[SOLVED] Question about DMMs and pulsed signals

[d123]

Hi,

One DMM user manual says it refreshes the LCD every 2 to 3 seconds, but it is much faster than that regarding measurements. No information about sample rate.

Another DMM manual says the sample rate is twice a second, DC and AC from 50 to 300 Hz.

With a 100 Hz, 10% on:90% off duty cycle pulsed square wave signal, might the readings displayed on the DMMs be meaningless for DC currents and DC voltages?

e.g. I thought an 100mA 10% on, 90% off signal would read as 10mA, or a 1V signal as 100 mV, but am getting readings I do not understand such as 70mA.

What am I not understanding?

Possibly the problem is in part my not re-scaling/re-sizing the sense resistor and current clamp reference voltage adequately for a pulsed signal rather than for a pure DC signal for it to fit the goal (LED brighness, in this case)?

Thanks.

 

[Carry for cents bazar]

Hi d123!

Please post circuits and part numbers when you ask questions. This makes it so much easier for us. I see you mean a digital multemeter.

I don't understand the question?

What do you mean by this:

With a 100 Hz, 10% on:90% off duty cycle pulsed square wave signal, might the readings displayed on the DMMs be meaningless for DC currents and DC voltages?

The manual means that it measures twice a second and you have to wait until the measurement is updated. If you have a 300Hz signal which changes 4 times per second to 20Hz, 50Hz, 200Hz and 300Hz, the multimeter will measure twice per second and not be able to display all the changes.

 

[ads-ee]

Unless you have a good quality True RMS DMM then you shouldn't even attempt to measure a none sinusoidal signal with that meter. A 10% duty cycle square wave is not sinusoidal and won't give you meaningful results on a cheap meter.

Fluke does a nice job of why you need to use a True RMS meter for none sinusoidal measurements.

 

[KlausST]

Hi,

There are various measurement methods currently used.
* average
* rectified average
* true RMS including DC
* true RMS excluding DC
None of them is right or wrong. Sometimes one needs the one, sometimes the other.

I thought an 100mA 10% on, 90% off signal would read as 10mA

100mA / 0mA 10% duty cycle, square signal will be*
* Average: 10mA = (10% × 100mA)
* rectified average: 10mA = (10% × 100mA)
* RMS including DC: 31.6mA = 100mA × sqrt(10%)
* RMS excluding DC: 30.0mA = sqrt(( 90mA × 90mA x 10%) + (10mA x 10mA x 90%))

Besides this there are many "non standard" measurement methods, mainly cheap DVMs where the measurement method gives correct results with some waveforms only, like pure sine without DC, square wave 50% duty

70mA may come from a "non standard" peak measurement method..

Klaus

 

[d123]

Hi,

Thanks, everyone.

One DMM for AC voltage is: 'Response: average responding, calibrated in rms, of a sine wave.'

No AC current measurement option, only DC current.

Other DMM for AC voltage and current is: 'AC conversion type: Average sensing rms indication. AC conversions are AC-coupled, true rms responding, calibrated to the sine wave input.'

I think from replies to the thread, neither DMM is adequate for this purpose without back-calculating with formulas Klaus provided.

Relevant respective pages attached as images, in case they are of interest.

Thanks a lot.

 

[FvM]

I understand the original question so that it's exclusvely about measurements in DC range. In so far the references to RMS capability are unrelated.

Correct measurement of pulsed DC current and voltage average value is a different problem. Unfortuntely it's rarely excactly specified in multimeter specifications.

In case of autoranging instruments, you should care that autoranging is disabled and a range according to the expected peak value is selected.