DMM behaviour - Page 1
Forum section: electronics / general

author: donescamillo
author_rank: Contributor
timestamp: February 29, 2024, 08:09:32 am
content: 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

author: Kleinstein
author_rank: Super Contributor
timestamp: February 29, 2024, 08:15:14 am
content: 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.

author: donescamillo
author_rank: Contributor
timestamp: February 29, 2024, 08:22:10 am
content: 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.

author: tggzzz
author_rank: Super Contributor
timestamp: February 29, 2024, 10:49:55 am
content: Quote from: donescamillo on February 29, 2024, 08:22:10 amMeters 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.

author: donescamillo
author_rank: Contributor
timestamp: February 29, 2024, 10:57:35 am
content: 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.

author: tggzzz
author_rank: Super Contributor
timestamp: February 29, 2024, 01:09:31 pm
content: Quote from: donescamillo on February 29, 2024, 10:57:35 amI've never seen a meter indicate that, including Flukes.

author: Veteran68
author_rank: Frequent Contributor
timestamp: February 29, 2024, 02:54:56 pm
content: Quote from: donescamillo on February 29, 2024, 10:57:35 amI 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.

author: TimFox
author_rank: Super Contributor
timestamp: February 29, 2024, 03:16:47 pm
content: 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.

author: coromonadalix
author_rank: Super Contributor
timestamp: February 29, 2024, 03:20:03 pm
content: some meter may get OL if you mistakenly put voltage on the inputs, some beep tooor 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 fusessome fluke copied this behaviour toobut if you are not over the meter ranges an applied voltage will display a value ..... my brymen do that loll

author: Kleinstein
author_rank: Super Contributor
timestamp: February 29, 2024, 04:47:37 pm
content: 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.

author: donlisms
author_rank: Frequent Contributor
timestamp: February 29, 2024, 08:40:47 pm
content: 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.

author: Harfner
author_rank: Contributor
timestamp: February 29, 2024, 09:39:00 pm
content: 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.

author: tggzzz
author_rank: Super Contributor
timestamp: February 29, 2024, 09:48:53 pm
content: Quote from: Harfner on February 29, 2024, 09:39:00 pmSome meters do that in 4wire mode, if you connect the sense wires the wrong way round