4-24
views A and B of figure 4-20.) Consequently, when the pulsating voltage is first applied to the LC
choke-input filter, the inductor (L1) produces a CEMF which opposes the constantly increasing input
voltage. The net result is to effectively prevent the rapid charging of the filter capacitor (C1). Thus,
instead of reaching the peak value of the input voltage, C1 only charges to the average value of the input
voltage. After the input voltage reaches its peak and decreases sufficiently, the capacitor C1) attempts to
discharge through the load resistance RL). C1 will only partially discharge, as indicated in view B of the
figure, because of its relatively long discharge time constant. The larger the value of the filter capacitor,
the better the filtering action. However, because of physical size, there is a practical limitation to the
maximum value of the capacitor.
Figure 4-20A.—LC choke-input filter (charge and discharge paths). CHARGE PATH
Figure 4-20B.—LC choke-input filter (charge and discharge paths). DISCHARGE PATH
The inductor (also referred to as the filter choke or coil) serves to maintain the current flow to the
filter output (RL) at a nearly constant level during the charge and discharge periods of the filter capacitor.
The inductor (L1) and the capacitor (C1) form a voltage divider for the ac component (ripple) of the
applied input voltage. This is shown in views A and B of figure 4-21. As far as the ripple component is
concerned, the inductor offers a high impedance (Z) and the capacitor offers a low impedance (view B).
As a result, the ripple component (Er) appearing across the load resistance is greatly attenuated (reduced).
The inductance of the filter choke opposes changes in the value of the current flowing through it;
