Multimeters With Low Ohms Function

[ermionesrl]

AVGresponding:
I don't think you will ever succeed with a LCR meter. I would try to use the oscilloscope. Knowing the Z of a x10 probe is 10Mohm, put any voltage source on the meter, disconnect it abruptly (even by hand, we aren't on the picoseconds, but a reed relay, or a series of them not to add further capacitance, could help ) measure the decay time and calculate C from it.

I'll try later. It should work even with 10Mohm input multimeters, just knowing the resulting resistance will be 5Mohm. To measure effective input capacitance of the ohm function could be much more tricky.

[AVGresponding]

I'll give it a try with the 289 in parallel with the 8840, and use the data logger function of the 289, should give broadly similar results.

[Kleinstein]

I just saw one more case where a relatively low test current in the ohms mode is helping / needed: The on-resistance of JFETs is somewhat nonlinear and thus changes with current if the votlage grop gets high. I just got one such example reading 1.02 K and 1.14 K depending on the direction - so the effect is visible, though still good enough. In this example the test current was around 0.19 mA.

[Neutrion]

Here's a couple of pics of the BM869S high ohms output as seen by an oscilloscope.

The high Z meters have a high input resitance, but often also some input capacitance that may start in some not so well defined state. So one can still see an initial change in the voltage from the capacitance and just a possible slow setting to a final voltage. For the open circuit voltage the ouput impedance of the meter under test may no longer be that high in resistance even with a low current setting.

Yes, I might have a go at measuring the input capacitance, since it's not specified in the manual (for DC anyway, AC specifies <100pF).

Thanks for checking! It seems that the scope reading is about the same as the high Z multimeter reading.
So maybe my scope reading is correct as well.

I just saw one more case where a relatively low test current in the ohms mode is helping / needed: The on-resistance of JFETs is somewhat nonlinear and thus changes with current if the votlage grop gets high. I just got one such example reading 1.02 K and 1.14 K depending on the direction - so the effect is visible, though still good enough. In this example the test current was around 0.19 mA.

So we are having PTC/NTC and JFET so far. Never any problems with micros? With microscopic components I would think the effect should be more dramatic.

[ermionesrl]

AVGresponding, Kleinstein:
I finally took the time to do the measurement as promised. I tested the 34401A input in >10G DCV mode with oscilloscope and 10V source. Fall time seems consistent enough and average to 16.5ms, so input capacitance should be 750pF (fall time defined as 90% to 10% and tau~=fall time/2.2). It's a reasonable value, so perhaps the experiment is correct. I tried 10M DCV mode and fall time got to 8.2ms as expected, I tried ACV mode and (this was expected but not to this extent) 34401A is awful here with a fall time of 5.14s, that translates to an effective 240nF, not according to 34401A specs that say 1Mohm in parallel with 100pf. In the schematic (page 158 of the document 34401-90013) I see C301, a 220nF capacitor in series with a 1Mohm resistor going to a OPAMP virtual GND, thus we are reading the 220nF discharging to an effective R of 11Mohm, measuring it through a resistance divider 11M/10M. Taking everything in account, effective measured C is 230nF. Good, it's C301.

I also tested the Fluke 177. DCV: wonderful 590us resulting to a 53pF (effective Zin 5Mohm). Here the oscilloscope probe should be taken in a account with its 15pF, so we have 38pF, something less because also the cables count in this case. ACV: the circuit looks similar to the 34401A, but with a 10Mohm in series to a C that looks to be 12nF.

[AVGresponding]

You had better luck than me! Trying it with the F289 data logger yielded no usable results. I might try it with a scope this weekend if I get time.

[Alex Nikitin]

FWIW, my MetraHit 28S has the o/c voltage on all Ohms ranges ~0.62V at 23C room temperature, dropping to 0.3V with 10M connected on 30M range. The current on 300 Ohm range is ~0.2mA . The diode test mode has ~2.9V o/c (which I suspect is just the battery voltage at the time of measurement) and <1mA current (looks like a 3K resistor in series).

