Products > Test Equipment

Multimeters With Low Ohms Function

<< < (10/13) > >>

AVGresponding:
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).

ermionesrl:
Kleinstein, AVGresponding:

What I expect to be inside the multimeters is a voltage limited, step controlled current source. The current is controlled (influence measurement) and different on many ranges, while the voltage is just limited (by giving a limited voltage input to the current source) and has not any requirement for accuracy or stability (in the 34401A the voltage is derived from +18V power with a simple 5.1V zener, I don't think in the U1401B took care of deriving it from a reference).

Unless there is a reason (like not to push semiconductors in conduction on some ranges, or the opposite) I expect it to be the same for all ranges even if might seem to have changed, in case multimeter insert a parallel resistor in highest range.

About falling time, if total capacitance is 10nF and total "leakage" resistance is 10Gohm, we would have a tau of 100s. Long, in effect, but I suspect both values being lower, in particular capacitance, so you should see voltage to settle in a reasonable time, if it is really stabilized.

It would be interesting to test again the U1401B with different condition batteries, to see if and how much the testing voltage change, as well as in the last but one range.

Kleinstein:
For the input capacitance I would expect maybe 100 pF to 1 nF. When at the limit the current source would also be no longer very high impedance. This is what limits the voltage.
depending on how the current shouce is made, the voltage may change. A common configuration is to start the current source from the positive supply and than get a defined drop on a switchable resistor. The voltage drop can change with ranges, with possibly less voltage lost in the lower current ranges in a way of switching rangs.  The 34401 uses such a circuit. So there can be a slightly different maximum voltage with the ranges.

Most handheld meters have a fixed voltage (like 200 mV with 2000 count) for the full scale reading on the ranges.
The fixed range corresponds to the configuration with a fixed votlage range that can be measured without a divider in front and thus with very high impedance.
Better bench DMMs have a higher voltage with the higher resistance ranges (e.g. 10 V for 20 M). This avoids very small current sources for the high ohms part.

David Hess:

--- Quote from: Kleinstein on January 20, 2022, 12:39:51 pm ---A common configuration is to start the current source from the positive supply and than get a defined drop on a switchable resistor. The voltage drop can change with ranges, with possibly less voltage lost in the lower current ranges in a way of switching rangs.  The 34401 uses such a circuit. So there can be a slightly different maximum voltage with the ranges.
--- End quote ---

In older multimeters, the configuration I regularly see bootstraps the switchable input divider (1k, 10k, 100k, etc.) to apply a fixed voltage across it.  So the operational amplifier buffers the voltage at the high input, and applies that voltage plus a fixed voltage to the switchable input divider and applies it back to the high input.  The example below is comparable.

Autoranging multimeters which use a grounded input divider must be doing it in a different way.

AVGresponding:
Well, I haven't charged the battery in the U1401B for some time, so I'll top it off, but I doubt it'll make any measurable difference.

I'm warming up my main LCR meter for a couple of hours, then I'll get a decent reading on the Fluke 8840A input capacitance.

Tried to get a nS reading on the Fluke 8840A to work out the input impedance, but both the Fluke 289 and Fluke 87V gave a settled reading of 0.00nS, and the Brymen 869S wouldn't settle, fluctuating between 0.02 and 0.07nS. Either way, It looks like the ≥10GΩ impedance is more > than =.

Also had to edit the spreadsheet, as I realised the Tek 912 had a LV ohms mode...   :palm:



EDIT: The Agilent U1401B with freshly charged battery peaked at 4.2788V, with an identical 41nA current. The voltage drops off at two or three hundred microvolts per minute, much more slowly than if the Fluke 8840A had no input.

EDIT 2: It looks like I should have had more patience the first time around with the Agilent!   :-DD  It stabilised at 4.2767V after around 10 minutes, then climbed back to the original figure over the next 20 minutes.
I'll leave it to run for a while and see where it ends up. Looks like it's just drifting around a bit under warm-up.

EDIT 3: It peaked at 4.3001V after about 3 hours and 40 minutes, then started a very slow (10-20uV/minute) drift down again. Not really a practical warm up time for a battery DMM! The current was still 41nA.
Naturally I had to unplug from the Fluke 8840A measuring voltage to plug into the Keithley 197 measuring current to check that, and when I plugged back into the 8840, the voltage went to 4.2995 pretty quickly. I'm confident to call it warm up drift and nothing else at this point.

EDIT 4: Hopefully the last edit for this post, but well...  Tried measuring the input capacitance of the Fluke 8840A; much more problematical than I expected. My main LCR meter (Iso-Tech LCR819) says slightly under 51nF, however checking with other meters to look for a mean value threw up various non-responses. Fluke 87V and Fluke 289 both went over-range,  Metrix 57 said about 430nF, Brymen 869S and Tek 912 both cycled through capacitance ranges without giving a reading, and the 328-based component tester said 150uF...
I have recently acquired a GenRad 1657 DigiBridge but I haven't yet had a chance to fettle it and check its reliability, so haven't tried it with that.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod