RMS and Average Current

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

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.

Yes.

Yes.

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

"John Larkin"

P = E . I

Where E = diode conduction voltage

and I = average amps.

......... Phil

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

...Jim Thompson

What strange dialect of gibberish is this ?

Not like a * magic carpet * at all then.

........ Phil

He was, in fact correct in saying that Phil was not even wrong (since Phil was right.) Maybe he meant it as a compliment.

John

That was a convincing bit of oratory, NOT.

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.

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

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.

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:

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.

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

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.

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.

Explanation?

"John Larkin" a écrit dans le message de news: snipped-for-privacy@4ax.com...

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

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.

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.

"Ignoramus29795" a écrit dans le message de news:i9zuf.10795$ snipped-for-privacy@fe65.usenetserver.com...

DC

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

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.

HEY. I simply said, "OK, I'm sure you're right, what are some good examples?"

What's the matter with you man, have you gone totally daft? The topic was/is "RMS and Average Current," and that's what I invited you to discuss for the last four posts. IIRC, you were strongly arguing for the value of average measurements (BTW, which I agree with for DC and low-frequency measurements - as well argued by John), and I suggested you might have some good examples for us? Come on, I wasn't interested in talking about the Nazis and the Jews in WW2, or whatever other issue you raised.

And you would have to use a peak responding meter to get E and I and some other method to determine n. For the pulse type waveform described, knowing E (peak) and the average value of e(t), you could determine n. Or use a scope to do it all.

Not necessarily. How will you know the diode conduction voltage (which we're assuming is constant)? Suppose you don't have a scope, just meters. The voltage across the diode varies with time with the same waveshape as the current through the diode, so the conduction voltage can't be measured with an average responding or RMS responding meter; you'll have to use a peak responding meter which is somewhat uncommon. If we don't have a peak responding meter, then what? We can measure the RMS voltage across the diode and the RMS current through the diode and multiply them.

An RMS ammeter is appropriate if an RMS voltmeter is also used.

Phil presupposes that you know the diode conduction voltage, which is not constant when a pulse current is applied; some sort of device to measure the peak voltage across the diode would be needed. If you have a DVM which can measure RMS (AC+DC) voltage, it almost certainly will be able to measure RMS (AC+DC) current, and that's all you need. No need to make peak measurements.