4-37. There are many different types of filter circuits in use today. We will look some of
the more common ones.
4-38. The simple capacitor filter is the most basic type of power supply filter. The
application of the simple capacitor filter is very limited. It is sometimes used on extremely
high-voltage, low-current power supplies for cathode-ray and similar electron tubes that
require very little load current from the supply. The capacitor filter is also used where the
power-supply ripple frequency is not critical; this frequency can be relatively high. The
capacitor (C1) (see Figure 4-15) is a simple filter connected across the output of the
rectifier in parallel with the load.
4-39. When this filter is used, the RC charge time of the filter capacitor (C1) must be
short and the RC discharge time must be long to eliminate ripple action. In other words, the
capacitor must charge up fast, preferably with no discharge at all. Better filtering also
results when the input frequency is high. Therefore, the full-wave rectifier output is easier
to filter than that of the half-wave rectifier because of its higher frequency.
Figure 4-15. Full-wave Rectifier With a Capacitor Filter
4-40. To help understand the effect that filtering has on Eavg, a comparison of a rectifier
circuit with a filter and one without a filter is shown in Figure 4-16, views (A) and (B). The
output waveforms in Figure 4-16 represent the unfiltered and filtered outputs of the half-
wave rectifier circuit. Current pulses flow through the load resistance (RL) each time a
diode conducts. The dashed line indicates the average value of output voltage. For the half-
wave rectifier, Eavg is less than half (or approximately 0.318) of the peak output voltage.
This value is still much less than that of the applied voltage. With no capacitor connected
across the output of the rectifier circuit, the waveform in view (A) has a large pulsating
component (ripple) compared with the average or DC component. When a capacitor is
connected across the output (view B), the average value of output voltage (Eavg) is
increased due to the filtering action of capacitor C1. A comparison of the waveforms (see
Figure 4-16) shows that the addition of C1 to the circuit results in an increase in the
average of the output voltage (Eavg) and a reduction in the amplitude of the ripple
component (Er), which is normally present across the load resistance.
4-41. The value of the capacitor is fairly large (several microfarads), so it presents a
relatively low reactance to the pulsating current and it stores a substantial charge. The rate
of charge for the capacitor is limited only by the resistance of the conducting diode that is
relatively low. Therefore, the RC charge time of the circuit is relatively short. As a result,
when the pulsating voltage is first applied to the circuit, the capacitor charges rapidly and
almost reaches the peak value of the rectified voltage within the first few cycles. The
capacitor attempts to charge to the peak value of the rectified voltage anytime a diode is
conducting and tends to retain its charge when the rectifier output falls to zero (the
23 June 2005