Cheers

Alex

[BlackFX]

Hioki 3238
Ohms 6v Max, 20nA (100meg range) - 1mA (200ohm range)
Low Ohms 0.45v Max, 100nA (2000k range) - 100uA (2000ohm range)

[alm]

I tried ACV mode and (this was expected but not to this extent) 34401A is awful here with a fall time of 5.14s, that translates to an effective 240nF, not according to 34401A specs that say 1Mohm in parallel with 100pf. In the schematic (page 158 of the document 34401-90013) I see C301, a 220nF capacitor in series with a 1Mohm resistor going to a OPAMP virtual GND, thus we are reading the 220nF discharging to an effective R of 11Mohm, measuring it through a resistance divider 11M/10M. Taking everything in account, effective measured C is 230nF. Good, it's C301.

That's the AC coupling capacitor in series that has forms a high-pass filter together with the input impedance. The ACV range is specified from 3 Hz - 300 kHz. At 3 Hz, the 220 nF capacitor has an impedance of about 240 Ohms. So I'm sure this won't affect the 1 MOhm +/- 2% spec. A fall time of 5.14s is well within the spec for a 7 second settling time in AC mode with the filter set to slow (see attached table from page 51 of the 34401-90004 user manual). Although I'm not sure how this relates to Ohms measurement?

[ermionesrl]

alm:
Yes, the 220nF capacitor is not a problem in real ACV measurements. It has nothing to do with ohm function but I found interesting it's possible to infer much about the input circuit with this simple setup.

It was interesting to know multimeter input capacitance to understand a supposed anomalous settling time in a measurement done by AVGresponding. Of course ideal would be to measure capacitance in ohm function, but I think it's very difficult to do it, so I did in DCV just to have at least an idea of the order of magnitude of the value.

[HKJ]

It was interesting to know multimeter input capacitance to understand a supposed anomalous settling time in a measurement done by AVGresponding. Of course ideal would be to measure capacitance in ohm function, but I think it's very difficult to do it, so I did in DCV just to have at least an idea of the order of magnitude of the value.

Capacity in DCV basically has nothing to do with ohm ranges.
I explain a bit about multimeters here: https://lygte-info.dk/info/DMMDesign%20UK.html
and here: https://lygte-info.dk/info/DMMDesignProtection%20UK.html#Ohm/Continuity/Diode/Capacity

The second link shows the typical input circuit for ohms (and other ranges), the first link shows the typical measurement circuit.

[AVGresponding]

It was interesting to know multimeter input capacitance to understand a supposed anomalous settling time in a measurement done by AVGresponding. Of course ideal would be to measure capacitance in ohm function, but I think it's very difficult to do it, so I did in DCV just to have at least an idea of the order of magnitude of the value.

Capacity in DCV basically has nothing to do with ohm ranges.
I explain a bit about multimeters here: https://lygte-info.dk/info/DMMDesign%20UK.html
and here: https://lygte-info.dk/info/DMMDesignProtection%20UK.html#Ohm/Continuity/Diode/Capacity

The second link shows the typical input circuit for ohms (and other ranges), the first link shows the typical measurement circuit.

Given the varied responses by different meters, I think it's fair to say that some at least may be atypical, in implementation if not broad theory of operation.

[HKJ]

Given the varied responses by different meters, I think it's fair to say that some at least may be atypical, in implementation if not broad theory of operation.

That is correct, but all DMM's has a high-z voltage input and at least one low-z path (current output) in ohms (Together with a very high z input path), i.e. very different.

[jasonRF]

I recently picked up a Fluke 8012a off of ebay for this feature, but unfortunately it doesn't work as well as the listing indicated...

Next time Dave collaborates with a company for a new DMM, he should include it.  I would buy one!

jason

[J-R]

I recently picked up a Fluke 8012a off of ebay for this feature, but unfortunately it doesn't work as well as the listing indicated...

Did you already try blasting the switches with contact cleaner and working them back and forth a bunch?