alternating components of the signal around the load (to ground). Resistors are used in

place of inductors in low current applications.

4-29. Reviewing the properties of a capacitor we will see that a capacitor opposes any

change in voltage. The opposition to a change in voltage is called capacitive reactance (XC)

and is measured in ohms. The capacitive reactance is determined by the frequency (f) of

the applied voltage and the capacitance (C) of the capacitor. Use the following formula to

make this determination:

1

.159

XC =

or

2 π fC

fC

4-30. You can see from the formula that if frequency or capacitance is increased, the XC

decreases. Since filter capacitors are placed in parallel with the load, a low XC will provide

better filtering than a high XC. To do this, a better shunting effect of the AC around the

load is provided (see Figure 4-10).

4-31. To obtain a steady DC output, the capacitor must charge almost instantaneously to

the value of applied voltage. Once charged, the capacitor must retain the charge as long as

possible. The capacitor must have a short charge time constant. You can do this by keeping

the internal resistance of the power supply as small as possible (fast charge time) and the

resistance of the load as large as possible (slow discharge time). Figure 4-10, view (A),

shows the fast charge time and view (B) shows the slow discharge time.

4-32. Remember learning from basic electricity that a one time constant is defined as the

time it takes a capacitor to charge to 63.2 percent of the applied voltage or to discharge to

36.8 percent of its total charge. This action can be expressed by the following equation:

t = RC