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| What is the internal resistance of the Uni-T UT139C (on the mV range)? |
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| bdunham7:
OK, 8V C-E gives me unmeasurably low (<2pA) current, 10V yields 2-3pA and 20V is 5pA. My analog picoammeter only goes down to a 100pA full-range scale, so I'm at the limits of precision. Putting a 1G resistor from E to B increases the current but the reading is very unstable (40-250pA) and I'm using a really messy long-leaded layin-on-the-bench sort of setup so I suspect this may be a noise issue. Still the first measurement is way less than 1.4nA. |
| Sredni:
Yes, I am starting to think I got a black swan here. Not one, but two consistent measurement that mimicked the expected voltage. It is raining now, so my next measurements outside will have to wait (I am made out of salt and I will dissolve myself if I catch some rain). So, I tried to see what I get inside, in the middle of the interference zone, just for the sake of curiosity. I read about 240 mV at 19.5 °C. (which I was not able to read last time - mind you this is the opposite side of the table) LTSpice says 328 mV, while Microcap says 215 mV (yes, I know there are variation just like in beta - I use simulations to get an idea of what I should see in the real world - few millivolts, tens of millivolts, hundreds of millivolts...). Now I'm wondering: is this another coincidence, the third one? I moved the voltmeter and the dut around, it reads 309 mV. Move it again, and it's 307 mV Move it again and it's 270 mV Move it again and it's 275 mV at 19.4°C Turn it off and on and it's 275 mv at 19.2°C Rotate voltmeter and repeat, it's 258.2 mV at 19.1°C (it should get higher, not lower with lower temps) And yes, the measurements takes several minutes - about five to ten minutes (I did not check) and the approach is most certainly exponential. I too thought about charging the base capacitance, but is it charging it to the value I was expecting to read and imposed by VCE? Funny thing, the measures I obtained in the middle of the garden in the dark of the night were almost instantaneous. What was I seeing? I was using normal probes, while now I used clips and no hands... EDIT: regarding the instrument's bias current, I figured that a few picoamps would not change VBE that much if the 'internally generated' value of hundreds of millivolts was to be seen. It's the impedance of the instrument that changes the configuration, altering the measure. |
| bdunham7:
--- Quote from: Sredni on March 09, 2023, 12:54:10 am ---(it should get higher, not lower with lower temps) --- End quote --- If the leakage current were irrelevant (or even zero if the power supply was off) you would still have the meter input bias current across the B-E diode. And since I get ~229mV with my meter with just a B-E connection and that voltage drops if you warm the transistor (as it should in that case), perhaps you are seeing the same thing only with a different meter, different bias current and a different transistor. 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. |
| Sredni:
--- Quote from: bdunham7 on March 08, 2023, 11:43:34 pm ---In your simulations with the 10M and 3G resistors in place, what is the leakage current then? Still 1.4nA or does removing that tiny bit of current from the base reduce it? --- End quote --- Without base resistor, 1.359 nA for IE and IC, with IB = 0A And VBE = 383 mV at 0°C With 10 meg base resistor to gnd IB = -7.9998 pA it's getting out of the base, into the resistor IC = 8.0024 pA but the current in the RC resistor, which shoud be equal is 7.9936 pA IE = 0.1974 fA With these values VBE is 79 uV. Microvolts! It's basically skipping the BE diode. And there are numerical errors here and there. (with 3 gigaohm VBE becomes 24 mV) With 70 gigaohm resistor to gnd IB = -5.072 pA out of the base, into the resistor IC = 422.196 pA IE = 417.119 pA With these values VBE is 355 mV, almost there to the open base value. |
| bdunham7:
--- Quote from: Sredni on March 09, 2023, 12:54:10 am ---EDIT: regarding the instrument's bias current, I figured that a few picoamps would not change VBE that much if the 'internally generated' value of hundreds of millivolts was to be seen. It's the impedance of the instrument that changes the configuration, altering the measure. --- End 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. |
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