I've been working to figure out an LED lighting project for my classroom for a while now, readings on this site has helped. One part of the solution is to combine in series computer power supplies (which we can get free) to get specific voltages. This will help us use the ideal voltage for the different needs of the different LED strips. For example, By adjusting resistance to the correct lower value and also set the power supply to 8V, Red LED strips can go from around 50% efficient to better than 80%.
It's well explained on this site that you can not connect say the 3.3V and the 5V rails from a single computer power supply to get 8.3V since they share the same negative terminal. I remember some page here (can't re-find it) that talked about combining separate computer power supplies to get combination voltage. BUT, I wonder, wouldn't the separate power supplies actually have a common ground (the grounded wire on the plug?) If this is problem, any easy work around? Any better way to get about 8 volts using computer power supplies?
UPDATE: PICTURE SHOWS CURRENT SETUP: Note that black wires (ground) are divided into 2 bunches and the yellow (12V) gathered into 1 bunch. Green and Black wire are connected (so that power is on) as per this instructable: I tested with an ohm meter and the mains earth ground IS electrically connected to the output DC power ground. Can I fix this problem?