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, 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.

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 put

The current goes one way. Polarity matters: swap the leads and the meter shows a minus sign.

AC: 120 V from the wall, 60 Hz

Sixty full cycles each second. There is no fixed + and −; the two wires swap roles 120 times a second.

Period = 1 s / 60 = 0.0167 s

One cycle takes about 17 thousandths of a second. That is the flicker nobody can see.

Peak = 120 V × 1.414 = 170 V

The 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

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.

Practise AC and DC on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.