The ripple value is given by the percent ratio between the rms value of all harmonics of AC component and the average value of the DC output. The AC component superimposed to DC output voltage can be measured, according to IEC recommendations, by an AC voltmeter indicating the true rms value parallel connected to output DC bars.
Therefore the ripple value is given by:
Where : Vrms = rms value of ripple voltage
VDC = actual output average direct voltage
It must be observed that the ripple value is not constant, but it increases if the output voltage decreases (according to the enclosed diagram).