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AC, DC and waveforms
Sine, square and triangle; peak, peak-to-peak and RMS.
Sine, square and triangle; peak, peak-to-peak and RMS.
Direct current (DC) flows one way at a steady level, like a battery. Alternating current (AC) swings back and forth, regularly reversing direction. The picture of voltage plotted against time is its waveform. A radio signal in an antenna is simply AC at a very high frequency.
The shape tells you what a signal does. A sine wave is the purest: one frequency, nothing else, and what mains power and a clean radio carrier look like. A square wave is a switch snapping on and off, as in digital circuits and clocks, and it contains many harmonics, whole-number multiples of its frequency. A rectified wave is AC with the negative half flipped over: it is no longer alternating, but it is not steady either, which is why power supplies filter it.
Frequency counts cycles in a second; the period is the time one cycle takes, so T = 1 ÷ f. US mains at 60 Hz has a period of 1 ÷ 60 = 16.7 ms. A 14.2 MHz signal repeats every 1 ÷ 14 200 000 = 70.4 ns. Mains frequency and voltage vary by country: much of the world uses 230 V at 50 Hz. Frequency and its link to wavelength continue in Frequency and wavelength.
An AC voltage keeps changing, so there is more than one way to quote it:
Power in a resistor goes as the voltage squared, so it is never negative and swings from zero up to its peak. A sine wave spends enough time near zero that the average is exactly half the peak power. Half the power means 1 ÷ √2 of the voltage: RMS = 0.707 × peak. Mains "120 V" is an RMS figure; its peak is 120 × 1.414 = 170 V and peak-to-peak is 339 V.
These factors only hold for a sine. For other shapes they differ: a square wave's RMS equals its peak, and a triangle's is 0.577 × peak. Cheap meters assume a sine wave when they convert their reading, so they misread other shapes; a true-RMS meter does the maths properly. See Multimeters.