the use of the universal time constant chart (Figure 27). The
horizontal axis is plotted in terms of time constants. The vertical
axis is plotted in terms of relative voltage or current where 100
percent corresponds to the applied voltage or current. With a
positive step voltage, the rising Curve A represents either the
current It or the voltage ER across the resistor. Curve B represents
the voltage EL across the inductor. For negative step voltages,
Curve B represents It, ER, and EL. However, EL is opposite in
polarity to ER and It. The same TC chart is used to analyze I and E
in RC and RL circuits.
b. The following discussion illustrates how the time constant
chart can be used. In a series RL circuit, if L is equal to 10 mH, R
is equal to 1,000 ohms, and the applied voltage is equal to 1 volt,
then 1 time constant is equal to 10 usec. The current will reach
63.2 percent of its final value at the end of this time (Figure 27).
At the instant E is applied, It and ER are equal to zero and EL is
equal to 100 percent of the applied voltage, or 1 volt. After 1
usec, onetenth of the time constant period has elapsed. At this
time, EL is equal to 90 percent of the applied voltage or 0.9 volt,
ER is equal to 10 percent of the applied voltage or 0.1 volt, and It
is equal to 10 percent of its maximum value or 0.1 mA. The values of
EL, ER, and It in Table I are obtained from the universal time
constant chart.
Table I. Voltage and current values when a positive step
voltage is applied to a series RL circuit
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