Complete Electronics Self-Teaching Guide with Projects

Capacitors in AC Circuits

1 An AC signal is continually changing, whether it is a pure sine wave or a complex signal made up of many sine waves. If such a signal is applied to one plate of a capacitor, it will be induced on the other plate. To express this another way, a capacitor will “pass” an AC signal, as illustrated in Figure 6.1.

Figure 6.1

Note Unlike an AC signal, a DC signal is blocked by a capacitor. Equally important is that a capacitor is not a short circuit to an AC signal.

Questions

A. What is the main difference in the effect of a capacitor upon an AC signal versus a DC signal? _____

B. Does a capacitor appear as a short or an open circuit to an AC signal? _____

Answers

A. A capacitor will pass an AC signal, whereas it will not pass a DC voltage level.

B. Neither.

2 In general, a capacitor will oppose the flow of an AC current to some degree. As you saw in Chapter 5, “AC Pre-Test and Review,” this opposition to current flow is called the reactance of the capacitor.

Reactance is similar to resistance, except that the reactance of a capacitor changes when you vary the frequency of a signal. The reactance of a capacitor can be calculated by a formula introduced in Chapter 5.

Question

Write the formula for the reactance of a capacitor. _____

Answer

3 From this formula, you can see that the reactance changes when the frequency of the input ...