Multimeters With Low Ohms Function

[EEVblog]

I have seen multimeters malfunction doing Ohms measurement on a DC rail with capacitance and semiconductors present.

Starting at the lowest Ohms range, beep and charging happens until the multimeter upranges with the corresponding lower compliance current then causing the capacitor's voltage to start dropping. This can also be aggravated due to semi's becoming biased on and current drain increases as the voltage goes up. It's a non-linear load. Auto-ranging will oscillate between ranges hunting all the time.

I believe the test current on the Brymen is quite low now at 0.3mA compared to 1-4mA on other multimeters. I just manual range to look for shorts.

Some old school meters have a "Low ohms" function that has a compliance voltage below 0.6V so it doesn't switch on semiconductor junctions in-circuit. Seems to have fallen out of favour since the 80's and 90's. My first digital meter, a Soar had it.

[EEVblog]

Some old school meters have a "Low ohms" function that has a compliance voltage below 0.6V so it doesn't switch on semiconductor junctions in-circuit. Seems to have fallen out of favour since the 80's and 90's. My first digital meter, a Soar had it.

e.g



And Metex:

[rsjsouza]

The Keysight U1272A/U1273A have a very low ohms voltage - I did a comparison a while ago with a few other multimeters (U1282A, 189, UT61E) and was quite pleased to find out this.
AFAIK it is not configurable like your examples.

[AVGresponding]

I have seen multimeters malfunction doing Ohms measurement on a DC rail with capacitance and semiconductors present.

Starting at the lowest Ohms range, beep and charging happens until the multimeter upranges with the corresponding lower compliance current then causing the capacitor's voltage to start dropping. This can also be aggravated due to semi's becoming biased on and current drain increases as the voltage goes up. It's a non-linear load. Auto-ranging will oscillate between ranges hunting all the time.

I believe the test current on the Brymen is quite low now at 0.3mA compared to 1-4mA on other multimeters. I just manual range to look for shorts.

Some old school meters have a "Low ohms" function that has a compliance voltage below 0.6V so it doesn't switch on semiconductor junctions in-circuit. Seems to have fallen out of favour since the 80's and 90's. My first digital meter, a Soar had it.

This sent me down the rabbit hole investigating the continuity compliance voltage on all my meters (no small task!     ).

I found some (to me) interesting results.

Most were in the range from 3-4.5V, with some outliers: F87V ~7V, Keithley 2015 6.6V, Gossen M242A 8.4V (running off 2 AA Ni-MH!), Fluke T5-1000 ~1.1V, UT139C 1.04V, and finally a winner-winner-chicken-dinner, the Mastech 2108A with 0.44V!
I had a strange result by accident when I tried my Fluke 99B S2, got around 500mV, but then realised it was in auto-ranging ohms mode, and not continuity. Put it in continuity, and it went right up to 3.2V!
Decided to try just ohms mode on a few other meters, and found pretty much the same thing; a compliance voltage of around 400-500mV, the one outlier I had there was the Tektronix DMM912 with ~850mV.
Obviously, using ohms instead of continuity has some drawbacks, like response speed, no audible signal (unless your meter has an audible output on %dev function, you could set that up), but it's worth bearing in mind, if you find something with continuity, check it with ohms too.

[EEVblog]

I'm thinking of a vidoe titled: "What happened to the Low Ohms Function?"
I could dig my original Soar out of the archives.
It would be interetsing to know what modern meters with have such a function, or have a complaince volatge under 0.6V?

[EEVblog]

This sent me down the rabbit hole investigating the continuity compliance voltage on all my meters (no small task!     ).

I found some (to me) interesting results.

Most were in the range from 3-4.5V, with some outliers: F87V ~7V, Keithley 2015 6.6V, Gossen M242A 8.4V (running off 2 AA Ni-MH!), Fluke T5-1000 ~1.1V, UT139C 1.04V, and finally a winner-winner-chicken-dinner, the Mastech 2108A with 0.44V!
I had a strange result by accident when I tried my Fluke 99B S2, got around 500mV, but then realised it was in auto-ranging ohms mode, and not continuity. Put it in continuity, and it went right up to 3.2V!

Yes, you might have to auto-range to get the desired result. the high ohms ranges have to have a higher compliance voltage otherwise the noise becomes an issue.

