A simple series inductor reduces both the peakand effective values of the output current and output voltage. On the other hand a simple shunt capacitor filter reduces the ripple voltage but increases the diode current. The diode may get damaged due to large current and at the same time it causes greater heating of supply transformer resulting in reduced efficiency.
In an inductor filter, ripple factor increases with the increase in load resistance RL while in a capacitor filter it varies inversely with load resistance RL.
From economical point of view also, neither series inductor nor shunt capacitor type filters are suitable.
Practical filter-circuitsare derived by combining the voltage stabilizing action of shunt capacitor with the current smoothing action of series choke coil. By using combination of inductor and capacitor ripple factor can be lowered, diode current can be restricted and simultaneously ripple factor can be made almost independent of load resistance (or load current). Two types of most commonly used combinations are choke-input or L-section filter-and capacitor-input or Pi-Filter.
Choke Input Filter Rectified Waveform
Choke-input filter is explained below:
Choke-input filter consists of a choke L connected in series with the rectifier and a capacitor C connected across the load . This is also sometimes called the L-section filter because in this arrangement inductor and capacitor are connected, as an inverted L. ln figure only one filter section is shown. But several identical sections are often employed to improve the smoothing action. (The choke L on the input side of the filter readily allows dc to pass but opposes the flow of ac components because its dc resistance is negligibly small but ac impedance is large. Any fluctuation that remains in the current even after passing through the choke are largely by-passed around the load by the shunt capacitor because Xc is much smaller than RL. Ripples can be reduced effectively by making XL greater than Xc at ripple frequency. However, a small ripple still remains in the filtered output and this is considered negligible if it than l%. The rectified and filtered output voltage waveforms from a full-wave re with choke-input filter are shown in figure.
Capacitor Input Filter Circuit
Capacitor-Input or Pi-Filter.
Such a filter consists of a shunt capacitor C1 at the input followed by an L-section filter formed by series inductor L and shunt capacitor C2. This is also called the n-filter because the shape of the circuit diagram for this filter appears like Greek letter n (pi). Since the rectifier feeds directly into the capacitor so it is also called capacitor input filter.
As the rectified output is fed directly into a capacitor C1. Such a filter can be used with a half-wave rectifier (series inductor and L-section filters cannot be used with half-wave rectifiers). Usually electrolytic capacitors are used even though their capacitances are large but they occupy minimum space. Usually both capacitors C1 and C2 are enclosed in one metal container. The metal container serves as, the common ground for the two capacitors.
A capacitor-input or pi- filter is characterized by a high voltage output at low current drains. Such a filter is used, if, for a given transformer, higher voltage than that can be obtained from an L-section filter is required and if low ripple than that can be obtained from a shunt capacitor filter or L-section filter is desired. In this filter, the input capacitor C1 is selected to offer very low reactance to the ripple frequency. Hence major part of filtering is accomplished by the input capacitor C1. Most of the remaining ripple is removed by the L-section filter consisting of a choke L and capacitor C2.)
The action of this filter can best be understood by considering the action of L-section filter, formed by L and C2, upon the triangular output voltage wave from the input capacitor C1 The charging and discharging action of input capacitor C1 has already been discussed. The output voltage is roughly the same as across input capacitor C1 less the dc voltage drop in inductor. The ripples contained in this output are reduced further by L-section filter. The output voltage of pi-filter falls off rapidly with the increase in load-current and, therefore, the voltage regulation with this filter is very poor.
Salient Features of L-Section and Pi-Filters.
1. In pi-filter the dc output voltage is much larger than that can be had from an L-section filter with the same input voltage.
2.In pi-filter ripples are less in comparison to those in shunt capacitor or L-section filter. So smaller valued choke is required in a pi-filter in comparison to that required in L-section filter.
3.In pi-filter, the capacitor is to be charged to the peak value hence the rms current in supply transformer is larger as compared in case of L-section filter.
4.Voltage regulation in case of pi-filter is very poor, as already mentioned. So n-filters are suitable for fixed loads whereas L-section filters can work satisfactorily with varying loads provided a minimum current is maintained.
5.In case of a pi-filter PIV is larger than that in case of an L-section filter.