HV DMM probes and resistor voltage coefficient. - Page 1
Forum section: electronics / general

author: bdunham7
author_rank: Super Contributor
timestamp: April 21, 2019, 01:06:57 am
content: I bought an old Beckmann 50KV DMM probe--HV-211-22 (set up for a 22M meter input) intending to use it and perhaps adapt it work with my non-Beckmann, non-22M meters. Just trying it out as-is, I found that it seems a bit noisy and reads a bit low, even correcting for the impedance mismatch. This probe is 1000:1 and has a large spiral film resistor marked 955M (I think--hard to read) and a small resistor of about 0.95M. I replaced the small resistor with 2 510K Vishay high voltage 1/2W units and a 50K trimmer pot, which I had calculated should put me right in the ballpark. Starting out at 100V, I couldn't even get my 100mV so I had to change to a 100K trimpot. The highest voltage power supply I have is 1600 volts and I didn't want to fry myself right off the bat, so I tried calibrating it against a known good DMM (two different ones actually) at various voltages. Well, I was a bit surprised to see that it seems way non-linear and a bit jumpy/noisy as well. If I set it right on at 100 volts, then at 900 volts actual input I get 0.945xx out, a 5% non-linearity over only the bottom 2% of its range! If I set it at 500 volts, then at 100V I get 0.0985 volts and so on. I looked at the specs and it claims 2% at 25KV, 8% at 1KV and 50KV. I don't think self-heating is the issue because we're talking about 1 uA current, 1mW power on some physically large resistors (and 1 uW on the small ones). So the question--could voltage coefficient account for this at less than 1 KV? Can resistors degrade in a way that greatly increases their voltage coefficient? Any other ideas? I don't need 50KV nor do I need great accuracy--just want to be able to read CRT voltage and the like--but as things are I have no idea how far it would change even at oscilloscope CRT voltages (7-18KV).

author: David Hess
author_rank: Super Contributor
timestamp: April 21, 2019, 08:30:19 pm
content: Beckman might have relied on matching of the voltage coefficient of resistance in the divider resistors. Replacing the shunt resistor could compromise this.In practice, these high voltage probes are designed with an accuracy commensurate with the voltmeters they were used with.

author: bob91343
author_rank: Super Contributor
timestamp: May 03, 2019, 04:07:50 am
content: I don't know what instrumentation you are using but I have a General Radio 1644-A I think it is, Megohm bridge. It measures into the teraohms and varies the voltage on the unknown anywhere up to one kilovolt. It is useful for measurements of voltage coefficient.Perhaps making some tests with my bridge would answer some of your questions.

author: HighVoltage
author_rank: Super Contributor
timestamp: May 03, 2019, 11:23:32 am
content: Typical high voltage probes are very much inaccurate below 1000 V or sometimes below 2000 VA good linearity usually starts at 1kV at best, even for very good probes.It is best to calibrate your small resistor at the highest rated voltage and then look at the linearity.If you do not have a stable and verified 50 kV do the calibration at least at 10 kV and then look at the linearity from 10V to 10 kV and you will be surprised how how much fluctuation there is in the lower range.It seems that even good high voltage resistors need a certain voltage applied to them, before they stabilize. From my own experience, this voltage needs to be around 2-3% of the rated voltage.And then it depends also on the type of DMM you are using?

author: e61_phil
author_rank: Frequent Contributor
timestamp: May 03, 2019, 02:19:45 pm
content: Please find attached some HV probe measurements. The polts show the deviation from the calibration system. One can see the deviation changes over voltage.

author: bdunham7
author_rank: Super Contributor
timestamp: May 04, 2019, 01:19:06 am
content: Thank you all for the input. I was surprised by both the deviation I was seeing, and the actual specs which seem to expect the probe to deviate 8% over its range. The problem was it was deviating that much and more over 0 to 1 kV. I took it apart and found that the main resistor actually consisted of two sections that screwed together and were covered in a sort of clear wrap. There appeared to be some green staining near the separation point and perhaps a little looseness. So I cut the wrap and unscrewed it, and sure enough it was corroded at the facing surfaces. I cleaned it up, put a little DeOxit grease on it and put it back together. Now it is stable and appears in the ballpark from 0-1kV after adjustment. I also observed that this thing is useless as-is for anything but DC, AC reads way high even at 60Hz due to the parasitic capacitance of the main resistor, which seems to be 3-4pF.So now my project, for which I will solicit your suggestions, is to make my FreeBay probe into something I can use for HVDC and HVAC, at least at 60 Hz (think microwave ovens, etc) and perhaps higher. The first thing is that I do not have a 50kV source, or even a 10kV source. I also don't need to use it anywhere near this level, and if I do, I can live with it being inaccurate--or do something about it when that time comes. The best I can do is two precision 1.6kV sources that I might be able to tie together to give 2-3kV, if I'm careful. For now, I'd like to stick to using 1000 volts, because that is easy to do and is also the most I can accurately measure directly at this time. Will this result in higher kV readings being too high or too low?I realize that to make this work I need two circuits--one for a DC divider and one as an AC divider. So far I have 2 510K resistors plus a 100K trimmer pot, which gives me good results for 0 to 1kVDC. I need to parallel that with either a slightly small capacitor (3.3nF) plus a trimmer cap, or else a slightly large capacitor (4.7nF) plus a trimmer resistor in series. I'm open to suggestions for improving my plan.For those that asked, I have multiple DMMs, but I'm trying to set it up to work properly with a 10M DMM, for which I have either a handheld BK 391A, which is 10M consistently, or a Fluke 8842A with either a 10M shunt resistor or no shunt and using only the 200V range.