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# RMS or Effective Voltage

August 2nd Fact-of-the-Day

The amount of power being delivered to a DC circuit load can be calculated easily by dividing the load resistance into the applied DC voltage squared. However, power calculation is more complicated in an AC circuit, because the voltage level continually changes throughout each AC cycle. Power calculation based on peak AC voltage would be too high, because peak positive and negative voltages are each reached only for an instant during each cycle. Power calculation based upon a voltage midway between one peak and zero also would be wrong, because AC voltage generally changes sinusoidally from zero to peak, rather than linearly. The voltage value that gives the correct power result is called the 'Effective Voltage,' because it is has the same effect on a power calculation as does a DC voltage of the same value. Effective Voltage is equal to the square root of the mean value of the squares of all the instantaneous values of an AC voltage. Because of that, Effective Voltage is also known as the Root Mean Square or RMS Voltage. ©2005 Martek International All rights reserved.