198 Electrical Power Systems Technology
2.4 × 950 mA
= ——————
0.5 V
C = 4550 uF
It should be pointed out that as the value of C increases, the value of
the peak current through the diodes will also increase. There is, therefore,
a practical limit for determining the value of C that is reflected in the fore-
going equation.
The capacitor filter produces a high DC voltage with low AC ripple
for light loads. However, its major disadvantages are higher ripple and
lower Vdc at heavier loads, poor voltage regulation, and high peak cur-
rent through the diodes.
RC Filter
It is possible to improve upon the previous filter circuit by using an
RC filter. Figure 7-29 shows an RC filter stage. This filter has lower ripple
than a capacitor filter, but has a lower average DC voltage, because the
voltage is dropped across R1.
The purpose of R1 and C2 is to add another filter network, in addi-
tion to C1, to further reduce the ripple. This circuit also operates best with
light loads connected. It is possible to have many stages of RC filters to
further reduce AC ripple.
Pi- or π-type Filter
The use of the resistor (R1) in the RC filter is not desirable in many
cases, since it reduces the average DC output of the circuit. To compensate
for the DC voltage reduction, a pi-type (or π-type) filter may be used. Figure
7-30 shows this type of filter. The advantage of the choke coil (L1) over
the resistor (R1) of the RC filter is that it offers only a small DC series re-
Figure 7-29. An RC filter circuit