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

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Sredni:
What is the internal resistance of the Uni-T UT139C when used as a DC voltmeter in the 600mV range?
The manual does not specify - it just says 10 megaohm for all ranges. But this is not the case, since the UT139C is capable of reading the correct VBE in a transistor with disconnected base. (My UT61E cannot do it because the mV range stops at 200mV).

I can measure the resistance in the volt-autorange setting (it comes out as 13.5 megaohm, in the right ballpark) but if I try to measure the current in the 600mV range I can only read 0.00 microamps. Someone with serious instruments (easy to find in here) and a UT139C (which might be in a different disjoint set :-) ) has ever measured it with a decent accuracy?

It is probably like the input resistance of the UT61E which is > 3 gigaohm, but... why on earth it is not stated in the manual is beyond me.
I mean, it's a good feature so why not advertise it?

TimFox:
I do not understand your conclusion about the Vbe (open) measurement.
If a voltmeter has roughly 10 megohm input resistance and some finite input current (> 60 nA) which is positive (which it may be) and you place it across a diode (B-E) in the forward direction, you should see somewhat less than 600 mV (since the diode current is very low).
To measure the input resistance, wire a 20 megohm (or greater) resistor in series with a 1.5 V cell (alkaline or other) to the 600 mV input.
To get an accurate result, you can measure the cell with the voltmeter on an appropriate range.

Sredni:
Tim,

it's a transistor with a floating base. I applied 8.2 volts between collector and emitter with the base disconnected. Simulation (and theory) says that due to leakage currents the VBE will about 230-240 mV at 27°C but I could not measure it inside my house because of the capacitive coupling with the mains lines (I get a negative value of a few volts for VBE) or for too small voltmeter resistance (it acts as a shunt).

I went out in my garden, far away from any buried mains cable and with the UT139C in mV range I was able to have a reliable measure. It was in the middle of the night and it was cold; at 0°C the simulated value is 380mV (I measured about 400 mV stable). Redid it tonight at 7°C and read some 320-330 mV (I did not bother to jot it down).

10meg input resistance is not enough to get the correct reading.

As for the negative multi-volt VBE I get in my house, I simulated capacitive coupling with AC mains and I see the VBE is a multi-volt sinuosoid dangling from the 250-300 mV level. The BE diode and the coupling capacitor seem to behave as a clamping circuit.

Kleinstein:
I have the similar UT133A. A quick test with a 1 µF capacitor and switching from the Ohm to 600 mV range shows a moderately fast discharge curve. From the time it takes to get from 600 mV to 300 V, I would guess some 10 M ohm. The test is easy to repeat.


For the measurement with the diode, I am afraid one could still pick up some mains hum. Another factor is light hitting the diode, that can produce a tiny current and finally even of the input is high impedance there usually is a bias current from the meter.  Even with the good bench DMMs it is common to see some 10-50 pA in high impedance voltage mode.

Just a diode in equilibrium and no external power source will show 0 V  as there is always some residual parallel resistance and with no power there is no voltage.

Sredni:
I have yet to try the voltage divider method Tim suggested because I have to find first a multimegaohm resistor in my pile of junk. I was aware of the method but I thought that with the tolerances of my instruments I could not get a reasonably accurate value. I might get a fairly good indication though, and surely it will tell me right away if the internal resistance is 10 meg or much higher. I will do it as soon as I find a suitable resistor.

I am convinced that the 600 mV range of the UT139C has much more than 10 megs because I am getting reliable measures of the BE voltage that I do not get with other meters with 10-15 megs resistance. The diode is not unpowered. There is a leakage current and there is a voltage across it (predicted by both theory and simulation). The voltage I measure (in the middle of the night) far away from any mains cable (I mean farther than 15 meters from ANY cable) is not only consistent with the theoretical value, but it also follows the temperature dependance predicted by theory. This is not a random reading. (And I have to say, I was surprised I could measure it so consistently).

EDIT: the values I measured are slightly higher than what theory predicts and this could be the result of the current flowing in the meter's probes. I have to say, tho, that up to now I have only tested one transistor. I will try with different BJTs, maybe the one I got is cooked and gives by mere chance values that appear to agree with theory?

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