4-84. While the L1-C2 network greatly reduces AC ripple, it has little affect on DC.
Remember 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. Therefore, the LC filter overcomes the
disadvantages of the RC filter.
4-85. Aside from the voltage divider effect, the inductor improves filtering in another
way. Remember 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.
4-86. The LC filter provides good filtering action over a wide range of currents. The
capacitor filters best when the load is drawing little current. So, the capacitor discharges
very slowly and the output voltage remains almost constant. However, 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.
4-87. The two disadvantages of the LC filter are that it is more expensive than the RC
filter (because an iron-core choke costs more than a resistor) and of its size. The iron-core
choke is bulky and heavy, which usually makes the LC filter unsuitable for many
4-88. Shunt capacitors are subject to open circuits, short circuits, and excessive leakage.
Series inductors are subject to open windings and occasionally shorted turns or a short
circuit to the core.
4-89. The input capacitor (C1) has the greatest pulsating voltage applied to it, is the most
susceptible to voltage surges, and has a generally higher average voltage applied. As a
result, the input capacitor is frequently subject to voltage breakdown and shorting. The
output capacitor (C2) is not as susceptible to voltage surges because of the series protection
offered by the series inductor (L1). However, the capacitor can become open, leaky, or
4-90. A shorted capacitor, an open filter choke, or a choke winding which is shorted to
the core, results in a no-output indication. A shorted capacitor (depending on the
may result in a blown fuse in the primary of the transformer. An open filter choke results in
an abnormally high DC voltage at the input to the filter and no voltage at the output of the
filter. A leaky or open capacitor in the filter circuit results in a low DC output voltage. This
condition is generally accompanied by excessive ripple amplitude. Shorted turns in the
winding of a filter choke reduce the effective inductance of the choke and decrease its
filtering efficiency. As a result, the ripple amplitude increases.
23 June 2005