Real World Instrumentation with Python

AC Circuits

As mentioned earlier, current flow that changes direction over time is called alternating current, or AC. AC is a bit more complex than DC. As well as the expected characteristics of voltage and current, it has the additional characteristics of frequency and phase.

When talking about the power wiring in a house, for example, one would expect to hear terms such as “AC,” “AC voltage,” or “AC current” (which is somewhat redundant). These terms typically refer to the electrical power type in general, the voltage in the circuit, and the current in the circuit, respectively. However, when referring to the low-voltage, low-current AC typically found in instrumentation circuits, the common term is just “signal” or “AC signal.”

Sine Waves

AC signals can occur with any one of a number of types of waveforms, but the sine wave is the prototypical AC signal. A sine wave is “pure”; that is, it is composed of just one frequency. Other waveforms can be decomposed into a series of sine waves by means of Fourier analysis techniques (which we won’t delve into here), but a pure sine wave cannot be decomposed any further.

A generic sine wave is shown in Figure 2-14. The sine wave gets its name because mathematically it is defined by the sine function:

V(t) = A sin(2πft + θ)

where A is the amplitude, f is the frequency, t is time, and θ is the phase. Sometimes one might see this form:

V(t) = A sin(ωt + θ)

where ω, the angular frequency, is actually just 2πf.

Figure 2-14. Generic sine wave

A fundamental ...