Products > Test Equipment
Multimeters With Low Ohms Function
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alm:
Some Fluke manuals have a note to this effect in their manual:

--- Quote from: Fluke 87 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.

--- End quote ---

--- Quote from: Fluke 8060A manual ---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).

--- End quote ---

--- Quote from: Fluke 189 manual ---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.

--- End quote ---

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:

--- Quote from: EEVblog on January 16, 2022, 04:23:24 am ---
• 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
--- End quote ---
[/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.


--- Quote from: kripton2035 on January 16, 2022, 08:51:53 am ---self-promoting, but this is really kick-ass to locate short circuits...
http://kripton2035.free.fr/Projects/shorty-display.html

--- End quote ---

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.


--- Quote from: AVGresponding on January 16, 2022, 10:52:38 am ---
--- Quote from: Neutrion on January 16, 2022, 04:34:58 am ---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.

--- End quote ---

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



--- End quote ---

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:

--- Quote ---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.
--- End quote ---
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.  :-DMM
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