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mayeewa · 28th September 2008, 04:45 AM

Why are the square wave RMS readings using DMM different from the sine wave RMS readings using DMM ?

Mikebits · 28th September 2008, 04:47 AM

Average power.

crutschow · 28th September 2008, 05:50 AM

Standard types of DMMs convert the AC to DC using a simple full-wave rectifier circuit. This type of circuit generates a DC voltage that's proportional to the average value of a waveform. The meter displays the RMS value of a sinewave by multipliying this by the difference between the average value and RMS value of a sinewave (roughly .707/.637). Thus this meter will display the correct RMS value only of a sinewave. Other waveforms will be in error since they have a different ratio between their average and RMS value.

If a DMM is a true RMS meter it has more complex circuitry that performs the actual RMS calculation of the waveform and thus will accurately display the RMS value of any type of waveform including square and sine.

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28th September 2008, 04:45 AM	(permalink)
mayeewa	Digital Multimeter (DMM) Why are the square wave RMS readings using DMM different from the sine wave RMS readings using DMM ?

28th September 2008, 04:47 AM	(permalink)
Mikebits	Average power. __________________ Forget injuries, never forget kindnesses.

28th September 2008, 05:50 AM	(permalink)
crutschow	Standard types of DMMs convert the AC to DC using a simple full-wave rectifier circuit. This type of circuit generates a DC voltage that's proportional to the average value of a waveform. The meter displays the RMS value of a sinewave by multipliying this by the difference between the average value and RMS value of a sinewave (roughly .707/.637). Thus this meter will display the correct RMS value only of a sinewave. Other waveforms will be in error since they have a different ratio between their average and RMS value. If a DMM is a true RMS meter it has more complex circuitry that performs the actual RMS calculation of the waveform and thus will accurately display the RMS value of any type of waveform including square and sine. __________________ Carl