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What is the internal resistance of the Uni-T UT139C (on the mV range)?
Sredni:
Ok, I've found a 10meg resistor. Tried the volt autorange and the divider halves the supply voltage, confirming 10 meg or so for the input resistance of the UT139C. But when I switch to the mV range I read almost all the voltage (496mV with a 0.5V on my inaccurate supply display) confirming that the UT139C has a gigaohm sort of input resistance on the millivolt range.
I will update this post with detailed measurements of the value of the resistor, the actual supply voltage, and a reasonable estimate for Rmeter).
bdunham7:
--- Quote from: Sredni on March 08, 2023, 08:07:36 pm ---Ok, I've found a 10meg resistor. Tried the volt autorange and the divider halves the supply voltage, confirming 10 meg or so for the input resistance of the UT139C. But when I switch to the mV range I read almost all the voltage (496mV with a 0.5V on my inaccurate supply display) confirming that the UT139C has a gigaohm sort of input resistance on the millivolt range.
--- End quote ---
That's interesting, it seems to not be in the documentation. Or I missed it, it is poor documentation at that.
bdunham7:
--- Quote from: Sredni on March 08, 2023, 07:49:36 pm ---The voltage is there with exactly zero base current. The current drawn or supplied by the meter can only alter the measurement.
--- End quote ---
I understand, I would just like to characterize it with an approximate source impededance, which is of course not truly the model but will give an idea of how much the meter might affect it. Then you'd have to characterize your meter for input impedance and input bias current. In the simulation, can you tell me what the C-E current is with the base open, then with say a 5G resistor from B to E?
Sredni:
The emitter and collector current in the simulation is 1.4 nA and yes, if I simulate with a shunt resistor the voltage drops to a few millivolts with a 10 meg resistor and even with a 3 Gohm resistor. I had it simulated before my first measurement in the cold of the night and I went out there just to prove I would only read a few millivolts (and not the minus 0.6 V of userxxxx on SE). And I was surprised to read hundreds of millivolts, in the ballpark of the simulation with an open base.
I also ran a simulation stepping Rshunt and only when it reaches several teraohms the simulation gives a VBE of hundred of millivolts. This is a voltage that should require an electrometer, and that is why I was surprised of being able to read it with bottom of the barrel instrumentation (the UT139C is a 50 bucks DMM!)
I am still left with the doubt I cooked the transistor because - ahem - before connecting 8.2 V between collector and emitter I placed 8.2 V between emitter and collector and the little guy got lukewarm.
In a few hours I will go out and risk my life in the abysmally low temperature of + 10 °C and try to measure a couple of untouched transistors.
bdunham7:
--- Quote from: Sredni on March 08, 2023, 10:48:59 pm ---The emitter and collector current in the simulation is 1.4 nA and yes, if I simulate with a shunt resistor the voltage drops to a few millivolts with a 10 meg resistor and even with a 3 Gohm resistor. I had it simulated before my first measurement in the cold of the night and I went out there just to prove I would only read a few millivolts (and not the minus 0.6 V of userxxxx on SE). And I was surprised to read hundreds of millivolts, in the ballpark of the simulation with an open base.
I also ran a simulation stepping Rshunt and only when it reaches several teraohms the simulation gives a VBE of hundred of millivolts. This is a voltage that should require an electrometer, and that is why I was surprised of being able to read it with bottom of the barrel instrumentation (the UT139C is a 50 bucks DMM!)
--- End quote ---
Well, don't get sucked into the trap of believing an erroneous measurement just because it looks like what you expect. I'm not saying that is or isn't happening here, but since you haven't accounted for the input bias current of the meter there's more to be considered. I'm also a bit dubious that you could take these measurements in any reasonable amount of time because the currents are so low and there is input capacitance to deal with. But with no experience, I'm open to any possibility and I'll let you explain what you think are the relevant theories.
I started a quick experiment myself and have dragged out the picoammeter for the next steps. I'm trying analyze it a piece at a time and also sidestep the issues of noise and meter bias that come up with a direct measurement.
The first step was to look at just the B-E junction with a very low current. My 8842A has a bias current of approximately -38pA so I connected it to the 2N3904 base and emitter so that it was forward biased. After some time, the voltage settled at ~229mV. The thing is, 'some time' was about 90 minutes. I can lower the voltage a tiny bit by warming the transistor with my fingers.
Next I'm going to measure the C-E leakage current with the base open. I'm expecting much less than 1.4nA. 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?
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