Measuring high voltages

Things at the limits.

Measuring high voltages

Postby Doug Coulter » Wed Dec 01, 2010 5:03 pm

There are a number of issues around measuring high voltages that don't exist inside the normal voltmeter, so we need a place to discuss those.

One is that you don't want your measuring device (usually) to draw much power, and as volts go up, for the same current draw from the measured source, power goes up. At some point, this may either strain the source, or your dividing chain's ability to dissipate power, so you are walking a line with this. As you try to go ever farther down in current because of this, the current can become so low at the output of the divider that noise issues crop up there, and you might need a very low current low voltage measuring device to read the result accurately.

Unlike a lot of other situations, at high voltages we have to begin to seriously consider various losses in insulators, which can either raise or lower the result you measure. For example, if there is something covering your series resistor (like fingerprints) the value of some of the large resistors can be reduced significantly, making your result read too high. On the other hand, if there's corona current being drawn from someplace along the series resistor, you reading will be lower than true. And it doesn't take an amount of either you'd think would be a serious issue. Even the resistance of pyrex glass or most any surface in high humidity can significantly affect your reading.

For voltages up to about 40-50kv, a TV service HV probe is mostly likely the way to go. These (B&K) and variations made by Fluke, Hewlett Packard and so on can often be had at hamfests, and they are for the most part, quite well made. They've solved the above problems pretty well up to these voltages, and usually will drive a 10 meg input DVM with minimal error (but beware some Harbor Freight meters that don't really have 10 meg inputs....despite the specs). Most of these are 1000:1, so give 1v per kilovolt, fairly handy with standard meters. Most also have reasonable frequency response at least up to the 10's of khz, which isn't fantastic, but isn't too bad unless you're looking for RF, and the word here is -- test it with a scope and known waveform from a frequency generator.

When you get to really high voltages, over 50k or so, the issues mentioned above become quite serious and it becomes hard to make up a divider chain that will reliably tell the truth to better than 10% (and fairly hard to get to 10%). At this point, all the issues above really start to kick in, and precision high value resistors aren't cheap at all, if you can even find them. Vishay and a few others make them, but most retailers don't carry them, and they're not cheap at all in any event. Most outfits that specialize in high voltage things like power supplies will sell you a divider that's guaranteed accurate, but the price may be a major shock. It's not that they are ripping you off, it's a genuinely hard problem to solve, especially when you have to expect the user isn't an expert (or they'd make their own, after all).

We have made a few successful probes here. We use a whole bunch of 10 meg resistors, usually 1/2 watt precision ones (not real cheap) that will stand off on the order of 1kv each according to ratings, for a max current of 100 ua. That's too much, of course, for most situations, so we don't use them at 1kv each -- we tend towards using more of them to get the current to half that or less, because that's too many watts for a light load on the measured source. For example, for a 100kv "probe" at 100 uA you are talking 10 watts load, which is enough to warm the resistors a good bit, which eventually ruins their not-very-good accuracy. To help with this, and with corona issues, we put them end to end in long quartz or soda-lime glass tubing, usually 3/8" or better, 1/2" stuff, and fill that with mineral oil. you make this chain of resistors, just solder them end to end, clean off the flux(!) and solder them to the center conductor of some common coax.
This is then glued into one end of the tube with silicone RTV (or something better if you have it, but beware stiff/brittle glues that will break easily later on). Once this has dried fully, we then stand the tube on end with the lead (use stranded wire) coming out the end, and fill it most of the way up with very pure mineral oil (we get it at McMaster Carr, it's better than the stuff they call baby oil). This has to be done carefully so as not to get any oil on the glass near the top, so you can seal that end too. We use a little homemade funell that terminates in some 1/8" OD copper tubing that you can stick down in there, and are careful extracting it when done so as not to wipe oil onto the inside of the tubing -- difficult, but crucial, and being very careful means not having to start all over with a new piece of coax (which you know you got oil on) and so on. We leave a bit of air space in the top, so when we seal it with RTV or whatever, there's some room for expansion to take place when the thing warms up. You can then affix some sort of corona ball to the "hot" end if you like or have the thing terminate inside the one on the device (which is going to have one if you're doing 100kv or more), to avoid streamer arcs into the air off sharp points. In some cases, your power supply might be all in oil anyway, which means you can put this whole thing in there with it, and not need the glass at all, which is a lot simpler.

