Power Supply Circuit # 2

Figure 1.  Circuit Diagram for a Full-Wave Rectifier Circuit using 2 Diodes

The circuit in Figure 1 is a full-wave rectifier circuit that converts the household AC voltage into a DC voltage. The transformer is used to step the 220 VAC to a lower voltage (12 VAC) in this case.

The reduced AC voltage is rectified using the two diodes shown.  In one direction of the current, the upper diode is 'on' while the lower one is 'off', charging the capacitor. In the other direction,  the upper diode is 'off' while the lower one is 'on', also charging the capacitor. The voltage built up by the capacitor is the useable DC voltage. 

This is called a 'full-wave' rectifier because diode rectification occurs in both the negative and positive cycles of the AC waveform.  Note that this circuit works by using the center tap of the transformer as 'ground'.  Also, the resulting DC voltage is higher than the measured RMS value of the AC voltage by a factor of about 1.414.  Thus, in the example above, the voltage at C1 can exceed 16 V DC (assuming no real-world losses).