# Average/Mean and RMS value of AC signal

#### uetonian

Joined Mar 23, 2004
1
Average/Mean and RMS value of AC signal

#### Dave

Joined Nov 17, 2003
6,970
What specifics are you wanting to know?

For a general sinusoidal wave:

RMS = 1/√2 x the amplitude ~ 0.707 x amplitude

Average = 2/π x the amplitude ~ 0.636 x amplitude

For more arbitrary waves further analysis is required. Does this help? If not post back with what specifically is your problem and we'll see what we can do.

#### pawankumar

Joined Oct 28, 2009
42
avg.value of sine is 1/2∏∫sin θ.dθ..how do you get 2/∏.(integration gives 0)?

#### russ_hensel

Joined Jan 11, 2009
825
the average is 0. people usually mean the average of the absolute value.

#### KL7AJ

Joined Nov 4, 2008
2,225
the average is 0. people usually mean the average of the absolute value.

He he! That used to be one of the trick questions on the old FCC radiotelegraph test!

Eric

#### zgozvrm

Joined Oct 24, 2009
115
Average/Mean and RMS value of AC signal

RMS can be thought of as the "DC equivalent" value of the AC signal.

On a graph, it is the "straight-line" value such that the area bounded by it and the X-axis (at the top and bottom) from 0 degrees to 360 degrees is equal to the area bounded by the X-axis and the 2 half-cycles of the sine wave.

#### pawankumar

Joined Oct 28, 2009
42
thanks everyone...this was my first post in this forum.since i am new to electronics,my doubts might seem stupid.please bear with me..they are geniune.
thanks in advance to the senior members and everyone who are going to reply me for my queries
regds,
pawankumar

#### The Electrician

Joined Oct 9, 2007
2,778
RMS can be thought of as the "DC equivalent" value of the AC signal.

On a graph, it is the "straight-line" value such that the area bounded by it and the X-axis (at the top and bottom) from 0 degrees to 360 degrees is equal to the area bounded by the X-axis and the 2 half-cycles of the sine wave.
What you have described is the average of the absolute value of the AC signal, not the RMS value.

#### pawankumar

Joined Oct 28, 2009
42
when both values are more or less equal,why do we need two quantities describing it?and,what is absolute value?(i m weak in math..pls dont mistake me)

#### The Electrician

Joined Oct 9, 2007
2,778
For a sine wave, the average and RMS differ by more than 10%. When making electrical measurements, one should aspire to do much better than that.

For other waveforms, such as the very spiky current drawn by non-powerfactor corrected compact fluorescent lamps, the difference can be much more.

The average absolute value is appropriate when the quantity of charge transported is relevant, such as when charging batteries.

The RMS value is relevant when power is the parameter of interest.

The absolute value of a number is its value without considering sign. Negative numbers become positive and positive numbers remain positive.