case for this linear dual bench power supply that I used for the previous switching supply. Even though the switching power supply had larger toroidal transformers space was more of a problem this time since the regulator boards with their heat sinks were so much bigger than the switching modules. Here are three pictures of the entire unit so that the layout is more apparent.
The display units are only 3 digit and are just inexpensive $4.00 modules from China but they had the advantage of having internal current shunts this time so I did not have to try to make my own. Each display also had its own current and voltage calibration trimmers. Don't get me wrong if you need mA precission this is probably not the meter for you but for most of my kinds of experiments it will be more than adequate and there are always the more precision bench meters if I need better accuracy.
I had to wait about a week for the second CPU heat sink fan assembly to arrive. After it was installed however I was not happy with the noise that the two cooling fans made as each were running at the full 12 volts even when no load was being driven. The answer was to build a simpleTLE 2142 op amp circuit with a thermistor as a sensor and a MOSFET to drive the fan. I still wanted to be able to run the fans at the full 12 volts when things got hot so I had to upgrade the 7812 regulators on the boards to 7815. This gave me a 15 volt supply and the drop out of my little sensor driver circuit was about 3 volts. Here is a picture of the unit with the second heat sink installed as well as the two op amp circuits mounted above the lower left side of each regulator board.
Now we were in business. The unit under no load has the fans driven with 6 volts which ramps up to 12 volts as the heat sinks warm up. The front panel of this unit is very similar to the front panel of the previous build. In fact I used the same template to mark and cut it. Unfortunately I slipped and scratched it which inspired me to flip it over and use the mirror side for this unit. Here is a picture of the front panel.
Here is the completed unit in its case and also a picture of it with its brother.
Now I would like to tell you that this will be the last power supply for a while but son Mike just dropped by and brought 5 more dental camera units for me to salvage out. Aye! my work is never finished. 
John
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