Lessons · Electrical · AC and DC
DC goes one way; AC turns around sixty times a second
Direct current flows steadily in one direction, like a battery. Alternating current reverses over and over; in North America it completes 60 cycles every second, and the 120 V on the nameplate is its effective value -- the RMS, or root mean square -- not its peak.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed electrician. Practice material, not professional advice. What that means.
What it is for
A meter on the DC volts setting reads zero on a live AC wire. That is how a technician decides a panel is dead when it is not. Knowing which kind you are measuring is a safety fact before it is a physics fact.
How to think about it
Ask where the power comes from. Batteries, solar panels and the low-voltage side of most electronics are DC. Anything from the utility is AC. Set the meter to match before you touch a probe to anything, and treat every circuit as live until you have tested it.
Worked example
DC: 12 V from a battery, + and − stay putThe current goes one way. Polarity matters: swap the leads and the meter shows a minus sign.
AC: 120 V from the wall, 60 HzSixty full cycles each second. There is no fixed + and −; the two wires swap roles 120 times a second.
Period = 1 s / 60 = 0.0167 sOne cycle takes about 17 thousandths of a second. That is the flicker nobody can see.
Peak = 120 V × 1.414 = 170 VThe 120 V is the RMS value, the steady DC voltage that would heat a load the same amount. The wave actually swings to about 170 V at its top.
Your turn
Write the peak of a 240 V AC supply, using 1.414 for the square root of two.
Peak = 240 V × 1.414 = V
Solve one, graded on the server
The trap
Reading 0 V on a live AC circuit because the meter is on DC volts. The meter is not lying; it is answering a different question. Check the dial before you believe a zero.