Effective, Peak, and Average Values

Author: J.B. Hoag

The effective value of an alternating current is that number of amperes which will produce heat at the same average rate as that number of amperes of steady direct current flowing through a given resistance. It is the square root of the mean of the instantaneous current values squared, and is also known as the root-mean-square or r.m.s. value. A.C. ammeters and voltmeters are calibrated to read effective values. For sine curves, the effective value I is approximately 70 per cent of the peak or maximum value, as shown in Fig. 4 C. More accurately,

and similarly for voltages.

Fig. 4C. Effective, peak, and average values of sinusoidal currents and voltages

For rectified alternating currents, an average value is used. This is simply the average of all the instantaneous values. For a sine wave, it is given by

I_av = 0.636 I_max= 0.9I.