You can use the circuit in Figure 1 to obtain a low regulated voltage, such as 5V dc, from a higher voltage, rectified, sinusoidal voltage source without resorting to an electrically noisy dc/dc converter or wasting watts in a dropping resistor. This application requires a regulated 5V-dc source, but a transformer supplies 18V rms to a full-wave bridge rectifier. During the charging phase, two equal-value electrolytic capacitors, C₁ and C₂, receive charging current when connected in series through forward-biased diodes D₁ and D₂. An enhancement P-channel MOSFET transistor, Q₁, an International Rectifier IRF9530, remains off because its gate receives a slightly positive reverse-gate-bias voltage due to zener diode D₄'s forward-voltage drop. Each capacitor charges to approximately one-half the peak value of the rectified voltage minus the forward-voltage drops that D₁ and D₂ present. The full-wave bridge rectifier, D₅, or Graetz bridge, produces these drops (Reference 1).

When the discharge phase begins, D₁ gets reverse-biased, and capacitor C₂ discharges through the load that voltage regulator IC₁ presents. Subsequently, the anode voltage of diode D₁ continues to decrease, Q₁'s gate-to-source voltage becomes negative, and the transistor conducts, allowing C₁ to discharge into the load through forward-biased diode D₃. In effect, the two capacitors charge in series and discharge in parallel into the load, halving the raw rectified voltage and ripple voltage at IC₁'s input. During C₁'s discharge, zener diode D₄ protects Q₁ by clamping its gate-to-source voltage within its maximum rating.

To function properly, the circuit requires a minimum load current; the regulator's quiescent-current drain is usually enough. Otherwise, capacitor C₂ charges to the peak voltage available from D₅. The values of C₁ and C₂ and the ratings of the remaining components depend on the maximum load current required. The values of resistors R₁ and R₂ are not critical. Note that Q₁ functions as a switch; selecting a device with low on-resistance limits Q₁'s power dissipation.