Apprentice path · Stage 3: The theory you cannot fake later · Lesson 11

AC, frequency, and why it is not DC

DC pushes one way and stays there. AC keeps swapping which way it pushes, many times a second.

Why this matters on the job

Nearly everything you will work on is AC, and the meter setting, the test and the maths all change depending on which one you are looking at.

How to think about it

  1. AC: the voltage changes polarity, and in a resistive circuit the current reverses direction with it.
  2. DC: one polarity, current in one direction.
  3. Frequency is just how many complete cycles happen per second.
  4. Set your meter to match -- AC volts and DC volts are different settings.

Worked through

60 Hz means 60 complete cycles every second. Turn that around and one cycle takes one sixtieth of a second, which is about 16.7 milliseconds. That is the number behind the phrase "sixty cycle": the wave goes positive, comes back through zero, goes negative, and returns, sixty times in the time it takes to say the word.

Check yourself: A supply runs at 60 Hz. Roughly how long is one complete cycle?

Answer: About 16.7 milliseconds -- one sixtieth of a second.

Frequency is cycles per second, so one cycle takes 1 divided by the frequency. 1 / 60 = 0.0167 seconds, which is 16.7 ms.

Watch out for: Steady does not mean DC. A DC level can rise and fall without ever reversing direction -- that is still DC. The thing that makes it AC is reversing, not changing.
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Written from the owner's poster for this lesson. Nothing added beyond what the poster states.

Written for Texas apprentices as free study material. The Texas apprentice licence is an application, not an exam (Occupations Code 1305.161), and nothing here counts as apprenticeship hours or continuing-education credit. VoltMark is not affiliated with, endorsed by, or sponsored by NFPA, PSI, TDLR, or any state or local licensing authority. Always follow your employer's safety programme and your supervising electrician.