can someone recommend a really TRUE RMS meter?

P

panfilero

Guest
I have a very noisy square pulse wave that I'm trying to measure the
RMS current and RMS votlage of... my pulses can have up to 24 volt
peaks. and a frequency of 1kHz.... I was wondering if anyone could
reccomend a Multi-Meter for doing these measurements, or do I need to
use a Scope?

From what I've gathered in trying to find a meter is that meters are
made for Sinusoids, and True RMS isn't always very true... this
article was interesting http://www.enginova.com/true_rms_volts.htm .
And since there is noise on my signal, I'd have different spikes and
things happening that would be beyond my fundamental frequency of
1kHz, and I don't know what a multi-meter does with that.

I've heard some meters assume that your signal is centerd around the
zero-axis and therefore return bad results, I've heard other meters do
internal calculations assuming a sinusoid, and give erroneous
results.... anyone have any suggestions?



Much thanks to all who participate in this forum, great source of
information
appreciate the help
 
panfilero wrote:

I have a very noisy square pulse wave that I'm trying to measure the
RMS current and RMS votlage of... my pulses can have up to 24 volt
peaks. and a frequency of 1kHz.... I was wondering if anyone could
reccomend a Multi-Meter for doing these measurements, or do I need to
use a Scope?

From what I've gathered in trying to find a meter is that meters are
made for Sinusoids, and True RMS isn't always very true... this
article was interesting http://www.enginova.com/true_rms_volts.htm .
And since there is noise on my signal, I'd have different spikes and
things happening that would be beyond my fundamental frequency of
1kHz, and I don't know what a multi-meter does with that.

I've heard some meters assume that your signal is centerd around the
zero-axis and therefore return bad results, I've heard other meters do
internal calculations assuming a sinusoid, and give erroneous
results.... anyone have any suggestions?



Much thanks to all who participate in this forum, great source of
information
appreciate the help
Maybe what You need is a Scope ?
Click to expand...


--
"I'd rather have a bottle in front of me than a frontal lobotomy"

"Daily Thought:

SOME PEOPLE ARE LIKE SLINKIES. NOT REALLY GOOD FOR ANYTHING BUT
THEY BRING A SMILE TO YOUR FACE WHEN PUSHED DOWN THE STAIRS.
http://webpages.charter.net/jamie_5"
 
panfilero wrote:
I have a very noisy square pulse wave that I'm trying to measure the
RMS current and RMS votlage of... my pulses can have up to 24 volt
peaks. and a frequency of 1kHz.... I was wondering if anyone could
reccomend a Multi-Meter for doing these measurements, or do I need to
use a Scope?

From what I've gathered in trying to find a meter is that meters are
made for Sinusoids, and True RMS isn't always very true... this
article was interesting http://www.enginova.com/true_rms_volts.htm .
And since there is noise on my signal, I'd have different spikes and
things happening that would be beyond my fundamental frequency of
1kHz, and I don't know what a multi-meter does with that.

I've heard some meters assume that your signal is centerd around the
zero-axis and therefore return bad results, I've heard other meters do
internal calculations assuming a sinusoid, and give erroneous
results.... anyone have any suggestions?



Much thanks to all who participate in this forum, great source of
information
appreciate the help
Click to expand...
If it doesn't say "RMS" then it probably measures the peak & assumes a
sinusoid.

If it does say "True RMS", then it's like the Fluke meter and measures
the RMS of the AC portion of the signal -- but it's pretty easy to
measure twice and calculate sqrt(a^2 + b^2), so what's the problem?

Lots of spikes, and lots of frequency content above 60Hz, may cause
problems -- you're describing a signal with a high "crest factor", which
will limit the meter's abilities, and I wouldn't just assume that a
meter has good performance above a kHz or so, either.

