RMS and Average Current

[Don Lancaster]

You mean, it won't indicate average? OK, what *does* an
average-indicating ammeter indicate?

John

It indicates an incompetent individual deluding themselves while making
a measurement.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Jim Thompson]

It indicates an incompetent individual deluding themselves while making
a measurement.

Sheeesh! The has-been is the delusional one.

...Jim Thompson

 

[Don Lancaster]

Sorry about that.
The link copier apparently got off track.

The correct cite for the crucial differences between RMS and average
pulse power is http://www.tinaja.com/glib/muse112.pdf

The specific analysis of the trouble this caused with the PE "magic
lamp" is found at http://www.tinaja.com/glib/muse112.pdf

I'll eventually get it right. The PE cite is muse113.pdf

For those of you not up on the story: An EE-challenged epsilon minus
created a very late duty cycle half wave dimmer circuit (straight out of
a 1938 industrial electronics book) and connected a 32 volt light bulb
to it. When compared to a 110 volt light bulb of the same brightness,
their average responding meter showed a 3:1 voltage difference and a 3:1
current difference.

Multiplying the two together gave an obvious 10:1 power difference.
Which they patented (!) and published (probably in 1997) in a
(coincidentally) April issue of Popular Electronics as a way to save 90
percent on your power bills.

Naturally, they never TOUCHED the 32 volt bulb to see if it was any COOLER.

Even years later, that individual refused to believe they had committed
EE Lab student blunder 0001-A. Or that their patent was utterly
worthless on countless different levels. (even ignoring the illegal
waveform, brightness sensitivity, and the ease with which the bulbs
could be inadvertently burned out.)

Sure enough, at a 138 degree or so halfwave phase delay angle, the RMS
to average difference is a surprisingly high 3:1.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Fred Bartoli]

One might also note that it's rare to have a ammeter that actually
indicates RMS current on a DC range, unless you have an ancient Weston
electrodynamic thing with a mirrored scale and brass lugs. As far as
DC current measurements, average is usually more meaningful.

Huh. What the difference between RMS and average (and peak) in case of DC
current?
Please entertain us

 

[Ignoramus29795]

Huh. What the difference between RMS and average (and peak) in case of DC
current?
Please entertain us

Let's say that you have DC current that is 1 amp for 1 second, and
then 2 amps for 1 second, repeating ad infinitum.

The average is 1.5 amps.

The root mean square is sqrt( 1^2/2 + 2^2/2 ) = sqrt( 2.5 ) =
approximately 1.6 amps. A different value from average. The peak is 2.

i

 

[Fred Bartoli]

Let's say that you have DC current that is 1 amp for 1 second, and
then 2 amps for 1 second, repeating ad infinitum.

That is not my definition of DC, and I hope for a lot of people. It
shouldn't be for you too.

 

[Ignoramus29795]

That is not my definition of DC, and I hope for a lot of people. It
shouldn't be for you too.

Hm, what is your definition of DC? Are you saying that DC is the same
as "constant current"? Maybe I am missing something.

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

Check out "types of direct current" to the right. It mentions
differing definitions of "DC", some refer to constant current and some
refer to potential that does not change sign. Maybe I can learn
something here.

i

 

[John Larkin]

Huh. What the difference between RMS and average (and peak) in case of DC
current?

None. But I was talking about DC meters, not about DC current.

Please entertain us

Suppose you have an average current of 1 amp, with 0.2 amps RMS of
ripple superimposed. An average-reading ammeter will show 1.0, but a
true RMS meter would indicate 1.02 amps. There are a few DVMs that do
real dc-coupled RMS measurement, but they're kinda rare.

All the DVMs I've been able to see the guts of, seem to have a slow
integrating (dual slope, delta-sigma, or some variant) DC section,
with a switchable, usually AC-coupled averaging or RMS ac-to-dc
converter ahead of it. So why not use a single fast (successive
approximation) ADC and then DSP the data? That way you could extract
anything and feature-itus it to your heart's content. And get superb
wideband AC accuracy.

