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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
I need some help in developing this idea. i dont know if it will work or if its even useful. ProudMary, I and others have run into problems with resistors and there quoted value. i would like to measure high value resistors. typical DMM's max out at 40Mohms, the Hp34401A tops out at 100Mohms. we often need to measure 300+Mohms, so how are we to do such a thing? it gets even worse... as Proud Mary says here...
Proud Mary wrote ...
We all know that resistance can vary with voltage - especially high voltage - but quantifying dR:dV empirically might involve access to HV calibration standards not easily obtained.
Normal Ohm meter functions are carried out at relatively low voltage, like less then 20V or 9V, perhaps a wise user would fear the difference of a 100Mohm measurement at 9 volts vs. a 100Mohm measurement at the intended circuit voltage of 20+kV.
I am thinking of using a wheatstone bridge, which would be powered by 10-25kV DC, then there would be a known division side, and then the side under test, with the uA and V meter giving the data out. Im not sure about all this, but it has the advantage of testing the resistor more like the conditions it would be expected to operate at, thus perhaps if sig figs are considered we could figure out a useful ohm value? But maybe just a simple known Vin and measured uA out, is good enough, it would be less failure prone then a wheatstone.
I intend to use some of my many HV resistors for oscilloscope HV probes, so I would like to know several resistors are within +0.5% to -0.5% or a total uncertainty of 1% absolute. However, the measurement accuracy of the DMM worries me, having to consider statements like this: "Accuracy Specifications ± (% of reading + number of counts)" for "10.00000 V @ 0.0015% + 2 counts"
i presume that the meaning is the (+ percent of error , and then + # of counts) which sets the upper uncertainty, while the ( - percent of error, and then - # of counts) which sets the lower uncertainty. and thus the theoretical, exact, perfect value lies within this band of uncertainty? but when they say "counts", what does that mean for a 6.5 digit or a 3.5 digit meter? if 1.375V is displayed on a 3.5 digit meter are there 1375 counts?
i was thinking of using the icl7107cpl A/D conversion IC then i would be able to choose the circuit impedance and accuracy for a scale of 0 to 200.0 uA, and another for 0 to 2.000 mA.
Registered Member #3610
Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506
I use a 400V power supply and the microamp range on my multimeter then calculate the value with Ohms law. It has worked fine to measure a 200M resistor, that's the higest I've tried.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
James wrote ...
I use a 400V power supply and the microamp range on my multimeter then calculate the value with Ohms law. It has worked fine to measure a 200M resistor, that's the higest I've tried.
Well Ive done this too, for critical components. The problem is 400V/200M = 2uA so if I wanted 1% I would need to see 2.00 uA on the display. (from 2.02 to 1.98 uA) to see the 198M or 202M value of a labeled 200M resistor. So the 40 nanoAmp difference accounts for the 4Mohm variation.
ProudMary and I had a little snag when we both ran into nanoamp and picoamp quanties being used to develop a V drop, which was then measured. As there was insuffcient current for the saturation of the A/D silicon to function. trying to read less and less current, and then using math can be problematic, given leakage paths, silicon, and other limitations.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Patrick wrote ...
.ProudMary and I had a little snag when we both ran into nanoamp and picoamp quanties being used to develop a V drop, which was then measured. As there was insuffcient current for the saturation of the A/D silicon to function. trying to read less and less current, and then using math can be problematic, given leakage paths, silicon, and other limitations.
Can you not overcome this by using a parallel resistor of known, suitable value?
I guess this would (more or less) bring you back to the wheatstone bridge idea?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Before I forget, Patrick, measurements made with Coleman's Patent Galvanic Engine must be ultimately traceable to the Quantum Hall Effect Resistance Standard - "the QHE Ohm."
I'd expect to see a paragraph or two on primary and secondary resistance standards in your dissertation. Standards are the common language that makes scientific discourse possible. Without them, measurement is arbitary, and comparison nonsense.
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
There;'s a nice trick for measuring small currents to fairly high accuracy. You use a cheap digital multimeter.
I presume that you have access to a decent meter too. Use the good meter to verify/ calibrate the cheap one on it's lowest voltage range- typically 200 mV. Then check the input resistance on that range- it's typically very close to 1MOhm.