[bdunham7]

I'm thinking of a vidoe titled: "What happened to the Low Ohms Function?"
I could dig my original Soar out of the archives.
It would be interetsing to know what modern meters with have such a function, or have a complaince volatge under 0.6V?

Keithley 2010--low power ohms and dry circuit test <20mV OCV

https://download.tek.com/datasheet/2010.pdf

I have a dual-banana plug with a 10R resistor that I use for short circuit testing.  It keeps OCV down to 10mV with a 1mA test current and doesn't affect readings very much below 1R--this is an easy ad-hoc dry circuit testing method as well.

[EEVblog]

I'm thinking of a vidoe titled: "What happened to the Low Ohms Function?"
I could dig my original Soar out of the archives.
It would be interetsing to know what modern meters with have such a function, or have a complaince volatge under 0.6V?

Keithley 2010--low power ohms and dry circuit test <20mV OCV

https://download.tek.com/datasheet/2010.pdf

I have a dual-banana plug with a 10R resistor that I use for short circuit testing.  It keeps OCV down to 10mV with a 1mA test current and doesn't affect readings very much below 1R--this is an easy ad-hoc dry circuit testing method as well.

Interesting, front page brochure feature:

Unique Resistance Measurement Functions
Characterizing the resistance, linearity, or isolation of contacts, connectors, switches, or relays
completely and efficiently demands an uncommon combination of ohms measurement capabilities.
The 2010 offers:
• Low-power ohms measurement mode. Low-level resistance measurements can be made with
source current as low as 100µA, an order of magnitude lower than is possible with other DMMs,
so device self-heating is minimized. Among other benefits, this low-power measurement capability
makes the 2010 suitable for end-of-life contact testing per ASTM B539-90.
• Dry circuit test function. When measuring contact and connector resistances, it is important to
control the test voltage carefully in order to avoid puncturing any oxides or films that may have
formed. A built-in clamp limits the open circuit test voltage to 20mV to ensure dry circuit conditions.
• Offset compensated ohms function. This function eliminates thermal effects that can create
errors in low-level resistance measurements in system environments.
• Extended ohms measurement capability. The 2010 provides a 10W range for more precise
measurements of low resistances

[Neutrion]

This reminds me of the discussion somwhere about why the low vs high continuity voltages are good or bad.
Although I don't remember any positiv point for the high ones apart from getting through dirt easyer.

AVGresponding: Did you also measure the currents?


On the other hand the voltage vs. semiconductors is just one thing, to not to have false alarm on capacitances would reqiere an other solution.

[floobydust]

Low-power Ohms feature was well known and common in the early 1970's to 1980's. It was great for repairs involving BJT's, you could fast test for shorted components. I remember the boss in the repair shop harping at us about the feature, it was kind of a standard feature that was expected but not really known about. Some multimeter models advertised it, and others allowed you to select the feature. A few I can find:

Beckman Tech 310/310/320, HD110 max in-range voltage 250mV, OCV 500mV
Beckman HD110B, 320B max in-range voltage 200mV, test 300mV except 3V on 200R range
Sencore I forgot the model numbers
Simpson 260-6XLM and 260-XLMP  "low power resistance" Rx1 100mV 4.9mA, Rx10 100mV 490uA
Fluke 8010A, 8012A, 8050A, 8060A
Sperry AWS DM-3010, Elenco M1200B, Triplett 601/603, Viz WD758 thru 763, Omega HHM57
Tektronix DMM155, 157 450mV, normal is 900mV.
Agilent 34420A low power mode is 10% limited power 1mA vs 10mA on 100R, and voltage limiting 20,100,500mV selectable clamp
Keysight 34460A, 34461A, 34465A have "low power" ohms mode but poorly documented.
TeleDyne TSC805, TelCom TC818 DMM IC switchable 0.35V on all ranges above 200R.

So it was great for servicing TV's, stereos (not germanium) and any IC's without low V substrate diodes or Schottky diodes. You could test for shorts in-circuit, no need to pull parts.
I dislike the creep to lower and lower ohms test currents in multimeters- away from the pristine lab/bench conditions, these choke and die out in the field.

