Tab Electronics Guide to Understanding Electricity ... - Sciences Club

More About Capacitors and Inductors
381
age from the power transformer secondary is rectified, converting it to a
120-hertz pulsating DC waveform (provided the rectifier is a full-wave rectifier).
This pulsating DC rectifier output is applied to a filter capacitor,
which charges to the peak of the DC voltage and converts the pulsating
DC voltage to a smooth, continuous level of DC voltage. Assuming that
you were utilizing a 1000-F filter capacitor in a hypothetical power
supply, consider the capacitive reactance (X C
) of the filter capacitor to the
AC component of the pulsating DC:
1
1
1
X C
1.32 ohms
6.28 (120 Hz) (1000 F) 6.28 (120) (0.001) 0.7536
The previous calculation shows that the power supply filter capacitor
exhibits 1.32 ohms of “reactive opposition” (i.e., capacitive reactance) to
the 120-hertz AC component of the pulsating DC applied across it. Of
course, the pulsating DC coming from the rectifier also contains a DC
component. The frequency of DC is zero. Therefore, calculating the capacitive
reactance of the DC component, we have
1
1 1
X C
∞
6.28 (0 hertz) (1000 F) 6.28 (0) (0.001) 0
(where ∞ is the symbol for infinity). The previous two calculations show
that a 1000-F capacitor exhibits only 1.32 ohms of capacitive reactance
to a 120-hertz AC frequency, but exhibits an infinite resistance to DC
(remember, a good capacitor cannot pass DC, so its opposition to DC
would have to be infinite). Another way of stating this same phenomenon
would be to say that the filter capacitor allows the low-frequency component
(DC) to pass to the power supply load, but blocks the AC component
(120-hertz ripple) from passing on to the load. The AC component is
“blocked” in the sense that it is shorted to circuit common through the
filter capacitor, which, for all practical purposes, attenuates it to such a
low level that it can be considered negligible. (If any of this is confusing,
you should review the material contained in Chapter 5.)
It can be accurately stated that filter capacitors in DC power supplies
are low-pass filters; passing the zero-frequency DC on to a load while
blocking the higher-frequency AC ripple component. As a matter of convention,
however, they are not normally regarded from that perspective.
Figure 15-7a illustrates a common method of designing a low-pass
filter. A resistance (R) is placed in series with the input signal and output
signal. A capacitor (C) is placed in parallel with the output signal,