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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