MM0704, Lesson 1
From the Universal Time Constant Chart, you can see that a capacitor cannot charge instantaneously. The voltage
across the capacitor at time to then is zero. Hence, all of the applied voltage appears across the resistor.
The values of ER and EC at time to are shown on their respective wave shapes in figure 1-21. At the instant a DC
voltage is applied to a series RC circuit, the voltage drop across the capacitor is zero, and all of the applied voltage
appears across the resistor.
At time T1, just before the input drops to zero, the DC voltage has been applied for 2,000 sec. This has given the
capacitor time to charge exponentially to some voltage value. With a voltage other than zero across the capacitor, the
voltage across the resistor will be less than the initial voltage. The value of ER, at time T1, is found to be
time for one TC = (0.02) (20K) = 400 sec.
Voltage is applied for 2,000 sec at T1; therefore, there are 5 TCs at T1. Thus, the capacitor is fully charged (50V), the
current has ceased to flow,
Figure 1-21. Wave Shapes for Circuit in Figure 1-20.