John

 

[John Larkin]

It indicates an incompetent individual deluding themselves while making
a measurement.

Suppose I'm charging a battery from an unfiltered rectifier, and am
interested in the charge rate. Am I deluding myself if I use an
average-reading ammeter?

Suppose I'm measuring the voltage of a parts-per-million accurate DC
reference, and the meter leads may pick up a little RF and hum. Do I
really want to measure true RMS?

Suppose I know that the AC line is a pretty good sinewave (which is
always is) and I want to check the line voltages. Am I an incompetant
individual for using a cheap Fluke?

John

 

[John Larkin]

Sheeesh! The has-been is the delusional one.

...Jim Thompson

Well, that new years resolution didn't last long.

You may as well start insulting me... I'm starting to feel left out.

John

 

[Don Lancaster]

None. But I was talking about DC meters, not about DC current.




Suppose you have an average current of 1 amp, with 0.2 amps RMS of
ripple superimposed. An average-reading ammeter will show 1.0, but a
true RMS meter would indicate 1.02 amps. There are a few DVMs that do
real dc-coupled RMS measurement, but they're kinda rare.

All the DVMs I've been able to see the guts of, seem to have a slow
integrating (dual slope, delta-sigma, or some variant) DC section,
with a switchable, usually AC-coupled averaging or RMS ac-to-dc
converter ahead of it. So why not use a single fast (successive
approximation) ADC and then DSP the data? That way you could extract
anything and feature-itus it to your heart's content. And get superb
wideband AC accuracy.

John

Actually, just about all but the cheapest of new instruments now
routinely provide RMS capability. The cost of doing so has dramatically
dropped.

Primarily because average responders were so ludicrously bad.


--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[Don Lancaster]

Suppose I'm charging a battery from an unfiltered rectifier, and am
interested in the charge rate. Am I deluding myself if I use an
average-reading ammeter?
Yes.


Suppose I'm measuring the voltage of a parts-per-million accurate DC
reference, and the meter leads may pick up a little RF and hum. Do I
really want to measure true RMS?
Yes.


Suppose I know that the AC line is a pretty good sinewave (which is
always is) and I want to check the line voltages. Am I an incompetant
individual for using a cheap Fluke?

John

I seriously doubt that Fluke makes ANY average responding meters any
more because they were so bad.

--
Many thanks,

Don Lancaster voice phone: (928)428-4073
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
rss: http://www.tinaja.com/whtnu.xml email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com

 

[John Larkin]

I seriously doubt that Fluke makes ANY average responding meters any
more because they were so bad.

Well, one *could* actually look it up...

http://us.fluke.com/usen/products/prodSelection_Grouping.htm

John

 

[Jim Thompson]

John Larkin wrote:
[snip]

It indicates an incompetent individual deluding themselves while making
a measurement.

Sheeesh! The has-been is the delusional one.

...Jim Thompson

Well, that new years resolution didn't last long.

You may as well start insulting me... I'm starting to feel left out.

John

Poor baby! Just say something really dumb and I'll be there for you
;-)

...Jim Thompson

 

[Fred Bartoli]

None. But I was talking about DC meters, not about DC current.

Oh oh. I grant you thought so, but you wrote:
"As far as DC current measurements, average is usually more meaningful."

Suppose you have an average current of 1 amp, with 0.2 amps RMS of
ripple superimposed. An average-reading ammeter will show 1.0, but a
true RMS meter would indicate 1.02 amps. There are a few DVMs that do
real dc-coupled RMS measurement, but they're kinda rare.

Indeed. Just pulling your leg.

All the DVMs I've been able to see the guts of, seem to have a slow
integrating (dual slope, delta-sigma, or some variant) DC section,
with a switchable, usually AC-coupled averaging or RMS ac-to-dc
converter ahead of it.

I guess these are mainly handheld ones.

So why not use a single fast (successive
approximation) ADC and then DSP the data? That way you could extract
anything and feature-itus it to your heart's content. And get superb
wideband AC accuracy.

Battery life (IOW, average current consumption) ?

