remaining 3.68 volts or 1.36 volts. Using this same method, the
value of It at the end of 30 usec (three time constants) is found to
be 9.5 mA, ER is 9.5 volts, and EL is 0.5 volts. This process may be
repeated for any number of time constants.
b. The period of time required for the current in any RL circuit
to rise to 99.9 percent of the steadystate value is found to be
equal to 7 time constants. If the time constant is 10 usec, 99.9
percent of the steadystate value is reached in 70 usec or 7 time
constants later. If the time constant is 50 usec, 99.9 percent of
the steadystate value is reached in 350 usec, again 7 time constants
later. Regardless of the value of the time constant, 7 time
constants will always represent the time required for the current to
reach 99.9 percent of the steadystate value. When the time
constant is short, the current rises rapidly to its steadystate
value. When the time constant is long, the current rises slowly to
its steadystate value.
EFFECT ON TIME CONSTANT OF VARYING L AND R.
a. Varying L only. Inductance in a circuit prevents the current
from rising immediately to its steadystate value. The larger the
inductance, the greater the opposition to a change in current, and
the longer the period of time required to reach the steadystate
value. Increasing the value of L, therefore, increases the time
constant. Decreasing the value of L decreases the time constant.
b. Varying R only. If the value of the resistance is increased
without varying the inductance, the steadystate is reached in a
shorter period of time. Increasing the value of R decreases the
maximum value of current in the circuit. Since the rate of current
increase remains the same, the steady state will be reached in a
shorter period of time, thus decreasing the time constant of the
circuit. Decreasing the value of R increases the time constant.
c. Effect of varying R and L proportionately. Circuits with the
same time constants require the same period of time to reach the
steadystate condition. For example, if L is equal to 10 mH and R is
equal to 1,000 ohms, the time constant is equal to 10 usec. If the
values of L and R are doubled so that the values increase to 20 mH
and 2,000 ohms respectively, the time constant remains 10 usec. In
both cases, 70 usec or 7 time constants are required to reach the
UNIVERSAL TIME CONSTANT CHART.
a. When a step voltage is applied to a series RL circuit, it is
possible to determine the values of It, ER, and EL through