Now we come to frequency accuracy. The way this is done in say, a scope probe (which works here too) is to put X capacity across the 9 meg resistor, and ten times as much across the 1 meg load resistor. As often as not, this doesn't actually take any physical capacitors other than those that exist parasitically anyway. Here, we have a tougher problem. You could either put tiny caps across each of the 10 meg series resistors...not cheap and not easy, with regular 1kv or so caps (if you can find them still) if that part of the chain has less than the required capacity, or try some other way. Here we have put a sleeve over some of the hot end of the probe, connected to the HV end, so as to get some through the glass, oil, etc to the chain.
Of course, you could try to reduce the capacity across the low end resistor, but there's going to be a limit, and any wire is going to add to that, or even the package capacity of an opamp you drive with that end. Depending on the details of your design and how it's realized in practice, you could have to add capacity to either side. We find that with the oil, which has a high dielectric constant, it comes out not too far off with nothing -- good deal if you can get it. Driving into a virtual ground at the bottom might help in some cases -- the inverting input of an opamp that has a feedback resistor, can help a lot. But at 50uA or so full scale currents, it has to be a really good opamp, with very low input currents, or that and drifts are going to drive you crazy. So choose wisely, there are a few I might mention in a later post if asked that seem to work well here. We've not yet tried the techniques Charles W is using on his ion chambers, or those fancy opamps with a built in chopper for stability, but it's quite possible those would do the best. I've been using things like LMC 660's with reasonable results, and live with a little current drift and the lack of HF response, but like always, Your Mileage May Vary. For now, when I'm looking for RF on top of a big DC source, I just place a scope probe a safe distance away from it (all inside a big grounded shield screen) and let the natural capacitive dividier do the job. This isn't quantitative by any means, but more often than not, good enough.

Some people find that for low accuracy situations, they can simply measure the voltage somewhere along the power supply's voltage multiplier. This will read high under load, due to increased drops in the multiplier stack, as well as those due to any ballast resistor (which you most probably need for the application anyway). But maybe you don't need extreme accuracy, and can back-calculate those losses anyway, from measuring current as well -- that's one of those "if it works, why not?" sorts of things.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Measuring high voltages

Postby William A Washburn » Thu Dec 02, 2010 7:51 pm

Doug,

Thanks for the education. I've been trying to build my own 40KV divider with ordinary resistors. The reason I tried was that I had seen
some lab photos with panel meters saying 10K to 100K. Therefore, I thought nothing of it. Just use your own resistors. After reading a bit
Today I called OHMCraft about their divider resistive elements and found out how expensive they are and how many I'd have to buy for them to sell to me.

Maybe the $170.00 for Fluke's probe for my Fluke 77 would save me a great amount of troubling issues. Now I realize I cannot make a Fluke 40KV probe from my junk box.

Thanks Doug
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Re: Measuring high voltages

Postby William A Washburn » Thu Dec 16, 2010 9:10 am

Doug,
I'm trying to build your little yellow multiplier.
I also found this:

Will this work for finding out out Distance Vs Voltage?
Thanks, Bill Washburn
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Re: Measuring high voltages

Postby Doug Coulter » Thu Dec 16, 2010 9:16 am

Ah, the (in) famous table. It's ok as a last ditch measurement technique if you have no other choice. Dumb things like surface finish topology at tiny scales, humidity make this vary around a lot, but it will always or nearly always get you within factor of a few -- if there's a bunch of series impedance in the supply, that will affect whether a corona streamer can go to a spark or not if the R is high enough to drop the actual voltage at the gap during the streamer. I'll have to do that thread soon on CW multipliers off CCFL, just been too busy with a bunch of other projects to get to it yet.

Digikey and others sell 10 meg resistors, 1/4 or 1/2 watt so cheap it's hardly worth worrying about -- you'll have to order something else to help amortize the shipping. You can use the ones not recommended for HV if you keep the volts across each one down, say 50ua apiece at full scale. It just takes a lot of them, but then you get a lot anyway for not much money.

Without extreme measures you're not going to have a lot of digits precision no matter what, due to things like corona, but if you get it repeatable, that's generally good enough.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Measuring high voltages

Postby William A Washburn » Wed Feb 02, 2011 9:26 pm

My Fluke 80K-40 HV probe arrived this week. I didn't want to pay $170.00 (this price showed up everywhere I looked).
Went to this site and found a rebuilt for $70.00. This will get me going for now up to 40 kV:

http://www.used-line.com/c6828030s1260-FLUKE_80K.htm
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Re: Measuring high voltages

Postby JonathanH13 » Mon Jul 18, 2011 5:56 pm

Somewhat of a trivial issue, but something that has been bothering me for a long time - my inability to measure over 40kv.

I have attempted to build a KV probe on two previous occasions, and both attempts have failed with wildly fluctuating and inaccurate measurements. I have finally sorted this out with attempt number three (by carefully following the instructions in this post). The result works great! Here is the schematic:

probe cct.jpg


For R1 I chose twenty 50 meg resistors, each one capable of holding off 5KV. So the entire string in series should stand off at least 100KV.

