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Measuring PWM current?

Started by Spare Change April 29, 2019
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
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
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.
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
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...
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
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