Being the benevolent tiger that I am, I felt the need to share some of my experience in the art of DC power supply design, on a budget.
A friend of mine who works in an industrial setting brought home some 3-phase motors, transformers, and meters that were being scrapped. He asked if I had any use for them. I asked him if the bear takes a dump in the woods. Among the spoils were some 277/120 and 480/120 transformers. What these do is take 480 volt (typical delta connected 3-phase voltage) or 277 volt (typical wye-connected 3-phase voltage) and steps it down to a safer-to-handle 120 volts. The secondary of these transformers had been connected to some Gould/Crompton meters to passively monitor the voltages present throughout the plant.
When you see 277-to-120, just think of it as a 2.31:1 step-down transformer. Some of the transformers he gave me had big "288" numerals on the sides of them. These are 2.4:1 step-down reduction.
I set about to test the transformers this morning by crimping a couple of screw lug terminals to the end of an old appliance cord. I attached the 120 VAC cord to the PRIMARY of the transformer (which normally would be receiving 277 or 480 volts) and measured the output voltage. The 480-to-120 transformers were indeed generating 30.1 VAC at the secondary, as expected.
The 277-to-120 transformers were generating 56 VAC at their secondaries. This seemed a few volts too high, as if the ratio of windings was off a bit. The 288-to-120's were generating 50.6VAC.
With these available AC voltages, I figured that using them to make rectified DC yields a DC voltage approximately 1.4142 times higher. That is the square root of 2.
Here's a picture of one of the low-power (25 VA) 277-to-120 transformers, whose secondary is connected to the "AC" leads of a 200V/2amp full-wave bridge rectifier, which in turn has its "+" and "-" leads screwed down to the corresponding leads of a 26,000uF 80WVDC electrolytic capacitor. This here is the simplest linear power supply you can make. A transformer, rectifier, and capacitor. The measured secondary AC voltage was 56VAC. The measured DC voltage at the capacitor was 76VDC. As stated above, 25 volt-amps isn't enough to do anything really useful with. It's just to illustrate the principle.
For my setup, I have 80V Geckos and 75V servos. A 72VDC power supply would be perfect. To achieve that, I would use one of the 288-to-120 step-down transformers, to get 2.4:1 reduction in AC voltage. Wall voltage being about 121 volts at my house, divided by 2.4, yields a secondary voltage of 50.4VAC, which once rectified and filtered, should give about 71 volts DC. Perfect!
Check out the picture here. The machinery it is sitting on is my air compressor in my garage. It was the only available free space at the moment That's sad, isn't it?
Enjoy!!!
http://i7.tinypic.com/525vmna.jpg