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 tends to hold the current steady. This, in
turn, helps to hold the voltage across the load constant.
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 components ensures good filtering over a wide range of current when size
of components is a factor.
The LC filter has two disadvantages. The first is cost. The LC filter is more expensive than the RC
filter because its iron-core choke costs more than the resistor of the RC filter. The second disadvantage is
size, since the iron-core choke is bulky and heavy. Thus, the LC filter may be unsuitable for some
applications but is still one of the most widely used.
Q30. What is the most commonly used filter in use today?
Q31. What are the two main disadvantages of an LC capacitor filter?
Several problems may cause the LC capacitor filter to fail. 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.
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), but the capacitor can
become open, leaky, or shorted.
A shorted capacitor, an open filter choke, or a choke winding that is shorted to the core, results in a
no-output indication. A shorted capacitor, depending on the magnitude of the short, may cause a shorted
rectifier, transformer, or filter choke and 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 an 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.
Ideally, the output of most power supplies should be a constant voltage. Unfortunately, this is
difficult to achieve. There are two factors that can cause the output voltage to change. First, the ac line
voltage is not constant. The so-called 115 volts ac can vary from about 105 volts ac to 125 volts ac. This
means that the peak ac voltage to which the rectifier responds can vary from about 148 volts to 177 volts.
The ac line voltage alone can be responsible for nearly a 20 percent change in the dc output voltage.
The second factor that can change the dc output voltage is a change in the load resistance. In
complex electronic equipment, the load can change as circuits are switched in and out. In a television