Part 1: AC Voltages and Current: Definitions

So far, we have dealt with direct current, which is abbreviated as DC. DC voltages and currents are characterized by having a fixed value that does not change with time. Strictly speaking, the term DC refers to a voltage that does not change in polarity. An alternating current, or AC, voltage has the characteristic where the polarity periodically changes. When the polarity changes, the current that was flowing out of a terminal begins to flow back into it. One simple way to imagine this, is to think about taking a DC voltage source of the type used do far, and flipping the “+” and “-“ terminals back and forth repeatedly.

What if you have a voltage that varies with time but its polarity doesn’t change? Technically this type of signal is a DC voltage. For lack of a better term, we will call these time variant voltages for now. This is not an official term, it is used here because it is convent. Later we will see that it acts like an AC voltage in most circuits.

The following figure depicts the three types of voltages we have been discussing as graphs. Each graph represents the amplitude of the voltage on the vertical axis and time on the horizontal axis. These plots show how the voltage varies over time.

Figure 1. Voltage characteristics over time.

Although it is frequently possible to describe and AC signal using a few values, in the same way as it is possible to describe a DC voltage with a single value, the use of graphs of this type when working with AC voltages is common. One possible way to produce a figure such as this would be to take a voltmeter, and repeatedly measure the values you get at some point in a circuit. For each value you measure, you write down the time at which the reading was taken. After you have taken a large number of readings, you would plot each voltage you measured (in the vertical axis) against the time it was measured (in the horizontal axis). This of course would only be practical if the voltage changed very slowly and you had a lot of patients. In practice, images like these are produced by an oscilloscope. An oscilloscope, colloquially referred to as a scope, is a very common piece of equipment found in an electronics lab. They display images of the voltage being measured versus time.

The voltage versus time plots shown in Figure 1 depict the voltage’s waveform. The term waveform is used to describe how the voltage looks over time. Conversationally, if you said, “the waveform increases with time”, you are communicating that the amplitude of the signal increases as time goes on.

The AC waveform in Figure 1 is depicted as a sine wave (waveforms with a sinusoidal shape are called sine waves). It does not have to be. The primary feature that makes it an AC waveform, is the fact that it alternates in polarity (the voltage varies from positive to negative) and therefore the current reverses direction periodically. The DC and Time Variant waveform are shown as always being positive. The voltages could be negative as well as long as it was consistently negative. Remember that the term Time Variant is an invented term you probably won’t find elsewhere.