Author Topic: DMM behaviour  (Read 1530 times)

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Offline donescamilloTopic starter

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DMM behaviour
« on: February 29, 2024, 08:09:32 am »
Hello,
I just bought an AstroAI digital multimeter (M6KOR model) about 30 pounds form amazon UK.
I set it on measuring resistance and connect the test leads to a 9V battery.
When the test leads match the battery polarity it measures 0.L, when they don't it measures 0 Ohms.
Is that the right behaviour?
I thought in both cases should be 0.L or some indication of error.
Thank you
 

Online Kleinstein

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Re: DMM behaviour
« Reply #1 on: February 29, 2024, 08:15:14 am »
The resistance measurement is not meant to measure a voltage source. The 9 V are more than the meter normally gets there. So it is normal to get the OL indication.
For the reverse direction it would be the wrong polarity. So one may expect a negative overflow, at lieast internally. It looks like the meter decides that a negative resistance is not right and shows 0 instead.
Also showing overflow would make more sense though.
 

Offline donescamilloTopic starter

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Re: DMM behaviour
« Reply #2 on: February 29, 2024, 08:22:10 am »
Thank you for your timely reply.
I thought so too, multimeter should treat voltage on RESISTANCE selection as an error and be consistent and show something 0.L.
« Last Edit: February 29, 2024, 08:24:50 am by donescamillo »
 

Offline tggzzz

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Re: DMM behaviour
« Reply #3 on: February 29, 2024, 10:49:55 am »
Thank you for your timely reply.
I thought so too, multimeter should treat voltage on RESISTANCE selection as an error and be consistent and show something 0.L.

Meters measure resistance by forcing a known current through the device, and measuring the resulting voltage.

Your meter is indicating that the voltage is too high to measure, i.e. an overload "O.L.".

Decent meters won't be damaged by such overloads.
« Last Edit: February 29, 2024, 10:53:01 am by tggzzz »
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
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Offline donescamilloTopic starter

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Re: DMM behaviour
« Reply #4 on: February 29, 2024, 10:57:35 am »
I absolutely understand this, I used to work in the field long ago.
My point was that a good multi-meter would somehow indicate an error when set for resistance and sensing voltage or at least be consistent no matter how you connect to a voltage source.
I have some recalls that previous multi-meters I used would indicate 0.L no matter how I connect to a voltage source.
This one thinks it is measuring voltage disregarding that RESISTANCE is selected by the rotatory switch.
Oh, well, next one will be a FLUKE, if the better half allows it.
 

Offline tggzzz

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Re: DMM behaviour
« Reply #5 on: February 29, 2024, 01:09:31 pm »
Oh, well, next one will be a FLUKE, if the better half allows it.

I've never seen a meter indicate that, including Flukes.
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 
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Offline Veteran68

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Re: DMM behaviour
« Reply #6 on: February 29, 2024, 02:54:56 pm »
This one thinks it is measuring voltage disregarding that RESISTANCE is selected by the rotatory switch.
Oh, well, next one will be a FLUKE, if the better half allows it.

I have over 35 DMMs in my collection, including several Flukes. I don't recall any of them indicating any kind "error" if they measure something out of spec while in resistance mode. Some might say O.L. while others might say 0 while others might just show a random value. It's undefined behavior, because what you're doing is not a proper use of the resistance function.

The user of the DMM should know what they're doing and understand, for example, that you can't measure resistance directly across the terminals of a battery with a regular DMM. DMMs can't conceivably account for every mistake or misguided uses. That just adds cost and complexity to a meter to protect the few idiots out there. Heck many cheap meters won't even have basic input protection to protect the meter from incorrect usage on mains -- they'll just go bang or die silently.
 
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Offline TimFox

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Re: DMM behaviour
« Reply #7 on: February 29, 2024, 03:16:47 pm »
As pointed out above, when the rotary switch is set to resistance, the meter itself is reading voltage (having switched in a current source).
There is no physical difference between the voltage from a too-high resistance (or open circuit) and a similar voltage directly imposed on the terminals.
How would a practicable meter distinguish those two cases from each other:  they both result in an off-scale (“overload”) voltage.
 
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Online coromonadalix

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Re: DMM behaviour
« Reply #8 on: February 29, 2024, 03:20:03 pm »
some meter may get OL  if you mistakenly put voltage on the inputs, some beep too

or some "ol"    gossen metrawatt 28 29 series, i have,   if you are in the noob / auto mode lolll   it will simply switch to vdc or vac if it detect something beyond normal ohms values,    i've done many mistakes in normal ranges, and the meter never failed,  no blown fuses

some fluke copied this behaviour too


but if you are not over the meter ranges an applied voltage will display a value .....    my brymen do that  loll

« Last Edit: February 29, 2024, 03:22:36 pm by coromonadalix »
 
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Online Kleinstein

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Re: DMM behaviour
« Reply #9 on: February 29, 2024, 04:47:37 pm »
Most meters will convert the voltage to a resitance in some way. With 9 V this would be overload, but with 1.2 V from a NiMh cell or similar this could be a more normal value in the range.

I just did the same test with a UT133A, and it also shows OL with the neg. side to COM. and 0 with the opposite polarity. This happens both with 9 V and 1.2 V. So the bahavior is not so unique.
 

Offline donlisms

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Re: DMM behaviour
« Reply #10 on: February 29, 2024, 08:40:47 pm »
Another way to look at is (1) these are two different error conditions, and (2) the meter is indeed giving an error indication for each.
 

Offline Harfner

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Re: DMM behaviour
« Reply #11 on: February 29, 2024, 09:39:00 pm »
I am not aware of DMMs trying detect that sort of error (having a voltage while switched into resistance mode). It would not work reliably for a small voltage and the user is supposed to know that stuff.
In fact I saw a negative resistance at least once, with the feeling of "damn, you did not check there is no external voltage".
At least some isolation meters however do make a check for voltage before measuring resistance. But they look for mains voltage before putting out 1000V. That is easier to do. 
 

Offline tggzzz

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Re: DMM behaviour
« Reply #12 on: February 29, 2024, 09:48:53 pm »
In fact I saw a negative resistance at least once, with the feeling of "damn, you did not check there is no external voltage".

Some meters do that in 4wire mode, if you connect the sense wires the wrong way round :)
There are lies, damned lies, statistics - and ADC/DAC specs.
Glider pilot's aphorism: "there is no substitute for span". Retort: "There is a substitute: skill+imagination. But you can buy span".
Having fun doing more, with less
 


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