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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