CAPACITANCE
DC Circuits
Example 3:
Find the total capacitance of three capacitors in parallel, if the values are
15 µF-50 V, 10 µF-100 V, and 3 µF-150 V (Figure 17). What would be the
working voltage?
Figure 17 Example 3 - Capacitors Connected in Parallel
CT
C1
C2
C3
15µ F
10µ F
3µ F
CT
28µ F
The working voltage of a group of
capacitors in parallel is only as high as
the lowest working voltage of an
individual capacitor. Therefore, the
working voltage of this combination is
only 50 volts.
Capacitive Time Constant
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.
TC = RC
(3-13)
where
TC = 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).
ES-03
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