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Sci.Electronics.Basics -> can someone recommend a really TRUE RMS meter?
There are 48 messages in this thread.
You are currently looking at messages 1 to 20.
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Author: panfileroDate: 18:03 02-06-08
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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
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Author: JamieDate: 18:20 02-06-08
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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 ?
--
"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";
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Author: Bob MyersDate: 18:26 02-06-08
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"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.
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Author: Tim WescottDate: 19:34 02-06-08
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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
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
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Author: Dave PlattDate: 20:18 02-06-08
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> 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?
A digital scope with a true-RMS calculation feature could give you the
numbers, I suppose... or, just capture the data with a fast digital
scope, upload it to your PC and do the math there.
Or, look around to see if you can find somebody who has an olde style
HP 3400 true-RMS voltmeter (I got mine for a song at a local hamfest
a couple of years ago). To measure RMS current you'd either need to
have a low-impedance sense resistor installed in the current path, or
use a current probe (HP 456A, or some sort of home-brew current
transformer with resistive termination, and then some work to
calibrate it).
The 3400 uses the two-thermocouples-and-a-feedback-loop method of RMS
detection.
--
Dave Platt <dplatt@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
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Author: Phil HobbsDate: 20:42 02-06-08
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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.
>
>
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
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Author: Robert BaerDate: 21:32 02-06-08
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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
the thermal method.
Some have a 100Khz bandwidth, and others are better.
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Author: Phil AllisonDate: 22:54 02-06-08
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"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
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Author: Phil HobbsDate: 23:00 02-06-08
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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
>
>
>
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
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Author: Phil AllisonDate: 23:03 02-06-08
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"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
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Author: Phil AllisonDate: 23:14 02-06-08
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"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.
Will not read combined AC +DC.
What did it cost originally ?
A new, digital display one with high accuracy costs??
...... Phil
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Author: Bob MastaDate: 07:12 03-06-08
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On Mon, 2 Jun 2008 15:03:15 -0700 (PDT), panfilero
<panfilero@gmail.com> 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?
SHAMELESS PLUG:
My Daqarta sound-card software has a true RMS Voltmeter. It actually
does the root-mean-square computation. If you can trigger on the
waveform (Daqarta has lots of trigger controls, so you probably can
unless it is really noisy), then the computation will take place over
an integer number of cycles. Otherwise, on an untriggered source the
computation will be done over 1024 samples.
Daqarta also includes a signal generator, which (depending upon your
particular situation) you may be able to use to initiate your pulses.
In that case, Daqarta can use Gen Sync to trigger off its own
generator and insure it knows the exact cycle length.
Or, if you have an external trigger source available (that is driving
the signal), you can feed that into the other input channel and just
use that as the trigger.
In any case where you can get a stable trigger, you can also use
Daqarta's synchronous waveform averaging to reduce noise.
IMPORTANT: Note that although the RMS computation handles DC
correctly, the sound card is AC coupled.
Also note that you'll need to calibrate your system first. Daqarta
has an auto-calibration option that determines the relative step sizes
of the sound card mixer using a loopback connection, but you'll also
need a full-scale range measurement. If you already have a meter that
reads AC Volts, you can use that as your reference, with a sine wave
input (that can be provided by Daqarta, if you wish). You just take a
reading and do a simple calculation to get the value to enter into
Daqarta's Full Scale Range dialog. All described in great detail in
the Help system.
Since your signal can run larger than sound cards can handle, you
should also provide an input attenuator to keep things in the +/-3V
range or so. (The card probably won't be damaged by +/-12, but
the input ranges will clip.)
Daqarta is US$29 to purchase (Personal/Hobby license), but it sounds
like this may be only a short-term need. If so, you may be able to do
your whole job within the 30-day/30-session trial period. Enjoy!
Best regards,
Bob Masta
DAQARTA v3.50
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!
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Author: Phil HobbsDate: 07:49 03-06-08
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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.
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
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,
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??
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
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Author: Phil AllisonDate: 08:06 03-06-08
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"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.
>
> 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%20HP_3400A
** The HP 400A is an antique, non true rms millivolt meter.
http://oak.cats.ohiou.edu/~postr/bapix/HP400A.htm
>> What did it cost originally ?
> 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
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Author: panfileroDate: 09:59 03-06-08
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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
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Author: MarkDate: 10:17 03-06-08
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On Jun 3, 9:59=A0am, 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? =A0Why 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
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Author: John LarkinDate: 10:48 03-06-08
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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.
John
>
>...... Phil
>
>
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Author: panfileroDate: 11:31 03-06-08
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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
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Author: Rich GriseDate: 16:02 03-06-08
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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.
You could sample the (instantaneous) voltage and current at some fast
rate, and calculate it in a micro.
Good Luck!
Rich
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Author: Phil AllisonDate: 19:35 03-06-08
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"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
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