 

[The Phantom]

"John Larkin"


** The power produced by such a rectangular pulse can be measured with an
average responding meter.

P = E . I

Where E = diode conduction voltage

and I = average amps.

Actually, the power delivered by *any* current waveform (for which an
"average" exists) into a constant voltage drop can be measured by an average
responding meter.

The canonical computation of average power for a time T is:

1/T * Integral from 0 to T of [v(t)*i(t)] dt

If v(t) is constant (call it V), then it comes out of the integrand and we're
left with:

V/T * Integral from 0 to T of [i(t)] dt

The integral is now just the average value of the current waveform.

And, of course, if the current is constant and the voltage is time-varying,
the same formulation applies using the average voltage.

 

[The Phantom]

Not even wrong.

I think you should stop using this phrase, Don. You've worn it out, and it
has lost its impact.

An average responding meter will ALWAYS lie like a rug.

An average responding meter won't lie about the average value of a current,
will it? It's not a question of whether an average responding meter lies or
not. It's whether the average value is appropriate in a given situation.

 

[The Phantom]

Yes.

Rather than just saying "yes", how about an explanation? It seems to me that
John is exactly right here. If by "charge rate", one means "coulombs per second
delivered to the battery", then an average responding meter is just what you
want, isn't it?

What could "charge rate" mean such that an average responding meter wouldn't
be appropriate? I suppose it could mean "power (or energy) delivered to the
battery". But then just measuring current by any definition wouldn't do it;
you'd need to also measure the battery voltage.

 

[The Phantom]

None. But I was talking about DC meters, not about DC current.


Suppose you have an average current of 1 amp, with 0.2 amps RMS of
ripple superimposed. An average-reading ammeter will show 1.0, but a
true RMS meter would indicate 1.02 amps. There are a few DVMs that do
real dc-coupled RMS measurement, but they're kinda rare.

The older Fluke 45 does it, and the latest handhelds (the 187, 189) do it.
I suspect it will become common because everybody will start doing it all in DSP
as you suggest below.

All the DVMs I've been able to see the guts of, seem to have a slow
integrating (dual slope, delta-sigma, or some variant) DC section,
with a switchable, usually AC-coupled averaging or RMS ac-to-dc
converter ahead of it. So why not use a single fast (successive
approximation) ADC and then DSP the data? That way you could extract
anything and feature-itus it to your heart's content. And get superb
wideband AC accuracy.

I suspect that that's what the latest Fluke meters do. If you look at the
specs for the 189, for example, they say that the rated accuracy can be had for
AC measurements with crest factor less than 3, but if the maximum reading is
half or less than full scale, the crest factor may be up to 6. Sounds like
clipping the A/D may be the limiting factor. I'll bet they're doing it all with
DSP these days.

 

[The Phantom]

One might also note that it's rare to have a ammeter that actually
indicates RMS current on a DC range, unless you have an ancient Weston
electrodynamic thing with a mirrored scale and brass lugs. As far as
DC current measurements, average is usually more meaningful.

The old hot-wire ammeters indicated RMS of AC+DC. Some of the thermocouple
type RF ammeters do it too. I always buy RF ammeters that I find at hamfests
just because they can do this. The construction of the RF ammeters sometimes
adds a small fraction of the applied current drop in series with the
thermocouple output, and these can't accurately measure RMS AC+DC. They will
often read backwards with applied DC of one polarity, and way too high with the
other polarity. The ones I covet don't have this problem, and will read
correctly with DC or AC (up to 65 MHz typically). I use them to measure
currents in switching supplies. The modern DVMs are often fooled by all the RF
flying around in a switcher, but the thermocouple RF ammeters aren't.

Another type of meter that will correctly read RMS of AC+DC is the moving iron
meter. These are often in the form of a panel meter, intended to be connected
to the secondary of a current transformer, and typically are 5 amps full scale,
even though the printed scale may be different. Bring a magnet near one of
these and get a reading, so keep magnets away. They can be had surplus quite
often for a good price. They aren't good for high frequency AC as are the RF
ammeters, but for line frequency work, they are quite handy.