Now connect a known voltage- say 10.0 V in series with your cheap meter ( set on the 200mV range ) and the high resistor. Say its 200 M. OK with 10 V and 200+1 MOhms you will get 10/201000000= 49.8nA That will flow through the 1M resistor in the meter and give 0.0498V or 49.8 mV. That's the clever bit- the meter reads directly in nanoamps. Reading 2µa is easy- you just use the 2V range on the meter. You have to remember to subtract off the 1MOhm from the meter when you do the calculation- but that's no hassle. Incidentally, if your "good" meter is calibrated to the QHE ohm, (and the HQ volt) then the measurement is too. If you are prepared to take the meter manufacturer's word for it that your meter input resistance is what it says in the manual then you don't need a second meter. You only need that so you can use the good meter to measure the input resistance.
You still need to worry about leakage currents , but with high resistors like this you are going to be stuck with that anyway. At lest the equipment is cheap.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Proud Mary wrote ...
Before I forget, Patrick, measurements made with Coleman's Patent Galvanic Engine must be ultimately traceable to the Quantum Hall Effect Resistance Standard - "the QHE Ohm."
I'd expect to see a paragraph or two on primary and secondary resistance standards in your dissertation. Standards are the common language that makes scientific discourse possible. Without them, measurement is arbitary, and comparison nonsense.
Dam you Proud Mary! If I get my Ph.D., it will be in large part due to you Proud Mary. Someday were going to have to meet.
Bored Chemist wrote ...
There;'s a nice trick for measuring small currents to fairly high accuracy. You use a cheap digital multimeter.
I presume that you have access to a decent meter too. Use the good meter to verify/ calibrate the cheap one on it's lowest voltage range- typically 200 mV. Then check the input resistance on that range- it's typically very close to 1MOhm.
Now connect a known voltage- say 10.0 V in series with your cheap meter ( set on the 200mV range ) and the high resistor. Say its 200 M. OK with 10 V and 200+1 MOhms you will get 10/201000000= 49.8nA That will flow through the 1M resistor in the meter and give 0.0498V or 49.8 mV. That's the clever bit- the meter reads directly in nanoamps. Reading 2µa is easy- you just use the 2V range on the meter. You have to remember to subtract off the 1MOhm from the meter when you do the calculation- but that's no hassle. Incidentally, if your "good" meter is calibrated to the QHE ohm, (and the HQ volt) then the measurement is too. If you are prepared to take the meter manufacturer's word for it that your meter input resistance is what it says in the manual then you don't need a second meter. You only need that so you can use the good meter to measure the input resistance.
You still need to worry about leakage currents , but with high resistors like this you are going to be stuck with that anyway. At lest the equipment is cheap.
I was considering this too Bored Chemist, but it still uses low volatge for which dR:dV at HV, cannot be included.
I beleive I can calibrate the ICPl7017 and can be made to measure a High value resistence now, and of the QHE standard later.
Registered Member #311
Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253
wrote ...
I need some help in developing this idea. i dont know if it will work or if its even useful. ProudMary, I and others have run into problems with resistors and there quoted value. i would like to measure high value resistors. typical DMM's max out at 40Mohms, the Hp34401A tops out at 100Mohms. we often need to measure 300+Mohms, so how are we to do such a thing?
Many Fluke DMMs have a NanoSiemens conductance range, manually ranged from the ohms range, which will get you a bit further. Most DMMs have a default Rin of 10M, so this can be used as a voltage divider to easily measure into Gohms, possibly after some calibration to verify the Rin.
wrote ...
Normal Ohm meter functions are carried out at relatively low voltage, like less then 20V or 9V, perhaps a wise user would fear the difference of a 100Mohm measurement at 9 volts vs. a 100Mohm measurement at the intended circuit voltage of 20+kV.
I am thinking of using a wheatstone bridge, which would be powered by 10-25kV DC,
As soon as you start using that sort of voltage for low-current measurements I think you will start to run into major corona leakage problems.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
mikeselectricstuff wrote ...
As soon as you start using that sort of voltage for low-current measurements I think you will start to run into major corona leakage problems.
I dont see why. It will be under oil, with ball bearing spheres, and other anti-corona strategies for connections. besides the important "unplanned factors" are what i want to include in the measurement, ill give you two examples.
First, the dR:dV which is V dependent, and which needs to be part of the measured and operating conditions of each resistor.
Second, the surface, and body leakage paths must be "seen" as well if one is to expect 1% accuracy or preciscion. (Often leakage occurs at the interface layer between solid and liquid insulation.)
For both cases corona would be as great or greater of a loss, thus it must be effectivley suppressed.
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
"I was considering this too Bored Chemist, but it still uses low volatge for which dR:dV at HV, cannot be included." Then use a higher voltage and turn up the volts range on the meter. You are still going to be stuffed by corona losses at high voltages however you measure the current.
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High Voltage resistors, Measuring M Ohms to high accuracy.