The multimeter's internal impedance (R3) is 10 meg, and R1 is 1 gig, so to keep my multiplier to a nice round number, I chose R2 to be 10 meg as well. So I guess the total resistance here is R1+(R2||R3) = 1005 000 000 ohms.
At 100kV this should draw 99uA (that is 100 000/1005 000 000), which is a heavier load than I would like, but I could not really afford higher value resistors.

I recently tested the new probe, together with my old probe (which tops out at around 40kV). The two probes track together to within 1% up to 40kv (I have to multiply the reading on the new probe meter by 200 to get the actual voltage).

Results = -10 = -2KV, -50 = -10KV, -94 = -19KV, -222.3 = -45kV

I pushed it up to 56kV whereupon corona leakage off the power supply obliged me to go no further, but the probe itself was fine :)

probe4.jpg
Teflon spacers were added to keep the resistors away from the glass


probe3.jpg
Two lengths of glass tube were joined by an acrylic coupling to achieve the correct length


probe2.jpg
Probe tip was lathed from brass rod and rounded



probe1.jpg
Care was taken to keep all solder joints smooth and rounded
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Re: Measuring high voltages

Postby Doug Coulter » Thu Jul 21, 2011 2:35 pm

Great job Jon, keep up the good work! This really isn't as trivial a problem as most believe.
picture-3065.gif
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Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Measuring high voltages

Postby Joe Jarski » Sat Aug 27, 2011 11:58 pm

I came across this video on how to make a high voltage (5kV) scope probe. It's kind of a reiteration of the methods above, but for lower voltages. Since these things cost more to buy than a nice scope - I thought it'd be worth posting.

http://youtu.be/jUvSP3BQpvs
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Re: Measuring high voltages

Postby Doug Coulter » Sun Aug 28, 2011 11:57 am

As the guy in the background says, yep. This is not trivial. Of course, for something close to this, MPJ has a probe (I got one myself) that's not too shabby.
http://www.mpja.com/prodinfo.asp?number=18148+TE
Is 100x, 2kv (rated, I've pushed mine past that for short times) and decent priced - 40 bucks or so. The 100 meg input is nice, and high enough to use on a faraday probe in the tank direct to see what you can see that way. Time response is decent.

If you can tolerate a higher divide ratio and therefore use a pretty small value R at the bottom of the divider, you can comp frequency by making the shield connect to the probe output wire, which then acts like a cap to the resistors up the divider chain. You can then add an outermost shield at ground to keep pickup out of it, and provide some of the capacity to ground...FWIW, the resistors, oil in pyrex get pretty close to the right capacity ratio for AC with a couple feet of coax to the scope if you do nothing at all...depends on what you're trying to measure.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Measuring high voltages

Postby chrismb » Fri Jan 13, 2012 6:38 pm

I am, right now, in the middle of a 'distraction project' away from my 'research thread', and instead have let myself drift into building some HV power supplies. More on that in due course, but the immediate issue is that it has brought me into HV measurement, for which I am now acquiring a newly-earned experience.

So up to now, I have been measuring to 20kV with a simple setup - I take a meter with a 1Meg internal resistance, then put a 100Meg resistor in each of the DVM's sockets - 200Meg total - therefore multiply the reading by 200 and, bingo, I now have a reading for the voltage across the ends of those two resistors.

The HV supplies I am building are modules that can be added on to each other, and I have now evolved the design that each module is worth 30kV, so I can hit big, serious, voltages (with current) with little fuss by adding a few together.

Yesterday I finished the second such module so for the first time I could tie them together and run to 60kV. I figured the 2 x 1Watt/100M resistors would not be suitable for carrying a volt reading current (they'd overheat), so upped that to 400Meg (200 either side of the DVM). All was well to 35ish kV but then the DVM screen started going black!!

The liquid crystals in the meter just aren't working properly when there is 40kV or more on the ends of the resistors. I tried moving the DVM so that one side is close to ground potential (so the meter itself was not floating around), but this actually seemed to make it worse! I can't see why that would be, but, bottom line, it is a hat-tip to the fact that once I was heading up to 40kV, the meter readings just didn't work out properly any more!

This 'blackening' of the LCD display goes away after a few minutes. I mean, above 50kV it just goes completely black, but switch everything off and, gradually, 10 minutes later, the screen is back to normal!

An effect I have seen before, up to 20kV, is that the cheap DVMs I am using will only measure the HV current (that is, when measuring mA) for a few hours before they pack up too! I have tried isolating them as best I can on rubber sheet and wotnot, but they just don't take it. I did order some moving coil meters some time back but they never arrived.

Usefully, I can determine the total power output, because these modules I'm making effectively work as 'constant power' devices (rather than there being an explicit function between V, I and W) and the efficiency remains fairly constant. So perhaps a better move would be to work out how to measure the current on a terminal close to ground, then calculate voltage from the power output and circuit current?

So, if you have any anecdotes, and any suggestions on why my DVM behaves like this and how to avoid it, I would be interested.
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