MM0704, Lesson 1
A smooth curve connecting points A through F follows the capacitor charge and shows all the intermediate percentages
of the applied voltage found across the capacitor at both whole and fractional time constants.
You can find the voltage across the capacitor for 1.5 TCs on the graph. On the X axis find 1.5 TCs. From this point
move vertically up to the curve for the capacitor charge. Call this point Z. Now, move horizontally from point Z to the
Y axis where the percentage of applied voltage to which the capacitor has charged is given. The capacitor will have
charged to 78 percent of the applied voltage. Since the applied voltage in the circuit of figure 1-16 is 100 V, the
voltage across the capacitor for 1.5 TCs is 0.78 x 100V = 78V.
You can also find the voltage across a capacitor after any number of time constants. You can find the number of time
constants in any given time by dividing the product of R x C into the time, thus,
Figure 1-18 is a more accurate graph than the one in figure 1-17. It is used for computations in series RC circuits.
Known as a Universal Time Constant Chart, you can use it to determine EC, ER, and I as accurately as possible without
calculations. This chart, in addition to being accurate, provides curves for the capacitor voltage on charge (curve A),
and also for the resistor voltage on charge (curve B). On the graph of figure 1-18, you can also use curve B to
determine the capacitor discharge voltage and curve A the resistor voltage on discharge.
On the chart, the numbers 1 through 5 along the horizontal axis represent the number of time constants rather than
specific units of time. This means the graph can be used for computations in any series RC circuit regardless of the
length of time represented by each time constant.
Figure 1-18. Universal Exponential Curves for RC and RL Circuits.