[kripton2035]

self-promoting, but this is really kick-ass to locate short circuits...
http://kripton2035.free.fr/Projects/shorty-display.html

[Squarewave]

That Soar looks rather like the AVO DA212

Some old school meters have a "Low ohms" function that has a compliance voltage below 0.6V so it doesn't switch on semiconductor junctions in-circuit. Seems to have fallen out of favour since the 80's and 90's. My first digital meter, a Soar had it.

e.g

[Kleinstein]

One can still test for shorts in circuit, if the open circuit voltage goes higher. A diode junction would still not measure very low ohms, but what you get with some 0.6 V at a resistor and this is usually quite high.

There are 3 points with a limited voltage/power: one is protecting sensitive parts that can not withstand a higher voltage and could be damaged (e.g. BE junction in reverse from more than some 6 V, some LEDs - at least according to the abs. max specs and some were actually sensitive to > 5 V).

The other point is measuring sensors like PTCs / NTCs, where the test current may already cause significant self heating. A small PT1000 tested with 1 mA test current is not ideal. A 10 K NTC with 1 mA test current can be way off. 

The 3 rd point is dry contact testing, so testing the low votlage performance of contacts, where a higher voltage can destroy an oxide layer at the contact and this way hide a contact problem. This dry ohms testing wants a rather low maximum voltage, often realized with a prallel resistor.

A high voltage is nice for diode testing to also measure a blue / white LEDs and maybe a 5 V zener.
On the other side high resistor ranges may need a relatively high voltage to gets low senstivity to hum.

[AVGresponding]

This reminds me of the discussion somwhere about why the low vs high continuity voltages are good or bad.
Although I don't remember any positiv point for the high ones apart from getting through dirt easyer.

AVGresponding: Did you also measure the currents?


On the other hand the voltage vs. semiconductors is just one thing, to not to have false alarm on capacitances would reqiere an other solution.

I did not, but it's easily arranged. Give me a couple of days to perform the tests and tabulate the results though.

Regarding the issue of turning semiconductor junctions on, it's also worth mentioning that quite a few here like to tinker with older equipment, that may well be full of germanium transistors. None of the compliance voltages I've seen so far would be low enough to not turn a germanium junction on.

[nali]

My Meterman 38XR circa 2005 vintage is about 480mV on all ranges

[alm]

Some Fluke manuals have a note to this effect in their manual:

Most in-circuit resistance measurements can be made
without removing diodes and transistors from the circuit.
The full-scale measurement voltage produced on ranges
below 40 MΩ does not forward-bias silicon diodes or
transistor junctions enough to cause them to conduct. Use
the highest range you can (except 40 MΩ) to minimize the
possibility of turning on diodes or transistor junctions. Full-
scale measurement voltage in the 40-MΩ range does
forward-bias a diode or transistor enough to cause it to
conduct.

In all fixed resistance ranges (200Ω to 200 kΩ), the test voltage is less than
that required to turn on most semiconductor junctions. This feature,
sometimes referred to as “low power” ohms, aids in troubleshooting by
allowing you to measure resistors independent of the effects of in-circuit
transistors and diodes. For the fixed ranges the maximum full scale voltage
across the circuit being measured is less than 250 mV. The autoranging MΩ
ranges have enough voltage to turn on semiconductor junctions (maximum
2.5V full scale), but the current is very low (2.2 μA maximum).

The resistance function can produce enough voltage to
forward-bias silicon diode or transistor junctions, causing
them to conduct. To avoid this, do not use the 30 MΩ or
500 MΩ ranges for in-circuit resistance measurements.

It's not about the open circuit voltage, but about the maximum voltage for an in-range resistance reading. So as long as the meter is in a range below 40 MOhm (for this particular meter), and it gives an in-range reading, you won't be forward-biasing any normal semiconductor junctions. I guess that's why these meters don't have a separate low Ohms function, because any range below the tens of Megaohm ranges is effectively a low Ohms range?

[Neutrion]

• Low-power ohms measurement mode. Low-level resistance measurements can be made with
source current as low as 100µA, an order of magnitude lower than is possible with other DMMs,
so device self-heating is minimized. Among other benefits, this low-power measurement capability
makes the 2010 suitable for end-of-life contact testing per ASTM B539-90.
• Dry circuit test function. When measuring contact and connector resistances, it is important to
control the test voltage carefully in order to avoid puncturing any oxides or films that may have
formed. A built-in clamp limits the open circuit test voltage to 20mV to ensure dry circuit conditions.
• Offset compensated ohms function. This function eliminates thermal effects that can create
errors in low-level resistance measurements in system environments.
• Extended ohms measurement capability. The 2010 provides a 10W range for more precise
measurements of low resistances

[/quote]

In which arrangement could be more than 100µA be a problem because of device self heating?(Apart from the mentioned PTC from Kleinstein.)
Also I am not qiet sure if I understand the last two points exactly.

