# Measuring PWM current?

Started by April 29, 2019
```Probably not.

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

```
```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?
>

>  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 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.
```
```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 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

```
```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.
```
```whit3rd wrote:
>
>
>
> I've got a Kill-a-Watt meter that does measure RMS current,

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

```
```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
```
```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

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

```