across the capacitor increases, the voltage across the resistor

decreases. At the end of two and a half time constants, the

capacitor voltage, as determined by the universal time constant

chart, is equal to approximately 92 percent of the applied voltage,

while the resistor voltage has decreased to approximately 8 percent

of the applied voltage. At this time, the applied voltage falls to

zero, and the capacitor starts to discharge through the resistor.

This causes a negative voltage to be developed across R equal to 92

percent of the maximum applied voltage. The discharge curve is

slightly more gradual than the charge curve for the first pulse. The

reason for this difference is that the capacitor charges from zero

toward E during the pulse duration time (t1), and discharges from 92

percent of E toward zero during the pulse rest time (t2). At the end

of five time constants Ec is equal to approximately 7 percent of the

maximum value of E, while ER is equal to approximately 7 percent of

E. At this time, another positive pulse is applied, and the process

is repeated.

(3) Now consider the output waveshapes when a square wave is

applied to an RC circuit with a comparatively shorttime constant

(Figure 25). In this case, the time constant is equal to 0.02 of a

frequency cycle. Thus, in 20 percent of a half cycle (duration

time), five time constants occur. As a result, the capacitor charges

very quickly to the maximum applied voltage. The early rise of Ec to

the full, applied voltage and its rapid decrease to zero when E falls

to zero cause the voltage waveform across the capacitor to resemble

the squarewave input. The short duration of the high charging and

discharging currents affect the resistor voltage quite differently.

The rapid rise and drop of ER cause the voltage waveform across the

resistor to be peaked at each squarewave changeover (change from

maximum voltage to zero or from zero to maximum). The amplitude of

each curve, at any instant of time, can be readily determined from

the universal time constant chart (Figure 20).

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