Dave, if you do a video about it, might would be nice to see on the scope, how the different meters clamp back the voltage and how fast. Some people defend the 7V of the 87V with the claim that it would be clamped back fast. Now It would be interesting to see, if it could cause a problem in any arrangement.

self-promoting, but this is really kick-ass to locate short circuits...
http://kripton2035.free.fr/Projects/shorty-display.html

Does your circuit avoid the the false positives of fast continuity beepers caused by capacitances?
Because the actual original "problem" in the bm786 topic was that, I don't even know why we drifted away completely into the low voltage/current domain.

This reminds me of the discussion somwhere about why the low vs high continuity voltages are good or bad.
Although I don't remember any positiv point for the high ones apart from getting through dirt easyer.

AVGresponding: Did you also measure the currents?


On the other hand the voltage vs. semiconductors is just one thing, to not to have false alarm on capacitances would reqiere an other solution.

I did not, but it's easily arranged. Give me a couple of days to perform the tests and tabulate the results though.

Thanks, it would be nice to see.

From the bm789 manual:
"Beeplit diode tester test current typical 0,35 mA" But it can go up to 0,5mA at least if I attach it to a 10ohm multimeter input. No maximum is specified. But in continuity mode it stays around 0,35 mA.

"Short beep-alert treshold: Drop across 0,85V."   Is this the short false positive beep? Drop across the leads from the open circuit voltage?

"Beeplit  continuous ON treshold: <0,1V " Drop across the leads?  I measure 3,6mV with the arrangement above measured paralell with another meter.

Will do some more measurement soon.

[kripton2035]

Does your circuit avoid the the false positives of fast continuity beepers caused by capacitances?
Because the actual original "problem" in the bm786 topic was that, I don't even know why we drifted away completely into the low voltage/current domain.

don't know. on what kind of device(s) can I test that ?

[Neutrion]

If the beeping function is as fast as on the Brymen, you can just start to probe around any circuit for shorts, to see wether you get any short false beeps where there are no shorts. But again, this only happens if the beeper function is fast enough. But if you did not design your circuit to avoid it, it would be a wonder if it would work.

[gamalot]

I just did a simple test on my Kyoritsu KEW1021R. Its open-loop voltage is about 3V (the voltage of two AA batteries), the voltage drop across a 10M resistor is 180mV, and the voltage drop across a 1K resistor is 95mV. The smaller the resistor, the lower the voltage drop. 

[Per Hansson]

Should not the name of the thread (and any future video) include the word power or compliance voltage?
As I read it now it sounds like a meter that can show low resistances, like a 4-wire measurement or the dedicated low ohms mode of the Fluke 289?

[Fungus]

Dave, if you do a video about it, might would be nice to see on the scope, how the different meters clamp back the voltage and how fast. Some people defend the 7V of the 87V with the claim that it would be clamped back fast. Now It would be interesting to see, if it could cause a problem in any arrangement.

Yep. These are current sources so measuring the voltage into a high impedance doesn't tell us much.

I assume the clamping will happen at the speed of analog electronics (ie. sub-microsecond) but it would be good to test this.

[kripton2035]

If the beeping function is as fast as on the Brymen, you can just start to probe around any circuit for shorts, to see wether you get any short false beeps where there are no shorts. But again, this only happens if the beeper function is fast enough. But if you did not design your circuit to avoid it, it would be a wonder if it would work.

the bip on the shorty has an audio tone proportionnal to the value of the short. so if there is a charging capacitor, the tone is changing, and you can hear it.

[AVGresponding]

I just did a simple test on my Kyoritsu KEW1021R. Its open-loop voltage is about 3V (the voltage of two AA batteries), the voltage drop across a 10M resistor is 180mV, and the voltage drop across a 1K resistor is 95mV. The smaller the resistor, the lower the voltage drop. 

This is because meters use a constant current source, and measure the voltage across the resistor under test to derive its resistance.

[AVGresponding]

Ok the tests took less time than I thought, so here are the results.