Rectifier output with fitters:

When half-wave and full-wave rectification suffice to deliver a type of DC output, neither produces constant-voltage DC (direct current). To generate steady DC from a rectified AC supply, a smoothing circuit or filter is needed. In its simplest form this can be just a reservoir capacitor or smoothing capacitor, located at the DC output of the rectifier. There will still stay an amount of AC ripple voltage in which the voltage is not fully smoothed.

Figure: RC-Filter Rectifier

Sizing of the capacitor presents a tradeoff. For a specific load, a larger capacitor will get reduce ripple but will cost much more and will make higher peak currents in the transformer secondary and in the supply feeding it. In extreme cases in which many rectifiers are loaded onto a power distribution circuit, it may prove hard for the power distribution authority to keep a correctly shaped sinusoidal voltage curve.

For a specific tolerable ripple the needed capacitor size is proportional to the load current and proportional inversely to the supply frequency and the number of output peaks of the rectifier per input cycle. The load current and the supply frequency are usually outside the control of the designer of the rectifier system however the number of peaks per input cycle can be influenced by the choice of rectifier design.