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Electronics Forum / Basics / August 2006Voltmeter Question !!!!
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 ?????? > 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 >> Hello again, >> I have a vary basic voltmeter (mastech M-830B) and I have a question [quoted text clipped - 4 lines] > > It will be RMS for any Periodic wave Almost certainly NOT. See other responses for details. Bob M. >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 ?????? --- For the price, I doubt whether it's true RMS. Don't you have the manual/specifications ?  SignatureJohn Fields Professional Circuit Designer > 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 ?????? 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" > Unless it says "True RMS" and you paid big bucks, you aren't going to > get it. ** Many " true rms " DMMs are quite affordable nowadays. Only takes a single, analogue SMD chip to perform the conversion. > The basic AC meter is actually detecting peak voltage... * * Hardly a one does that. Nearly all DMMs and all moving coil meters detect the *average rectified value* of the input wave - allowing for frequency response limitations. > and > then the display is reduced by the magic number to convert a sine > wave peak value to RMS. ** 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 > 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 ?????? 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 > 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 ?????? No. This is the cheapest DMM that money can buy (around $5 US) and probably reads peak to peak. >> Hello again, >> I have a vary basic voltmeter (mastech M-830B) and I have a question [quoted text clipped - 5 lines] >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. SignatureJohn Fields Professional Circuit Designer > 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 ?????? 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 >> Hello again, >> I have a vary basic voltmeter (mastech M-830B) and I have a question [quoted text clipped - 5 lines] >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. 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. -- Australia isn't "down under", it's "off to one side"! stanblaz@netspace.net.au www.cobracat.com (home of the Australian Cobra Catamaran) http://groups.yahoo.com/group/cobra-cat/ |
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