Where:

R represents the resistance of the charge or discharge path.

C represents the capacitance of the capacitor.

A capacitor is considered fully charged after five RC time constants (see Figure 4-11). You

can see that a steady DC output voltage is obtained when the capacitor charges rapidly and

discharges as slowly as possible.

4-33. In filter circuits the capacitor is the common element to both the charge and the

discharge paths. Therefore, to obtain the longest possible discharge time, you want the

capacitor to be as large as possible. Another way to look at it is that the capacitor acts as a

short circuit around the load (as far as the AC component is concerned) and since the larger

following formula).

1

XC =

2πfC

4-34. Let us look at inductors and their application in filter circuits. Remember, AN

INDUCTOR OPPOSES ANY CHANGE IN CURRENT. A change in current through an

inductor produces a changing electromagnetic field. The changing field, in turn, cuts the

windings of the wire in the inductor and thereby produces a counterelectromotive force. It

is the cemf that opposes the change in circuit current. Opposition to a change in current at a

inductor is determined by the applied frequency and the inductance of the inductor (see the

following formula).

XL = 2 *π *f C