Careful reading of the specs may tell the truth -- look for crest factor
and bandwidth, and see what they say.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
 
Bob Myers wrote:
"panfilero" <panfilero@gmail.com> wrote in message
news:c8383e75-8139-4cad-9ff8-6c0a08332121@l28g2000prd.googlegroups.com...
I have a very noisy square pulse wave that I'm trying to measure the
RMS current and RMS votlage of... my pulses can have up to 24 volt
peaks. and a frequency of 1kHz.... I was wondering if anyone could
reccomend a Multi-Meter for doing these measurements, or do I need to
use a Scope?

Honest-to-Gawd true RMS meters are relatively uncommon,
but they do exist. Look for something like this:

http://www.ballantinelabs.com/323meter.htm

which actually measures the "heating power" of the signal
in question, and therefore is getting you as close as you're
likely to get to what "RMS" is supposed to be without
capturing the waveform and doing the math (which
generally would require a fairly sophisticated scope).

Bob M.
Click to expand...
A properly designed true-RMS meter can easily be more accurate than a
scope. Scopes are no better than 1-2% on a good day. Analog Devices
used to make a true-RMS sensor that used a heater and (iirc) a diffused
silicon resistor bridge.

Cheers,

Phil Hobbs
 
panfilero wrote:

I have a very noisy square pulse wave that I'm trying to measure the
RMS current and RMS votlage of... my pulses can have up to 24 volt
peaks. and a frequency of 1kHz.... I was wondering if anyone could
reccomend a Multi-Meter for doing these measurements, or do I need to
use a Scope?

From what I've gathered in trying to find a meter is that meters are
made for Sinusoids, and True RMS isn't always very true... this
article was interesting http://www.enginova.com/true_rms_volts.htm .
And since there is noise on my signal, I'd have different spikes and
things happening that would be beyond my fundamental frequency of
1kHz, and I don't know what a multi-meter does with that.

I've heard some meters assume that your signal is centerd around the
zero-axis and therefore return bad results, I've heard other meters do
internal calculations assuming a sinusoid, and give erroneous
results.... anyone have any suggestions?



Much thanks to all who participate in this forum, great source of
information
appreciate the help
There are a number of ICs and meters that measure true RMS by using
Click to expand...
the thermal method.
Some have a 100Khz bandwidth, and others are better.
 
Phil Allison wrote:
"Phil Hobbs"

A properly designed true-RMS meter


** Which would cost thousands of dollars when wide bandwidth (ie MHz) is
needed.


can easily be more accurate than a scope.


** Many (even budget model) digital scopes include " true rms " voltage
measurements - with bandwidths the same as the range in use.



...... Phil
Click to expand...
I dunno--I have an HP 400A with true-RMS, bandwidth ~ 10 MHz, cost on
Ebay ~ $75. About 1% accuracy at this point--still better than your
average scope. It was specified at (iirc) 0.25% of FS when new. It
also has an analogue output.

Cheers,

Phil Hobbs
 
Phil Allison wrote:
"Phil Hobbs"
Phil Allison
A properly designed true-RMS meter

** Which would cost thousands of dollars when wide bandwidth (ie MHz) is
needed.


can easily be more accurate than a scope.

** Many (even budget model) digital scopes include " true rms " voltage
measurements - with bandwidths the same as the range in use.

I dunno--I have an HP 400A with true-RMS, bandwidth ~ 10 MHz, cost on Ebay
~ $75.



** I think you mean the HP 3400A, has +/- 5% accuracy from 3 to 10 MHz.
Click to expand...
No, I have one of those at home, vintage probably 1972 by the look of
it. The one at work is quite definitely a 400A and is much newer. I
used to use 400ELs back in the day, but they were peak-reading.

The accuracy spec includes the 1-pole rolloff of the input
amplifier--when you correct for that, it's much better than a scope.
And the 400A is much better than the 3400A.

Will not read combined AC +DC.
Well, since the cross term contributes zero average power, that isn't
Click to expand...
too serious--if it's an issue, he can measure both and add
root-sum-square fashion.

