AC vs DC
DC, direct current, flows steadily in one direction, like the output of a battery or a USB port. AC, alternating current, reverses direction periodically; wall power swings back and forth 50 or 60 times a second. Nearly everything on a circuit board runs on DC internally, even when the device plugs into an AC outlet.
AC won the power grid for one reason: transformers. A transformer can step AC voltage up or down with very little loss, but it only works with changing current. Utilities transmit at hundreds of thousands of volts because high voltage means low current for the same power, so thin cables waste little energy as heat. The voltage gets stepped down near your house. When the grid was built, there was no efficient way to do any of that with DC.
One subtlety worth knowing: “120 V AC” is an RMS figure, an effective average, while the waveform actually peaks near 170 V. That matters when you pick parts that touch mains. For board-level work the takeaway is simpler. A power supply’s first job is turning wall AC into the clean low-voltage DC your circuit actually runs on.
Key points
- DC flows one way at a steady level. AC reverses direction periodically (50/60 Hz for mains).
- AC dominates power distribution because transformers can change its voltage efficiently.
- Boards run on DC internally; converting AC to DC is the power supply’s job.
- AC voltages are usually quoted as RMS. The instantaneous peak is about 1.4× higher.
Practice
0 of 2 answered · Not startedWrong answers just let you try again, and hints are there if you want them. Answering every question first time, without hints, is what earns mastery.
- 1
Why is household mains quoted as 120 V when its peak is closer to 170 V?
- 2
What defines the frequency of an AC waveform?
