Mains-THD meter

The circuit described, in conjunction with a digital voltmeter (DVM), may be used to measure the total harmonie distortion (THD) of the mains supply voltage. A knowledge of this may be useful when the effect of switch-mode supplies or dimmers on the mains supply is to be determined. This is important, because the same mains supply is used to power, say, an audio system of which low distortion is expected.