Kc9EYE AC Components and Values

The four terms that describe the ac component are:

1. Maximum Value
2. Instantaneous Value
3. Average Value
4. RMS Value

Maximum Value
This is the highest positive value reached in the cycle. It is sometimes called peak value. Most voltmeters cannot measure this value. It requires a peak reading rectifier/amplifier type voltmeter to measure. Most electronic voltmeters these days measure something close to RMS voltage, and peak voltage must be estimated from calculation. Many electronic circuits are adjusted on the basis of the maximum value of the AC signal.

Instantaneous Value
This is the value at any selected instant during the cycle. For a sine wave:
ε = E_max × sinθ
ι = I_max × sinθ
where:

• ε is the instantaneous voltage
• ι is the instantaneous current
• E_max is the maximum voltage of the signal
• I_max is the maximum current of the siganl
• θ is the phase angle of the instant that you want to solve for

Average Value
This is the simple average (arithmetic mean) of all the instantaneous values in one cycle, disregarding sign.
E_avg = 0.637 × E_max
I_avg = 0.637 × I_max
This is the value of voltage that most amplifier/rectifier type electronic voltmeters respond to. It is also the value of voltage delivered by an unfiltered full wave rectifier.

RMS Value
This is the root mean square value. It is also called the effective value, since this is the AC value that would cause the same amount of heat to be disipated in a resistor as an equivalent DC value. So, if a DC voltage of 120V is applied to a resistor and the heat disipated by that resistor is 100°, then an AC voltage applied to that same resistor that produced a heat disipation of 100° would be 120VAC_rms.
E_rms = 0.707 × E_max
I_rms = 0.707 × I_max
The rms value is the one which most AC voltmeters and ammeters read, whether or not they actually respond to this value. The widely used rectifier type meter, for example is average responsive. But, its scale reads in the more useful rms units.