I discovered a very cool thing to do with an old computer SMPS. I thought people would want to know. In brief, it involves converting the supply so instead of outputting a fixed 12V/5V/3.3V, it outputs a variable 7V-10V/3.5V-4.5V/2.2V-2.8V. I'm working on improving the range so I can get the outputs to go above 12V (with caution, as the filter caps are only 16V rated) and below 2.2V. The voltages are adjusted in harmony using a potentiometer. It is unfortunately not possible to control each rail individually. I used a cheap Elenco 120W power supply, from a very old computer, but any supply should work. The first thing you must note is that a computer PSU is mains operated equipment. Therefore, it is easy to get shocked. You must take appropriate precautions including discharging the main filter capacitor(s) before modifications commence. If not, you will get a nasty shock, it could even be fatal. Don't be lazy, and double check the caps with your multimeter! The second thing to note is you must have a special type of PSU. It must have a feedback path to the control IC on the secondary side. In my case, the 5V rail was the feedback path. It went through a voltage divider (very important) to a TL431 shunt reference, and to an optoisolator. The TL431's configuration is like a comparator: it conducts when the supply voltage exceeds a set point. This allows it to be used to produce the feedback signal, which, along with the controller IC, drives the primary side MOSFET. The feedback goes through an optoisolator to the control electronics. Some supplies do not have an optoisolator and instead use a control IC on the secondary side. These still can be modified, but it is trickier. The modification goes like this: after locating your feedback path, you must solder a potentiometer across one of the resistors. By adjusting the resistance of the potentiometer, you change the feedback range. If you insert a resistor in parallel, you can decrease the voltage; a resistor in series can increase it. Series resistors are more tricky because usually, a divider resistor must be removed or a track must be cut, and this runs the risk of damaging the supply. Another thing to be aware of is the fan usually runs off the 12V line, which can now vary from 7V to 9V. This can cause problems under heavier loads, because the supply may not be adequately cooled. You should take appropriate precautions to ensure the supply does not overheat. Enjoy.