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My new multimeter is here.

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Adam Munich

Thu Oct 07 2010, 09:55PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Alright, I did some DC offset testing and here's what I got;

With a 9V battery in series with the 5V function generator @5kHz (dc offset 9V) the readings were still 5V RMS. It appears DC offset does not bother the readings.

With the mains half wave rectified I got 61v.

It appears the meter overestimates voltages (5V = 7.5V) when it's measuring square waves, and underestimates triangle waves (5V = 4V). This is only when measuring RMS voltage, and not frequency. All waves measure the same to the freqmeter.

Ohm testing later tonight.

klugesmith

Thu Oct 07 2010, 10:08PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Grenadier wrote ...
It appears the meter overestimates voltages (5V = 7.5V) when it's measuring square waves, and underestimates triangle waves (5V = 4V). This is only when measuring RMS voltage, and not frequency. All waves measure the same to the freqmeter.

What is your reference for the actual RMS voltages of your square and triangle waves?
Perhaps the function generator voltage knob sets the -peak- voltage.
Can your function generator put out enough power to make a flashlight bulb glow,
so its brightness can be compared with an identical bulb running on DC?

[edit, to reduce proliferation] The new meter and/or the borrowed Fluke meter might have an easier time
measuring true RMS of sine, square, and triangle waves at 60 Hz rather than at 5 kHz.

Adam Munich

Thu Oct 07 2010, 10:14PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Actually I have no idea.

I just assumed that when the function generator is set to 5V it puts out 5V RMS on all waveforms. Maybe the meter is measuring correctly, I don't really know. I wouldn't rule it out though since the freqmeter doesn't mind the waveforms.

I don't think the knob sets peak voltage, because a fluke (also on loan) measured 5V RMS for sine, but went nuts on the other waveforms.

Ken M.

Fri Oct 08 2010, 03:45AM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

It sounds more like the meter takes a series of samples over a period of time, so perhaps try the DC offset with the square wave again but if you can try it with different duty cycles, and see if it gives a closer to 5v reading, since the triangle wave only gave you 4v and most likely only has a peak for maybe 10uS, where as the square wave (provided it has 50% duty cycle) read 7.5v and had 100uS on time. At some point the meter will ignore the offset and measure only the AC, or if your generator doesn't have a duty cycle adjustment try the square wave at higher frequencys to findout what the band pass actaul is.

Nicko

Fri Oct 08 2010, 08:56AM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

Klugesmith wrote ...

Nick: to use the RMS feature, you move the range selector to an AC scale. Is it then a surprise to get just the AC component of a mixed waveform? As your reference states, if the same signal gives a nonzero result on DC scale, you can externally do an RMS addition of the two readings.

Yup... I can just about turn my DMM to AC... The question centres around the definition of "True RMS". If its to be the DC equivalent voltage/current for heating purposes, then just using the AC component is not sufficient. Its disingenuous to suggest that because you are on AC you just get the AC component - if its a "True RMS" meter and does not have an "AC+DC" mode (see below) then what are you actually getting is not the "True RMS" value if the meter is capacitively coupled on AC ranges.

The DMM916 has several modes - A straight AC range, an AC+DC(offset) range, and then you can select either a TrueRMS reading or an Average AC reading. In TrueRMS mode, on AC+DC, it includes the square of the DC component along with the mean sum-of-squares of the AC part before taking the overall square root to get the final RMS value.

e.g. on the DMM916, a 1V P-P 50Hz sine wave with no DC offset from a Tek FG5010, reads 0.3428V on the AC range and 0.3492V on the AC+DC range.
If I then add a +0.5V DC offset to that signal, The AC range still reads 0.3429V but the AC+DC range now gives 0.6210V. Which would you say is the "True RMS" value?

The bottom line is not what the manufacturer calls the type of reading, but whether the user understands what they are getting. Its one of those things that many are not clear about and, anyway, it probably not important for most as they are only dealing with periodic waveforms centred around 0V with a 1:1 duty cycle...

However, the tendency is for users to blindly trust the magic number that is displayed on their equipment without wondering how that value was arrived at... A sanity check is always a good idea

.

Cheers

Tue Oct 19 2010, 05:40AM

Registered Member #3336 Joined:
Location:
Posts: 1

a multimeter is an important device that can perform the multiple functions of measuring different units, such as voltage, current, and resistance.

Thanks

Adam Munich

Tue Oct 19 2010, 05:52AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

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