Chapter 8 Tutorial

Measurement Fundamentals

j td bWc tyw °”u

measure the “heating” potential of an applied voltage. Unlike an “average responding” measurement, a true bWc measurement is used to determine the power dissipated in a resistor. The power is proportional to the square of the measured true bWc voltage, independent of waveshape. An average responding ac multimeter is calibrated to read

the same as a true bWc meter for z. For other waveform shapes, an average responding meter will exhibit substantial errors as shown below.

The internal MWW’s ac voltage and ac current functions measure the ac-coupled true bWc value. This is in contrast to the ac+dc true bWc value shown above. Only the “heating value” of the ac component of the input waveform is measured (dc is rejected). For sinewaves, triangle waves, and square waves, the ac and ac+dc values are equal since these waveforms do not contain a dc offset. Non-symmetrical waveforms, such as pulse trains, contain dc voltages which are rejected by ac-coupled true bWc measurements.

An ac-coupled true bWc measurement is desirable in situations where you are measuring small ac signals in the presence of large dc offsets. For example, this situation is common when measuring ac ripple present on dc power supplies. There are situations, however, where you might want to know the ac+dc true bWc value. You can determine this value by combining results from dc and ac measurements as shown below. You should perform the dc measurement using at least 10 power line cycles of integration (678 digit mode) for best ac rejection.

ac + dc = √‾‾‾‾‾‾‾‾ac 2 + dc2

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