4-34
Figure 4-29.—LC filter and waveforms.
C1 reduces the ripple to a relatively low level (view B). L1 and C2 form the LC filter, which reduces
the ripple even further. L1 is a large value iron-core induct (choke). L1 has a high value of inductance an
therefore, a high value of XL which offers a high reactance to the ripple frequency. At the same time, C2
offers a very low reactance to ac ripple. L1 and C2 for an ac voltage divider and, because the reactance of
L1 much higher than that of C2, most of the ripple voltage is dropped across L1. Only a slight trace of
ripple appears across C2 and the load (view C).
While the L1-C2 network greatly reduces ac ripple it has little effect on dc. You should recall that an
inductor offers no reactance to dc. The only opposition to current flow is the resistance of the wire in the
choke. Generally, this resistance is very low and the dc voltage drop across the coil is minimal. Thus, the
LC filter overcomes the disadvantages of the RC filter.
Aside from the voltage divider effect, the inductor improves filtering in another way. You should
recall that an inductor resists changes in the magnitude of the current flowing through it. Consequently,
when the inductor is placed in series with the load, the inductor maintains steady current. In turn, this
helps the voltage across the load remain constant when size of components is a factor.
The LC filter provides good filtering action over a wide range of currents. The capacitor filters best
when the load is drawing little current. Thus, the capacitor discharges very slowly and the output voltage
remains almost constant. On the other hand, the inductor filters best when the current is highest. The
complementary nature of these two components ensures that good filtering will occur over a wide range
of currents.
The LC filter has two disadvantages. First, it is more expensive than the RC filter because an
iron-core choke costs more than a resistor. The second disadvantage is size. The iron-core choke is bulky
and heavy, a fact which may render the LC filter unsuitable for many applications.
