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.

Integrated Publishing, Inc. |