What did it cost originally ?
You're moving the goal posts on me. I doubt the OP has a time machine,
Click to expand...
so it doesn't have much practical significance what it cost originally.
(I forget where I saw the line, "Time travel is no longer a thing of
the past." ;)

A new, digital display one with high accuracy costs??
Click to expand...
Dunno. I'm happy with my 400A until they pry it from my dead fingers,
and the OP can easily get one too, I think.

Cheers,

Phil Hobbs
 
On Tue, 3 Jun 2008 12:54:05 +1000, "Phil Allison"
<philallison@tpg.com.au> wrote:

"Phil Hobbs"


A properly designed true-RMS meter


** Which would cost thousands of dollars when wide bandwidth (ie MHz) is
needed.


can easily be more accurate than a scope.


** Many (even budget model) digital scopes include " true rms " voltage
measurements - with bandwidths the same as the range in use.
Click to expand...
As far as I know, most DVMs still put an analog RMS-to-DC converter in
front of their regular DVM stuff. But fast SAR ADCs are cheap these
days, so one could just random or dither sample the input and do the
RMS as math. That would be cheap and deadly accurate, well into the
MHz range where the analog things quit.

John





...... Phil
Click to expand...
 
Whoa, ok sounds like the opinions on this issue are all over the
place, from people who think a SCOPE would be better, others a
METER...etc are these old thermal-based rms meters really better than
today's digital ones for measuring RMS? Why would a METER be better
than a SCOPE... don't SCOPES have much higher bandwidth and more
capable of measuring this?

What I'm trying to do: measure the current and voltage going into each
winding of a brushless fan motor, each winding is recieving some ugly
looking pulses in order to run the motor, I'm intersted in find the
power per each winding.

Thanks
J
 
On Jun 3, 9:59 am, panfilero <panfil...@gmail.com> wrote:
Whoa, ok sounds like the opinions on this issue are all over the
place, from people who think a SCOPE would be better, others a
METER...etc are these old thermal-based rms meters really better than
today's digital ones for measuring RMS?  Why would a METER be better
than a SCOPE... don't SCOPES have much higher bandwidth and more
capable of measuring this?

What I'm trying to do: measure the current and voltage going into each
winding of a brushless fan motor, each winding is recieving some ugly
looking pulses in order to run the motor, I'm intersted in find the
power per each winding.

Thanks
J
Click to expand...
then you also need to know the phase angle betwen the voltage and
current..

I suspect in this case, getting a wide bandwidth TRUE RMS reading is
the least of your problems.....

Analog devices or someone makes ICs that can measure the REAL power of
an AC circuit.,.. I think that is the way you need to go,,,if you
make independent RMS measurments of the voltage and current, you still
have the phase angle problem... You need to measure them both in a
combined device that can multiply them together on a point by point
basis to account for the phase angle..., not just multiply the overall
RMS numbers...

Mark
 
On Jun 3, 9:17 am, Mark <makol...@yahoo.com> wrote:
On Jun 3, 9:59 am, panfilero <panfil...@gmail.com> wrote:

Whoa, ok sounds like the opinions on this issue are all over the
place, from people who think a SCOPE would be better, others a
METER...etc are these old thermal-based rms meters really better than
today's digital ones for measuring RMS? Why would a METER be better
than a SCOPE... don't SCOPES have much higher bandwidth and more
capable of measuring this?

What I'm trying to do: measure the current and voltage going into each
winding of a brushless fan motor, each winding is recieving some ugly
looking pulses in order to run the motor, I'm intersted in find the
power per each winding.

Thanks
J

then you also need to know the phase angle betwen the voltage and
current..

I suspect in this case, getting a wide bandwidth TRUE RMS reading is
the least of your problems.....

