Use a transformer with a center-tapped secondary. The center tap is one of the output terminals. Connect the anodes of two rectifier diodes to the ends of the secondary and connect the cathodes together. The cathodes are the positive terminal. (Reverse the diodes to reverse the polarity of the output.) The average voltage will be about 90 percent of the rms voltage of the half-winding, minus about .7 volts. The rms voltage will equal the rms voltage of the half-winding, minus about .7 volts.
Use a simple secondary into the ~ terminals of a bridge rectifier. The + and - terminals are the output. The average voltage will be about 90 percent of the rms voltage of the winding, minus about 1.5 volts. The rms voltage will equal the rms voltage of the total winding, minus about 1.5 volts.
If you want filtered dc, put an electrolytic capacitor across the output. The peak-to-peak ripple voltage will be approximately 8333 times the current drawn, in amperes, divided by the capacitance in microfarads. The output voltage will be about 141 percent of the half-winding or winding, respectively, minus about .7 or 1.5 volts, respectively. The capacitor's working-voltage rating should be at least as high as the filtered output voltage.
Notice that the dc output of the center-tapped transformer and its unrectified ac can share a common terminal, which is not possible with the bridge rectifier.
