Power supply :
DC Power Supply by 78xx
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Variable Regulator
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I didn't realize till the other day that I have never shown a circuit for a standard power supply. Shown below is a supply that will use any of the LM78XX series of voltage regulators.
The transformer in the circuit will vary depending on which regulator you use. For voltages from 5 to 12 use a transformer with output of 18vac. With voltages from 15 to 24 use a transformer of 30vac. The first capacitor in the circuit may need to vary if you are supplying more current to the load. Typically it will be 2000uf for every amp of current.
Source :: http://home.maine.rr.com/randylinscott/nov98.htm