ELECTRONIC PRINCIPLES - OD1647 - LESSON 1/TASK 1
specific value. As shown in figure 21, one L,/R time constant
is the time required for the current in an inductor to increase
to 63% (actually 63.2%) of the maximum current. Each time
constant is equal to the time required for the current to
increase by 63.2% of the difference in value between the current
flowing in the inductor and the maximum current. Maximum
current flows in the inductor after five L/R time constants are
completed. The following example should clear up any confusion
about time constants. Assume that the maximum current in an LR
circuit is 10 amperes. As you know, when the circuit is
energized, it takes time for the current to go from zero to 10
amperes.
FIGURE 21. L/R TIME CONSTANT.
(a) When the first time constant is completed, the current in
the circuit is equal to 63.2% (.632) of 10 amperes. Thus the
amplitude of current at the end of 1 time constant is 6.32
amperes.
(b) During the second time constant, current again increases
by 63.2% (.632) of the difference in value between the current
flowing in the inductor and the maximum current. This
difference is 10 amperes minus 6.32 amperes, and equals 3.68
amperes; 63.2% of 3.68 amperes is 2.32 amperes. This increase
in current during the second time constant is added to that of
the first time constant. Thus, upon completion of the second
time constant, the amount of current in the LR circuit is 6.32
amperes + 2.32 amperes = 8.64 amperes.
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