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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