Therefore, the RC charge time constant is short compared to its discharge time. Figure

4-20, views (A) and (B) shows the comparison in RC charge and discharge paths.

Consequently, when the pulsating voltage is first applied to the LC choke-input filter, the

inductor (L1) produces a cemf that opposes the constantly increasing input voltage. The net

result is to effectively prevent the rapid charging of the filter capacitor (C1). 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

(B) shows that C1 will only partially discharge 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.

4-53. 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 (see

Figure 4-21, views (A) and (B)). As far as the ripple component is concerned, the inductor

offers high impedance (Z) and the capacitor offers low impedance (view B). As a result,

(reduced). The inductance of the filter choke opposes changes in the value of the current

flowing through it. Therefore, the average value of the voltage produced across the

capacitor contains a much smaller value of ripple component (Er) than the value of ripple

produced across the choke.

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