36.8 percent of its maximum value. The capacitor voltage (Curve A)
EER) is then equal to 63.2 percent of the applied voltage. At the
end of two time constants, It and ER are equal to 12.9 percent of
their maximum values, while Ec is equal to 87.1 percent of its
maximum value. The rate of current change decreases rapidly until at
the end of five time constants there is little change in current
flow. Thus, the capacitor charges, and the current and resistor
voltage decrease at a gradual rate. Theoretically, the capacitor
never becomes fully charged, it is normally considered fully charged
at the end of 7 time constants, when it has obtained a charge equal
to 99.9 percent of the applied voltage.
Figure 20. Universal time constant for a series
RC circuit
b. Negative step voltage. When the negative step voltage is
applied, E drops instantly to zero, and the voltage drop across the
resistor is equal to the voltage drop across the capacitor, but is of
opposite polarity. The current is equal to its maximum value, but is
now flowing in the opposite direction. The instantaneous values of
Ec are represented by Curve B in Figure 20.
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