c. At 40 usec, the applied voltage begins to decay. Current
begins to flow in a direction opposite to that of the original
current flow, and causes a small negative voltage to be developed
across the resistor. The rate of discharge of the capacitor quickly
reaches the rate of decrease of the applied voltage, and Ec closely
follows the decay of the pulse.
d. At 60 usec, the applied voltage is equal to zero and the
output voltage Ec is equal to ER but is opposite in polarity. Ec
continues to discharge at the same rate; and at 67 usec, Ec and ER
are equal to zero.
23.
LONGTIME CONSTANT.
a. When the time constant is long compared with the pulse rise,
duration, and decay times, the capacitor charges to only a small
fraction of the total applied voltage. Most of the voltage appears
across R, therefore, the output voltage Ec does not resemble the
input waveform.
b. The pulse shown in Figure 36B is applied to the lowpass RC
filter in Figure 36A. The time constant of the circuit is equal to
100 usec. Since the rise time of the input pulse represents only 10
percent of 1 time constant, the capacitor charges only slightly
during this time, while the current and ER closely follow the rise of
the input pulse.
c. After 10 usec, the applied voltage remains at 10 volts for 30
usec and the capacitor charges slowly toward the applied voltage, at
a rate determined by the universal time constant chart. During this
time the current and ER are decreasing at the same rate.
d. After 40 usec, the applied voltage starts to decay. The
output voltage continues to rise at a reduced rate until E decays to
a value equal to Ec, When E is equal to Ec, the current and ER are
equal to zero. As E decreases toward zero, Ec also decreases, but
follows a more gradual decay curve. After the applied voltage
reaches zero, Ec continues to decay following the capacitor discharge
curve of the universal time constant chart. After 7 time constants
have elapsed, that is, after 760 usec, the output voltage Ec is equal
to zero.
70
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