Alternating Current (AC)

Objectives

  • Define Alternating Current (AC)
  • Explain AC timing parameters
  • Define AC amplitude parameters
  • Explain the meaning of root mean square

The Basics

DC Current
AC Current

Unlike a direct current (DC), which flows in only one direction, alternating current (AC) flows one way, then the other way, constantly changing in magnitude and reversing direction.

As result an AC voltage is also continually changing (alternating) between positive (+) and negative (-) polarities, and varying in amplitude with time.

When plotted over time, an AC signal takes the shape of a sine wave, crossing the zero line every time the direction of current flow is changed.

An AC signal does not always take the shape of a sine wave.
For example, an AC waveform may be in the shape of a triangle or square wave.

Period and Frequency

A cycle is a single repetition of "back and forth" alternating current flow.

The time it takes for one complete cycle of the AC signal is called the period.

The unit of measurement for the period is seconds (s).

The frequency of alternating current signal is the number of cycles in a single second.

Frequency is measured in hertz (Hz).

In North America, the power line frequency is 60 cycles per second, or 60 Hz.

In the rest of the world 50 Hz is more common.

And if we know the frequency we can go back and calculate the period.

For example, using 60 Hz...

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