Q26. What is the most commonly used filter today?
Q27. What are the two main disadvantages of an LC capacitor filter?
FALURE ANALYSIS OF THE LC CAPACITOR-INPUT FILTER.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 which 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 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 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 receiver, the
load on a particular power supply may depend on the brightness of the screen, the control settings, or even
the channel selected.
These variations in load resistance tend to change the applied dc voltage because the power supply
has a fixed internal resistance. If the load resistance decreases, the internal resistance of the power supply
drops more voltage. This causes a decrease in the voltage across the load.
Many circuits are designed to operate with a particular supply voltage. When the supply voltage
changes, the operation of the circuit may be adversely affected. Consequently, some types of equipment
must have power supplies that produce the same output voltage regardless of changes in the load
resistance or changes in the ac line voltage. This constant output voltage may be achieved by adding a
circuit called the VOLTAGE REGULATOR at the output of the filter. There are many different types of
regulators in use today and to discuss all of them would be beyond the scope of this chapter.
A commonly used FIGURE OF MERIT for a power supply is its PERCENT OF REGULATION.
The figure of merit gives us an indication of how much the output voltage changes over a range of load
resistance values. The percent of regulation aids in the determination of the type of load regulation
needed. Percent of regulation is determined by the equation: