can someone recommend a really TRUE RMS meter?

[panfilero]

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

 

[Jamie]

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 ?


--
"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"

 

[Bob Myers]

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.

 

[Phil Allison]

"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

 

[Phil Allison]

"panfilero"

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....

** What are you really trying to measure ???

I suspect you have no clue about when knowing the "rms" current value is
useful and when it is NOT useful.




....... Phil

 

[Phil Allison]

"Phil Hobbs"
Phil Allison

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.

Will not read combined AC +DC.

What did it cost originally ?

A new, digital display one with high accuracy costs??




....... Phil

 

[Phil Allison]

"Phil Hobbs"
Phil Allison

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.

This is a 3400a.

http://www.valuetronics.com/Details.aspx?ProdID=7135&Model=Agilent HP_3400A


** The HP 400A is an antique, non true rms millivolt meter.

http://oak.cats.ohiou.edu/~postr/bapix/HP400A.htm

You're moving the goal posts on me.

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

Post a link to your example.




..... Phil

 

[panfilero]

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

 

[Mark]

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

 

[panfilero]

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

 

[Rich Grise]

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.

You could sample the (instantaneous) voltage and current at some fast
rate, and calculate it in a micro.

Good Luck!
Rich

 

[Phil Allison]

"panfilero"

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.

** See, I knew he did not have a clue when rms values are used.



....... Phil

 

[Phil Allison]

"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

 

[krw]

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.

Video converters run well into the tens of MHz. The Analog one I'm
using is 14bits at 75MHz (though it's double correlated).

 

[Phil Allison]

"Phil Hobbs"

Phil Allison wrote:



I'm travelling at the moment, but I'll see when I get back to the lab. I
bought it in 1991-ish.

** Not off eBay you didn't.

This is a HP 3400A
http://www.valuetronics.com/Details.aspx?ProdID=7135&Model=Agilent HP_3400A

This is a HP400A
http://oak.cats.ohiou.edu/~postr/bapix/HP400A.htm


WTF is Hobbs on about ???



....... Phil

 

[JosephKK]

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.

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.

 

[JosephKK]

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

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.

 

[JosephKK]

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

There is a significant and important difference between VA and watts
for motors. Study up.

 

[panfilero]

"panfilero"


**  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.

much thanks.
J

 

[Richard Henry]

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

In principle that will work, but if the waveform is complicated, so
will be the calculations.