When a capacitor is connected to a DC voltage source, it charges very rapidly. If no resistance was present in the charging circuit, the capacitor would become charged almost instantaneously. Resistance in a circuit will cause a delay in the time for charging a capacitor. The exact time required to charge a capacitor depends on the resistance (R) and the capacitance (C) in the charging circuit. Equation (3-13) illustrates this relationship.

T_C = RC (3-13)

where

T_c= capacitive time constant (sec)

R = resistance (ohms)

C = capacitance (farad)

The capacitive time constant is the time required for the capacitor to charge to 63.2 percent of its fully charged voltage. In the following time constants, the capacitor will charge an additional 63.2 percent of the remaining voltage. The capacitor is considered fully charged after a period of five time constants (Figure 18).

Figure 18 Capacitive Time Constant for Charging Capacitor

The capacitive time constant also shows that it requires five time constants for the voltage across a discharging capacitor to drop to its minimum value (Figure 19).

Figure 19 Capacitive Time Constant for Discharging Capacitor

Example: Find the time constant of a 100 F capacitor in series with a 100 resistor (Figure 20).

Figure 20 Example - Capacitive Time Constant

Summary

The important information on capacitors is summarized below.

Capacitance Summary

A capacitor is constructed of two conductors (plates) separated by a dielectric.

A capacitor will store energy in the form of an electric field caused by the attraction of the positively-charged particles in one plate to the negativelycharged particles in the other plate.

The attraction of charges in the opposite plates of a capacitor opposes a change in voltage across the capacitor.

Capacitors in series are combined like resistors in parallel.

Capacitors in parallel are combined like resistors in series.

The capacitive time constant is the time required for the capacitor to charge (or discharge) to 63.2 percent of its fully charged voltage.