c. After 10 usec, the applied voltage remains constant for 30
usec. During this time, the inductor continues to oppose any change
in current flow. However, the current and the voltage drop across
the resistor continue to increase at a rate determined by the
universal time constant chart. As ER increases, EL decreases; and at
40 usec, approximately 70 percent of the applied voltage appears
across the inductor and 30 percent is developed across the resistor.
d. After 40 usec, the applied voltage starts to decay and the
inductor opposes any change in current flow. As E decreases, EL
decreases proportionally until 60 usec, when the applied voltage is
equal to zero, ER and the output voltage EL are equal to
approximately 3 volts (30 percent of the applied voltage), but are
opposite in polarity. After this time, ER and EL decay to zero at a
rate determined by the universal time constant chart.
Section VI. LOWPASS RL CIRCUIT
27.
GENERAL.
The output of a lowpass RL filter is taken across the
resistor. The effect of the time constant on the output waveform
differs entirely from that of a highpass RL filter. When the time
constant of a lowpass RL filter is short compared with the reference
period, the output waveform closely resembles the input waveform.
When the time constant of a lowpass RL filter is long compared to
the reference period, the output waveform is only a fraction of the
input waveform.
28.
SHORTTIME CONSTANT.
a. The pulse shown in Figure 39B is applied to the lowpass RL
filter in Figure 39A. The time constant is equal to 1 usec, which is
short compared to the pulse rise (10 usec), duration (30 usec), and
decay (20 usec) times. Since the time constant of this circuit is
short compared with the pulse rise time, the current and, therefore,
the output voltage across the resistor can increase almost as rapidly
as the input voltage increases, while EL levels off at a low value.
b. At 10 usec, the applied voltage reaches the maximum value of
20 volts and remains constant for 30 usec. The rate of current
increase remains the same until the output voltage ER is equal to E.
ER is considered to be equal to E after 11 usec. EL
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