 - Also see peak and peak-to-peak.
In a direct current (DC) circuit, voltage or current is simple to define, but in an alternating current (AC) circuit, the definition is more complicated, and can be done in several ways. Root-mean-square (rms) refers
to the most common mathematical method of defining the effective voltage or current of an AC wave.
To determine rms value, three mathematical operations are carried out on
the function representing the AC waveform:
(1) The square of the waveform function (usually a sine wave) is
determined.
(2) The function resulting from step (1) is averaged over time.
(3) The square root of the function resulting from step (2) is found.
In a circuit whose impedance consists
of a pure resistance, the rms value of an AC
wave is often called the effective value or DC-equivalent value.
For example, if an AC source of 100 volts rms is connected across a resistor, and the
resulting current causes 50 watts of heat to be dissipated by the resistor, then 50 watts
of heat will also be dissipated if a 100-volt DC source is connected to the resistor.
For a sine wave, the rms value is 0.707 times the peak value, or 0.354 times the
peak-to-peak value. Household utility voltages are expressed in rms terms.? A
so-called "117-volt" AC circuit carries about 165 volts peak (pk), or 330 volts
peak-to-peak (pk-pk).
| LAST UPDATED: |
31 Jul 2001
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