Hello again, I have a vary basic voltmeter (mastech M-830B) and I have a question about the AC measurement setting. I know that when you use it to measure an AC wave form (sine wave) it shows you the RMS value, what I was wandering was does the meter show the RMS value for any periodic wave or just sine waves ??????
It will be RMS for any Periodic wave
-- For the price, I doubt whether it\'s true RMS. Don\'t you have the manual/specifications ?
Unless it says "True RMS" and you paid big bucks, you aren't going to get it. The basic AC meter is actually detecting peak voltage and then the display is reduced by the magic number to convert a sine wave peak value to RMS.
"Kurt Krueger"
** Many " true rms " DMMs are quite affordable nowadays.Only takes a single, analogue SMD chip to perform the conversion.
Nearly all DMMs and all moving coil meters detect the *average rectified value* of the input wave - allowing for frequency response limitations.
** Wrong again - of course.The "true rms" value of a sine wave is 1.11 times the average rectified value.
One multiplies the reading shown on a standard DMM by 0.9 to obtain the average rectified value of **any** input waveform. Allowing for frequency response limitations - as always with AC meters.
....... Phil
Almost certainly NOT. See other responses for details.
Bob M.
Even if your meter is a "true RMS" type, it will probably not give accurate readings for all non-sinusoids. The shape of an input signal can have a substantial effect on the meter's accuracy.
If you are interested in this, do a search on "crest factor" and TRMS.
Chuck
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No. This is the cheapest DMM that money can buy (around $5 US) and probably reads peak to peak.
-- It _measures_ peak-to-peak (or peak), but it _displays_ the RMS value of a sine wave with that peak-to-peak (or peak) voltage.
Unless is says "true RMS" odd are it'll only give RMS for sine waves the cheaper meters measure mean voltage and then scale the result to get RMS.
Bye. Jasen
The 'really cheap' ones don't even decouple the DC so if you're trying to read (say) a 12vAC voltage on something that has 50vDC on it, you'll get a reading of about 70V. Not much use if you're trying to measure the ripple/hum on a DC supply.
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