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| Multimeters With Low Ohms Function |
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| 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:
--- Quote from: AVGresponding on January 19, 2022, 07:22:37 pm ---Here's a couple of pics of the BM869S high ohms output as seen by an oscilloscope. --- Quote from: Kleinstein on January 19, 2022, 03:02:16 pm ---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. --- End quote --- 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). --- End quote --- 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. --- Quote from: Kleinstein on January 23, 2022, 09:33:50 pm ---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. --- End quote --- 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. |
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