Analog devices or someone makes ICs that can measure the REAL power of
an AC circuit.,.. I think that is the way you need to go,,,if you
make independent RMS measurments of the voltage and current, you still
have the phase angle problem... You need to measure them both in a
combined device that can multiply them together on a point by point
basis to account for the phase angle..., not just multiply the overall
RMS numbers...

Mark
Click to expand...
I need the phase angle? This might be going off on a tangent a bit,
but the way I'm finding the power used by each of the 3 windings is by
putting a shunt resistor (R) in series with the winding and measuring
the votlage across it (Vrms,shunt) and then dividing Vrms/R to find
the current (Irms) going to each winding, then I am going to measure
the voltage at the winding (Vrms) with respect to ground. And then
just take the product Pavg = Vrms*Irms .... I don't see why I would
need the phase angles to find the power.

Thanks
J
 
On Tue, 3 Jun 2008 08:31:34 -0700 (PDT), panfilero <panfilero@gmail.com>
wrote:

On Jun 3, 9:17 am, Mark <makol...@yahoo.com> wrote:
On Jun 3, 9:59 am, panfilero <panfil...@gmail.com> wrote:

Whoa, ok sounds like the opinions on this issue are all over the
place, from people who think a SCOPE would be better, others a
METER...etc are these old thermal-based rms meters really better than
today's digital ones for measuring RMS? Why would a METER be better
than a SCOPE... don't SCOPES have much higher bandwidth and more
capable of measuring this?

What I'm trying to do: measure the current and voltage going into each
winding of a brushless fan motor, each winding is recieving some ugly
looking pulses in order to run the motor, I'm intersted in find the
power per each winding.

Thanks
J

then you also need to know the phase angle betwen the voltage and
current..

I suspect in this case, getting a wide bandwidth TRUE RMS reading is
the least of your problems.....

Analog devices or someone makes ICs that can measure the REAL power of
an AC circuit.,.. I think that is the way you need to go,,,if you
make independent RMS measurments of the voltage and current, you still
have the phase angle problem... You need to measure them both in a
combined device that can multiply them together on a point by point
basis to account for the phase angle..., not just multiply the overall
RMS numbers...

Mark

I need the phase angle? This might be going off on a tangent a bit,
but the way I'm finding the power used by each of the 3 windings is by
putting a shunt resistor (R) in series with the winding and measuring
the votlage across it (Vrms,shunt) and then dividing Vrms/R to find
the current (Irms) going to each winding, then I am going to measure
the voltage at the winding (Vrms) with respect to ground. And then
just take the product Pavg = Vrms*Irms .... I don't see why I would
need the phase angles to find the power.

Thanks
J
Click to expand...
This page explains why you need "...the phase angles to find the (real)
power.":
http://en.wikipedia.org/wiki/Power_factor

In your case, the current waveform is not sinusoidal, so the notion of
phase angle becomes more complicated.

A 3-phase wattmeter would give the results you want, but as a practical
matter, you should probably use an oscilloscope with trace math to carry
out the measurement of your 3-phase power. See:

http://www.yokogawa.com/tm/tr/tm-tr0605_02.htm
 
On Tue, 03 Jun 2008 06:59:52 -0700, panfilero wrote:

Whoa, ok sounds like the opinions on this issue are all over the place,
from people who think a SCOPE would be better, others a METER...etc are
these old thermal-based rms meters really better than today's digital ones
for measuring RMS? Why would a METER be better than a SCOPE... don't
SCOPES have much higher bandwidth and more capable of measuring this?

What I'm trying to do: measure the current and voltage going into each
winding of a brushless fan motor, each winding is recieving some ugly
looking pulses in order to run the motor, I'm intersted in find the power
per each winding.
Click to expand...
You could sample the (instantaneous) voltage and current at some fast
rate, and calculate it in a micro.

Good Luck!
Rich
 
Phil Allison wrote:


** No - because HP 400As are simply not true rms meters.

Post a link to your example.
Click to expand...
I'm travelling at the moment, but I'll see when I get back to the lab. I
bought it in 1991-ish. It's a thing of great beauty in its way.

