Measuring PWM current?

It depends on the meter. It is certainly possible, if it samples the curren t frequently enough to get quasi-stable individual current samples whose va lues have to be squares and summed before the square root of the mean value is extracted.

If the sampling period is long enough to allow a lot of variation in curren t during the sampling period, so that the number squared is the linear aver age of that particular chunk current there's an obvious source of error (wh ich may not be all that big if the current doesn't get close to or cross ze ro during the sampling period)

Allegro do a range of Hall effect current sensors which provide a couple of kilovolts of isolation between the 1.2 milliohm shunt through the IC and t he current output derived from the Hall effect device.

They aren't all that cheap - a couple of dollars per part - and the bandwid th is only about 80kHz. If the pulse width modulated current has been filte red enough to minimise radiated RF before it gets measured, this should be plenty.

--
Bill Sloman, Sydney

** Yes, with two conditions.

1. The clamp meter is a DC /AC type, using Hall effect sensing.

2. The frequencies involved do not exceed the meter's range, which may be only 1kHz.

** A Hall effect current sensor followed by a true RMS meter - both with adequate bandwidth which must be several times the PWM frequency.

... Phil

** Correct me if I am mistaken, but *sampling* RMS clamp meters do not exist.

** Current sensors from Lem Heme or Honeywell simply have a hole that a current carrying wire can be passed through - eg:

This one has 100kHz, 1dB bandwidth and high accuracy.

** The OP wants to make a measurement, he is not doing a design.

BTW:

Shame the OP did not say what the app was nor the PWM frequency or current levels involved. Usual PITA novice stuff.

..... Phil

M

rrent frequently enough to get quasi-stable individual current samples whos e values have to be squares and summed before the square root of the mean v alue is extracted.

ist.

You may be able to see further inside the meters than I can, but my intenti on was to high-light the fact - that you have also emphasised - that the me ter has to be fast enough to square the actual signal rather than some aver age value of a signal over a period.

If I were building such a meter, I'd digitise the current and do the squari ng, averaging and root mean square extraction in the digital domain, and ea ch digitisation of the current would be a sample.

Sampling doesn't have to mean stroboscopic sampling (which I have done).

er?

e of kilovolts of isolation between the 1.2 milliohm shunt through the IC a nd the current output derived from the Hall effect device.

urrent carrying wire can be passed through - eg:

dwidth is only about 80kHz. If the pulse width modulated current has been f iltered enough to minimise radiated RF before it gets measured, this should be plenty.

Even if he only wants to do a measurement, it does help to have some idea o f what might be going on.

t levels involved. Usual PITA novice stuff.

That's pretty common - we usually have to ask a few extra questions before we can get on to answering the question that the OP should have asked in th e first place.

--
Bill Sloman, Sydney

It's gonna be interesting to see the pissing contest that happens over this one. Hows about you state exactly what you're trying to do, how accurate you need, how much current we talking about?

Read the specs on the meter you are considering.

gives some relevant information for 50-60 Hz. use as long as 100mA resolution and 2% +/- 5 counts accuracy is what you need.

A typical hall-effect current sensor, including the sensor for a DC clamp meter, creates a current through a secondary coil, that nulls the magnetic field at the Hall sensor. If the sensed current changes much faster than the speed of the Hall-sensor current loop, isn't it likely the loop responds to the average of the input current? If the OP's PWM system is DC, of one polarity, he'd get an average measurement. But if he's got an AC current, then yes, the Hall sensor needs to be much faster than the effective AC frequency, to follow. Any RMS calculations would be performed after the sensor. In both cases high-frequency PWM could be averaged by the sensor.

--
 Thanks, 
    - Win

No. That's because a PWM scheme is intended to produce DC current, and RMS measurement is inappropriate. RMS meter technology is poorly adapted to the task, though some individual meters might be good enough...

