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What is the internal resistance of the Uni-T UT139C (on the mV range)?

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Sredni:

--- Quote from: bdunham7 on March 09, 2023, 01:07:31 am --- Try reversing the multimeter leads.  If the effect were not dominated by the meter, then you should see similar results with just a polarity change indicated on the meter.

--- End quote ---

256 mV at 18.9°C
invert terminals and... it does not change sign!!! Ah no, it's decreasing and needs to go through zero first.
-293 mV at 18.8°C
invert again and...
+251 mV at 18.8°C

Well, not bad for a sub-50bucks multimeter...


--- Quote ---I wouldn't assume anything about the impedance and bias current unless you can characterize them.  I've no idea how high the UNI-T bias current could be and the input 'resistance' is often not really ohmic and for meters with a spec of >10G that can mean >>10G. 
--- End quote ---

I agree, and that is why I asked if someone with decent instruments had measured it. I tried the divider method but when I measured the voltage at the source I get the same voltage at the meter under test - actually a mV less, since my meters are not accurate enough to allow me to get enough figures to compute Rmeter reliably. It sure is big and, as you say, it is not a resistance but it is probably some FET frontend.

bdunham7:

--- Quote from: Sredni on March 09, 2023, 02:06:47 am ---Well, not bad for a sub-50bucks multimeter...

 It sure is big and, as you say, it is not a resistance but it is probably some FET frontend.

--- End quote ---

It occurred to me that the supply voltages for the front end might be a lot lower for an instrument with 600mV range than one with 20V, so the leakage actually could be pretty low.  Very high end DMMs get down in the single-digit pA range, but those are typically on input ranges of 12-20V.

On the 8842A, following your lead I tried a ~5V supply with a 1G resistor in series.  Shorting the resistor I get 5.0614V, with the resistor in series I get 5.0560V.  That works out to the meter having a 'resistance' at this level of about 937G. 

Kleinstein:

--- Quote from: bdunham7 on March 09, 2023, 02:42:54 am ---
--- Quote from: Sredni on March 09, 2023, 02:06:47 am ---Well, not bad for a sub-50bucks multimeter...

 It sure is big and, as you say, it is not a resistance but it is probably some FET frontend.

--- End quote ---

It occurred to me that the supply voltages for the front end might be a lot lower for an instrument with 600mV range than one with 20V, so the leakage actually could be pretty low.  Very high end DMMs get down in the single-digit pA range, but those are typically on input ranges of 12-20V.

On the 8842A, following your lead I tried a ~5V supply with a 1G resistor in series.  Shorting the resistor I get 5.0614V, with the resistor in series I get 5.0560V.  That works out to the meter having a 'resistance' at this level of about 937G.

--- End quote ---
The test with just the resistor and shortening it is measuring the input current. With these meters the input current is not directly propoertional to the voltage, there is quite some bias.
The input resistance is more descring how the input current changes with input voltage. So one would have to do the test also with a 2nd voltage (e.g. -5 V, maye 0 V).

aristarchus:
Not sure if this be of your help, just letting you know my experience with my UT139C.
I remember that it had difficulties making proper readings in low values (cant recall mV or mA), it was ok otherwise but in low readings it was giving improper results.
Then I found somewhere some pcb photos and compared the part values.
To my surprise one resistor was not the same value on pcb with what was on the photo, the pcb one was 1K IIRC (been a few years and forgot what but I think somone in R33, R4, R5).
When I changed it, the multimeter started working ok.
Could be a bad batch, most likely.

Either way I do not know if this could be your case, just saying my experience with this multimeter and placing the pcb photos I found at the time that helped me.

Sredni:
Thanks for the heads up, but I don't think the meter is the problem here. The (absence of a proper) measurement procedure most surely is. I don't think the meter is giving erroneous values on its own. I measured 0.7 mV both on the UT61E and the UT139C and they both agreed.

Right now I am mostly curious about how on earth I was able to get those two series of fast, clean and repeatable measures the first and second night. It is clear that the meter needs some 10 minutes to stabilize the reading, but I am absolutely certain the measures I did in 0°C and 4 (or was it 7)°C were very fast, but they shouldn't have been.
I tried again, same place, same time of the night (but different day and temperature, but most importantly different probes) and I get the same slow rising to the equilibrium value I get inside the house. I wonder if the relative humidity is playing a dominant role in this. Since the measurements are so dependent on small amounts of charges on the base, an environment that does not allow for charge to accumulate or deposit (if not driven) might explain what I saw, but ...

To add to the list of coincidences, here is a plot showing how - in numberland - the simulated BJT VBE with an open base changes with VCE, while stepping the shunt resistor from 1gigaohm to 71gigaohm in 5gigaohm steps. The first curve on the left is the simulation for actually open base at 18°C.


https://i.postimg.cc/jS5kcmz6/screenshot-10.png

The dots are the measurements I took with 2.9V(*), 4.36V, 8.3V, 16.3V at 19.5°C. Of course there are errors associated with each measure and the actual VBE, like beta, would be different in simulation and actual transistor but... I am getting a trend with VCE that mimicks what should be expected.
(It's true that all exponentials look alike and this might be a different exponential process, but... another coincidence?)

(*) the 2.9 VCE measurement was not exactly reproducible. I was able to get this trend starting from 0 and then going up in voltage, but when I got back (by switching batteries by hand, therefore messing with charges in the worst possible ways) reducing the voltage, the higher voltages were close to the ones shown, but the 2.9V VBE was much higher than that shown.

I plan to redo this measurement with appropriate shielding, clean probes, shorting all terminals before each measurement and in an electrically quiet environment to see if I can get a reproducible curve that fits the expectations. I would appreciate if someone in this forum, with better instruments and setups could try to see if this is a feasible measurement or not.

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