Cheers,

Phil Hobbs
 
In article <47ma449pgo6bl6lbj21sdmt9atu5ilv03l@4ax.com>,
jjlarkin@highNOTlandTHIStechnologyPART.com says...
On Tue, 3 Jun 2008 12:54:05 +1000, "Phil Allison"
philallison@tpg.com.au> wrote:


"Phil Hobbs"


A properly designed true-RMS meter


** Which would cost thousands of dollars when wide bandwidth (ie MHz) is
needed.


can easily be more accurate than a scope.


** Many (even budget model) digital scopes include " true rms " voltage
measurements - with bandwidths the same as the range in use.



As far as I know, most DVMs still put an analog RMS-to-DC converter in
front of their regular DVM stuff. But fast SAR ADCs are cheap these
days, so one could just random or dither sample the input and do the
RMS as math. That would be cheap and deadly accurate, well into the
MHz range where the analog things quit.
Click to expand...
Video converters run well into the tens of MHz. The Analog one I'm
using is 14bits at 75MHz (though it's double correlated).


--
Keith
 
On Mon, 2 Jun 2008 16:26:14 -0600, "Bob Myers"
<nospamplease@address.invalid> wrote:

"panfilero" <panfilero@gmail.com> wrote in message
news:c8383e75-8139-4cad-9ff8-6c0a08332121@l28g2000prd.googlegroups.com...
I have a very noisy square pulse wave that I'm trying to measure the
RMS current and RMS votlage of... my pulses can have up to 24 volt
peaks. and a frequency of 1kHz.... I was wondering if anyone could
reccomend a Multi-Meter for doing these measurements, or do I need to
use a Scope?

Honest-to-Gawd true RMS meters are relatively uncommon,
but they do exist. Look for something like this:

http://www.ballantinelabs.com/323meter.htm

which actually measures the "heating power" of the signal
in question, and therefore is getting you as close as you're
likely to get to what "RMS" is supposed to be without
capturing the waveform and doing the math (which
generally would require a fairly sophisticated scope).

Bob M.
Click to expand...
While what you say is true those instruments tend to be expensive. For
most any purpose any "TrueRMS" class handheld DMM or small benchtop
TrueRMS DMM will do OP's task quite nicely. OP may have to read the
datasheet to verify that it includes DC in the "TrueRMS" calculation.
Well designed RMS to DC converter ICs have been around for over 20
years.
 
On Tue, 3 Jun 2008 06:59:52 -0700 (PDT), panfilero
<panfilero@gmail.com> wrote:

Whoa, ok sounds like the opinions on this issue are all over the
place, from people who think a SCOPE would be better, others a
METER...etc are these old thermal-based rms meters really better than
today's digital ones for measuring RMS? Why would a METER be better
than a SCOPE... don't SCOPES have much higher bandwidth and more
capable of measuring this?

What I'm trying to do: measure the current and voltage going into each
winding of a brushless fan motor, each winding is recieving some ugly
looking pulses in order to run the motor, I'm intersted in find the
power per each winding.

Thanks
J
Click to expand...
I just poxy hate it when OP does not state the problem clearly,
completely and correctly the first time. You are stuck with using a
digital Scope that does math. You will have to "simultaneously"
digitize both voltage and current. Then do the calculations.
 
On Tue, 3 Jun 2008 08:31:34 -0700 (PDT), panfilero
<panfilero@gmail.com> wrote:

On Jun 3, 9:17 am, Mark <makol...@yahoo.com> wrote:
On Jun 3, 9:59 am, panfilero <panfil...@gmail.com> wrote:

Whoa, ok sounds like the opinions on this issue are all over the
place, from people who think a SCOPE would be better, others a
METER...etc are these old thermal-based rms meters really better than
today's digital ones for measuring RMS? Why would a METER be better
than a SCOPE... don't SCOPES have much higher bandwidth and more
capable of measuring this?

