A square wave of 10 V peak is connected to an analogue multimeter that is set to measure altern

step 1 31 .52. So we have a volt meter with a large impedance like volt meters are supposed to have, an

A square wave of 10 V peak is connected to an analogue...

Key Concepts

Waveform Shape Dependency

For a square wave, the situation is different because the rectified average value is equal to the peak value. When the analog meter applies the sine wave calibration factor of 1.11 to this average, it produces an incorrect reading—in the case of a 10 V peak square wave, the meter will display approximately 11.1 V, reflecting a miscalculation introduced by the assumption of a sinusoidal waveform.

Analog AC Voltage Measurement

Analog AC voltmeters typically use a rectifier to convert the alternating current into a unidirectional (DC) signal. Then they measure the average value of the rectified voltage, which is later scaled by a fixed factor to display what is assumed to be the RMS value of a sine wave. This design means that the measurement is inherently based on the assumption that the waveform is sinusoidal.

Rectification and Averaging

The rectification process converts the AC signal, which alternates in polarity, into a signal that only maintains positive values. The meter averages these values over time. For a sine wave, the average of the rectified waveform is about 0.637 times the peak value, and by applying a scaling factor (approximately 1.11), the instrument displays the RMS voltage.

RMS and Average Value Conversion

The conversion factor used by analog AC voltmeters is predicated on the known relationship between the RMS value and the average of the rectified sine wave. Specifically, for a sine wave, multiplying the average by 1.11 gives the correct RMS voltage. However, if the wave shape deviates from a sine wave, this conversion no longer matches the actual RMS value.

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