A few Simple Power Supplies

The first supply uses a center-tapped transformer rated at 12.6 V at 1.2 to 3 A. The voltage after rectification resides at an 8.9-V peak pulse. The filter capacitor (C1) smoothes the pulses, and the 7805 outputs a regulated +5 V. C2 is placed across the output of the regulator to bypass high frequency noise that might be generated by the load. The diode placed across the 7805 helps protect the regulator from damaging

reverse-current  surges generated by the load. Such surges may result when the power supply is turned off. For example, the capacitance across the output may discharge more slowly than the capacitance across the input. This would reverse-bias the regulator and could damage it in the process. The diode diverts the unwanted current away from the regulator. The second power supply is similar to the first but uses a bridge rectifier.