What I'm trying to do: measure the current and voltage going into each
winding of a brushless fan motor, each winding is recieving some ugly
looking pulses in order to run the motor, I'm intersted in find the
power per each winding.

Thanks
J

then you also need to know the phase angle betwen the voltage and
current..

I suspect in this case, getting a wide bandwidth TRUE RMS reading is
the least of your problems.....

Analog devices or someone makes ICs that can measure the REAL power of
an AC circuit.,.. I think that is the way you need to go,,,if you
make independent RMS measurments of the voltage and current, you still
have the phase angle problem... You need to measure them both in a
combined device that can multiply them together on a point by point
basis to account for the phase angle..., not just multiply the overall
RMS numbers...

Mark

I need the phase angle? This might be going off on a tangent a bit,
but the way I'm finding the power used by each of the 3 windings is by
putting a shunt resistor (R) in series with the winding and measuring
the votlage across it (Vrms,shunt) and then dividing Vrms/R to find
the current (Irms) going to each winding, then I am going to measure
the voltage at the winding (Vrms) with respect to ground. And then
just take the product Pavg = Vrms*Irms .... I don't see why I would
need the phase angles to find the power.

Thanks
J
Click to expand...
There is a significant and important difference between VA and watts
for motors. Study up.
 
On Wed, 4 Jun 2008 08:50:11 -0700 (PDT), panfilero <panfilero@gmail.com>
wrote:

On Jun 3, 6:38 pm, "Phil Allison" <philalli...@tpg.com.au> wrote:
"panfilero"

And then just take the product Pavg = Vrms*Irms

**  The formula for Volt Amperes  or  VA.

.... I don't see why I would need the phase angles to find the power.

**  Fraid you do and there will not be a simple number either.

......   Phil

ok phil, so you are telling me that I can not put a shunt resistor in
line with one of my fan windings and simply measure the voltage
waveform across that shunt, and convert that waveform to an RMS
voltage and then divide that by the shunt resistance in order to get
my RMS current? And once I have both the RMS voltage and current,
that I can't multiply those values together in order to see how much
power the winding is consuming?

I'm not just directing this at Phil, if anyone thinks this would not
work please let me know, I don't see what is wrong with this approach.
Click to expand...
I gave you the answer in an earlier post.

Go read this page:

http://en.wikipedia.org/wiki/Power_factor

Then read this page for some more of the same explanation:

http://www.yokogawa.com/tm/tr/tm-tr0605_01.htm

followed by this page to explain how to actually measure the power
delivered to your 3 windings, except that you will probably need to use a
scope with trace math, rather than a wattmeter. The scope will have to be
able to multiply the instantaneous voltage and current and integrate
(average) that product. You can use the two-wattmeter method to get the
3-phase power delivered to the motor windings, with the scope taking the
place of the wattmeters.:

http://www.yokogawa.com/tm/tr/tm-tr0605_02.htm


much thanks.
J
Click to expand...
 
On Jun 3, 6:38 pm, "Phil Allison" <philalli...@tpg.com.au> wrote:
"panfilero"

And then just take the product Pavg = Vrms*Irms

**  The formula for Volt Amperes  or  VA.

.... I don't see why I would need the phase angles to find the power.

**  Fraid you do and there will not be a simple number either.

......   Phil
Click to expand...
ok phil, so you are telling me that I can not put a shunt resistor in
line with one of my fan windings and simply measure the voltage
waveform across that shunt, and convert that waveform to an RMS
voltage and then divide that by the shunt resistance in order to get
my RMS current? And once I have both the RMS voltage and current,
that I can't multiply those values together in order to see how much
power the winding is consuming?

I'm not just directing this at Phil, if anyone thinks this would not
work please let me know, I don't see what is wrong with this approach.

much thanks.
J
 
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