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Voltmeter Question !!!!

J

jo.jo

Jan 1, 1970
0
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 ??????
 
S

stevebriz

Jan 1, 1970
0
jo.jo said:
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
 
J

John Fields

Jan 1, 1970
0
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 ??????
 
K

Kurt Krueger

Jan 1, 1970
0
jo.jo said:
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.
 
P

Phil Allison

Jan 1, 1970
0
"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
 
B

Bob Myers

Jan 1, 1970
0
stevebriz said:
It will be RMS for any Periodic wave

Almost certainly NOT. See other responses for details.

Bob M.
 
C

chuck

Jan 1, 1970
0
jo.jo said:
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
 
H

Homer J Simpson

Jan 1, 1970
0
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.
 
J

jasen

Jan 1, 1970
0
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
 
S

Stan Blazejewski

Jan 1, 1970
0
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"!

[email protected]
www.cobracat.com (home of the Australian Cobra Catamaran)
http://groups.yahoo.com/group/cobra-cat/
 
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