Measuring PWM current?

[Spare Change]

Can an RMS current clamp meter accurately measure the current of a PWM
voltage?

How else to measure PWM current without a shunt or direct in-line meter?

Thanks.

 

On Monday, April 29, 2019 at 1:25:01 PM UTC+10, Spare Change wrote:

Can an RMS current clamp meter accurately measure the current of a PWM
voltage?

It depends on the meter. It is certainly possible, if it samples the current frequently enough to get quasi-stable individual current samples whose values 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 current during the sampling period, so that the number squared is the linear average of that particular chunk current there's an obvious source of error (which may not be all that big if the current doesn't get close to or cross zero during the sampling period)

> How else to measure PWM current without a shunt or direct in-line meter?

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 the current output derived from the Hall effect device.

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

--
Bill Sloman, Sydney

 

Spare Change wrote:

Can an RMS current clamp meter accurately measure the current of a PWM
voltage?

** 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.

How else to measure PWM current without a shunt or direct in-line meter?

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


.... Phil

 

bill....@ieee.org wrote:

Spare Change wrote:
Can an RMS current clamp meter accurately measure the current of a PWM
voltage?


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

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

How else to measure PWM current without a shunt or direct in-line meter?

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 the current output derived from the Hall effect device.

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

https://uk.farnell.com/lem/la-55-p/current-transducer-50a-pcb/dp/1617405

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

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

** 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

 

[Mike]

On 4/28/2019 8:24 PM, Spare Change wrote:

Can an RMS current clamp meter accurately measure the current of a PWM
voltage?

How else to measure PWM current without a shunt or direct in-line meter?

Thanks.

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.

https://www.industrialcalibration.co.uk/downloads/Fluke%2033x%20%28PN%201618765%20Rev.%203,%204-06%29%20Calibration%20Manual.pdf

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

 

On Monday, April 29, 2019 at 2:31:41 PM UTC+10, palli...@gmail.com wrote:

bill....@ieee.org wrote:

Spare Change wrote:
Can an RMS current clamp meter accurately measure the current of a PWM
voltage?


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


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

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

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

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

How else to measure PWM current without a shunt or direct in-line meter?

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 the current output derived from the Hall effect device.

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

https://uk.farnell.com/lem/la-55-p/current-transducer-50a-pcb/dp/1617405

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

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

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

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

BTW:

Shame the OP did not say what the app was nor the PWM frequency or current 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 the first place.

--
Bill Sloman, Sydney

 

[Winfield Hill]

pallison49@gmail.com wrote...

Spare Change wrote:

Can an RMS current clamp meter accurately measure
the current of a PWM voltage?

** 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.

How else to measure PWM current without a shunt or direct in-line meter?

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

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

 

[whit3rd]

On Sunday, April 28, 2019 at 8:25:01 PM UTC-7, Spare Change wrote:

Can an RMS current clamp meter accurately measure the current of a PWM
voltage?

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...

 

[Phil Hobbs]

On 4/29/19 3:32 PM, whit3rd wrote:

On Sunday, April 28, 2019 at 8:25:01 PM UTC-7, Spare Change wrote:
Can an RMS current clamp meter accurately measure the current of a PWM
voltage?

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

 

[Yzordderrex]

Probably not.

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

 

Some 1/2 wit called whit3rd wrote:

On Sunday, April 28, 2019 at 8:25:01 PM UTC-7, Spare Change wrote:
Can an RMS current clamp meter accurately measure the current of a PWM
voltage?

No. That's because a PWM scheme is intended to produce DC current,

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

RMS meter technology is poorly
adapted to the task, though some individual meters might be good enough...

** 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

 

Winfield Hill wrote:

pallison49@gmail.com wrote...

Spare Change wrote:

Can an RMS current clamp meter accurately measure
the current of a PWM voltage?

** 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.

How else to measure PWM current without a shunt or direct in-line meter?

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


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?

** Your point being ?

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.

** As alluded to in my post above.

Any RMS calculations would be performed after the sensor. In
both cases high-frequency PWM could be averaged by the sensor.

** 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

 

On Tuesday, April 30, 2019 at 5:32:52 AM UTC+10, whit3rd wrote:

On Sunday, April 28, 2019 at 8:25:01 PM UTC-7, Spare Change wrote:
Can an RMS current clamp meter accurately measure the current of a PWM
voltage?

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....

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

Don Lancaster's "magic sine waves"

https://www.tinaja.com/glib/msintro1.pdf

are a case in point.

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

The third harmonic content of the "stepped sine wave" is zero, and the fifth 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

 

[Clifford Heath]

On 30/4/19 12:12 pm, pallison49@gmail.com wrote:

Winfield Hill wrote:
pallison49@gmail.com wrote...
Spare Change wrote:
Can an RMS current clamp meter accurately measure
the current of a PWM voltage?
** 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.
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?

** Your point being ?

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.

> A clear explanation in plain English is much preferable

It seemed pretty clear to me.

Clifford Heath.

 

[whit3rd]

On Monday, April 29, 2019 at 8:10:46 PM UTC-7, bill....@ieee.org wrote:

On Tuesday, April 30, 2019 at 5:32:52 AM UTC+10, whit3rd wrote:
On Sunday, April 28, 2019 at 8:25:01 PM UTC-7, Spare Change wrote:
Can an RMS current clamp meter accurately measure the current of a PWM
voltage?

No. That's because a PWM scheme is intended to produce DC current,
and RMS measurement is inappropriate.

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

Don Lancaster's "magic sine waves"

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.

 

Clifford the Chirping Cricket Heath wrote:

pallison49@gmail.com wrote:

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

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?

** Your point being ?

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".

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

That was a useful contribution, to me anyway.

** Win can interpret his own posts, thankyou.

A clear explanation in plain English is much preferable

It seemed pretty clear to me.

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


..... Phil

 

whit3rd wrote:

I've got a Kill-a-Watt meter that does measure RMS current,

** 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.

https://www.keysight.com/en/pdx-2851345-pn-U1232A/handheld-digital-multimeter-3-digit?cc=AU&lc=eng&state=0



..... Phil

 

[Clifford Heath]

On 30/4/19 2:03 pm, pallison49@gmail.com wrote:

Clifford the Chirping Cricket Heath wrote:


pallison49@gmail.com wrote:

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

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?

** Your point being ?

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".


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


That was a useful contribution, to me anyway.


** Win can interpret his own posts, thankyou.

A clear explanation in plain English is much preferable

It seemed pretty clear to me.



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

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?

 

On Tuesday, April 30, 2019 at 5:38:14 PM UTC+10, Clifford Heath wrote:

On 30/4/19 2:03 pm, pallison49@gmail.com wrote:
Clifford the Chirping Cricket Heath wrote:


pallison49@gmail.com wrote:

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

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?

** Your point being ?

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".


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


That was a useful contribution, to me anyway.


** Win can interpret his own posts, thankyou.

A clear explanation in plain English is much preferable

It seemed pretty clear to me.



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

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

 

Clifford the Chirping Cricket know nothing wrote:

That was a useful contribution, to me anyway.


** Win can interpret his own posts, thankyou.

A clear explanation in plain English is much preferable

It seemed pretty clear to me.



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

Bullshit, stop being a dick.

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

I quoted *all text* prior to you asking "Your point being?". Check it
yourself!

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

I only deleted *blank lines* and the following context - which
I was not responding to.

** 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.

You don't like me calling you out for playing dumb ...

** Huh ??

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

and attacking Win

** No, my post was purely in self defence, cos I rightfully object to being treated like a pupil by anyone.

because you're envious of him having a real job where
someone cares about what he does?

** 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