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Reservoir and Smoothing Circuit

<^< Worksheet: Bridge Rectifier | Course Index | Worksheet: Reservoir and Smoothing Circuit >^>

Unfortunately, the output of simple half and full-wave power supplies is not a steady direct voltage but a voltage which pulsates (at the same frequency as the mains supply in the case of a half-wave rectifier or at twice the frequency of the mains supply in the case of a full-wave rectifier).

This pulsating direct voltage is unsuitable for use as the power supply for most electronic circuits and steps must be taken in order to keep the load voltage constant. This is achieved by connecting a large value of capacitance across the load. Since this capacitor acts as a reservoir for charge, it is known as a reservoir capacitor. It charges when the rectifier diode(s) conducts (at or near the peak of the positive voltage cycle from the transformer secondary) and then discharges into the load during the period for which the rectifier diode(s) is non-conducting.

In order to remove any residual ripple that might still be present on the direct voltage dropped across the reservoir capacitor, a further R-C smoothing filter may be fitted. This circuit acts as a low-pass filter - allowing direct current to pass through but severely attenuating any higher frequency ripple current that may be present.

(Note that on the simulation the light blue trace shows what the current through the load would be without the capacitor, and the red trace shows the current throught the load when the capacitor is in circuit.)

<^< Worksheet: Bridge Rectifier | Course index | Worksheet: Reservoir and Smoothing Circuit >^>