A true-RMS meter has a crest factor of at least 4, and good ones are around 10, based on the full scale range. If you choose a FSR that's at least a quarter of the PWM high level, and the meter has enough bandwidth, it should work fine.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

** Not always, PWM can be used to control heating and lighting.

** Any meter, analogue or digital, will correctly read the average DC value of a PWM current - but the OP specifically wants to know the RMS value.

.... Phil

Probably not.

A good scope with a math package and a reasonably fast current probe should get you close.

** Your point being ?

** As alluded to in my post above.

** That last sentence seems contradictory. The average value of an AC wave is zero.

FYI Win:

A clear explanation in plain English is much preferable to the pseudo-academic tone of your post - the OPs query is not a test question posed on same Engineering exam paper.

..... Phil

Win is saying that the field cancellation servo will easily LPF a much higher frequency DC field than the speed of the servo, as long as the sign doesn't reverse (AC) - in which case it would need to be "fast enough".

That was a useful contribution, to me anyway.

It seemed pretty clear to me.

Clifford Heath.

.

Pulse Width Modulation can produce an approximation to any waveform you lik e - not just DC current.

Don Lancaster's "magic sine waves"

are a case in point.

If you use two switches connecting to positive and negative rails respectiv ely, you can make the famous "stepped sine wave" which is at the positive r ail for one third of the time, at the negative rail for one third of the ti me, and at zero for a third of the time - as two equal length periods betwe en the periods when the output is connected to either rail.

The third harmonic content of the "stepped sine wave" is zero, and the fift h harmonic content is tolerably low. The higher harmonic content can damage gear that was designed for pure sine wave excitation, but a bit of passive filtering can help a lot.

--
Bill Sloman, Sydney

** The point of the remark in this context is still missing.

** Win can interpret his own posts, thankyou.

** You have way over-snipped and posted a comment about yourself - which is impossible for others to interpret with any certainty.

.... Phil

Yes, an excellent example! I've got a Kill-a-Watt meter that does measure RMS current, ad wouldn't expect it to handle (unfiltered) PWM of the 'magic sine waves' type, beause it has internal sampling frequencies that can beat against the various harmonics in that modulation. It works fine for house current, though, in the absence of pulses of the PWM flavor. It's only intended for 60 Hz AC...

It's not some DC-specificity that makes the problem, it's the RMS validity with a signal input that has an unknown modulation. Some RMS meters will work, some won't.

** FYI to readers.

I recently purchased a Keysight U1232A, 6000 count DMM from Element 14 for a little over A$200.

The AC volts and current ranges are specified as being "true RMS" but the usable bandwidth way *exceeds* the spec sheet numbers.

Sine wave testing showed flat response to 40kHz +/- 1% with a -3db point at 200kHz. The crest factor is approximately 4.5 near FSD.

.... Phil

Bullshit, stop being a dick.

I quoted *all text* prior to you asking "Your point being?". Check it yourself! I only deleted *blank lines* and the following context - which I was not responding to.

You don't like me calling you out for playing dumb and attacking Win because you're envious of him having a real job where someone cares about what he does?

And here I was thinking that this thread had been moderately civil and useful.

Phil Alison is easy to irritate, and we had seemed to be doing a decent job of not irritating him. Now you blow it, for absolutely no useful purpose.

And I very much doubt if Phil Alison gives a toss how Win Hill makes his money.

He does seem to care about having stuff stated clearly and simply, which is an attitude I share. An assumption of too much background knowledge for the people who read this group is a perennial problem.

--
Bill Sloman, Sydney

** Simple fact and you are the "dick".

** All of which Win wrote, it was MY stuff you over-snipped.

** That is a great big lie as anyone can see.

You posted an unsupported opinion which equates to comment about YOURSELF.

I have no clue why you think it, so no reply is possible.

** Huh ??

I never play dumb, I do sometimes call folk's bluff when they fail to explain their posts.

** No, my post was purely in self defence, cos I rightfully object to being treated like a pupil by anyone. ** Ok folks, I think we have a loyal "Win fan" here and I have upset him with my comments. Fraid they all went right over his head.

.... Phil