DIY High voltage probe - safety concerns - Page 1
Forum section: electronics / projects

author: MarkKruetzmann
author_rank: Contributor
timestamp: October 28, 2019, 08:53:38 pm
content: Hi!I recently bought a Tek 465 which is dead and while it isn't even here yet, I was asking myself on how to measure the 15.5kV in that oscilloscope. I didn't think they were that simple, but as it turns out, high voltage probes are basically just a high resistance high voltage resistor with a handle attached. You can see it here: Edit: at 2:33 (starting at a defined point does not work in the embedded player I think)Wait some seconds as he pulls out the resistor.As the cheapest probes I can find are still about 80€ or so, I figured I could do that myself quite easy with less than 15 bucks in parts. All I need is a ~1GOhm resistor capable of 20kV or more, maybe add a 10MegOhm or 1MegOhm resistor directly as to form a "worst case" voltage divider and hook it up to some (not even HV) cable and test leads.Worst case would be the GOhm-range input impedance of my HP3456A if it is accidentally in auto-range. The input capacitance might save it, but I better not count on that. Proper grounding beforehand is mandatory in any case.Anyway, my question is: Is a DIYed high voltage probe safely usable? I would do the following:Print a probe from PLA that "wraps around" a 1GOhm resistor in 2 halves. They will later be epoxied to each other.Make a probe tip out of a brass M4 screw as the resistor has a M4 mount on each side.Place the resistor away from the grip (as seen in the video).Add a 1MegOhm resistor (or 1.11MegOhm to account for 10Meg input resistance) to the mentioned 1GOhm resistor and get two leads out or some coax. One with the 1/1000 of the measured voltage and one ground lead.I figured that PLA has way better dielectric strength than air and therefore it should be just fine.I found a nice 35kV rated (70kV peak for 5 minutes) resistor with 1 GOhm I would use:https://highvoltageshop.com/epages/b73088c0-9f9a-4230-9ffc-4fd5c619abc4.sf/sec95a0d6e60c/?ObjectPath=/Shops/b73088c0-9f9a-4230-9ffc-4fd5c619abc4/Products/RES_35kV_1GThis thing certainly will be hard to use for anything but DC and very low frequency AC, but I guess the reasonably priced commercial ones don't have a particularly flat frequency response either. It will therefore be mostly a HV DC probe for meters with 10MOhm input impedance and with an attenuation factor of 1000. Quite a specialized product, but I guess that is just how it is in High voltage world. I will measure the resistance of the HV resistor with my HP3456A to tweak the second resistor or actually measure the attenuation with a 1kV source (Fluke 1653 installation tester. I know this would be kinda crude since this is just for insulation measurements but hey, it's the only more or less trustworthy HV supply I have and I can measure the voltage beforehand.)My important question is:Did I oversee anything safety-wise?Performance-wise, I would be happy enough with 5% DC. I opt for better than 1% DC and at least the capability to measure 50Hz High Voltage to say 10%, better would of course be better.My not so important question is: How to improve frequency response? I guess I have a hell of a lot of stray capacitance, which isreallybad with 1GOhm input. Is it possible to just add a capacitor over the 1.1MegOhm and get into kHz range? I think I do not want to try shielding the resistor to a defined input capacitance as this would severely impact safety if I am not cautious. This would also probably be anything but linear as the resistance is spread about 14cm or so of length. So, even with shielding, this sounds like a hard job for the compensation network.Any input is welcome, as this in its current state is nothing more than a thought project.

author: Ian.M
author_rank: Super Contributor
timestamp: October 28, 2019, 09:24:11 pm
content: The devil is in the details. I certainly wouldn't trust a 3D printed shell glued up from halves to provide safety critical insulation. All it takes is a small trace of semi-carbonised degraded plastic that had 'cooked' for too long stuck to the wall of the extruder head to be in a critical part of the print, or for the epoxy to be not quite fully and evenly mixed, and the insulation could break down without any warning.I would strongly recommend building any such probe inside commercially available clear (to avoid potentially conductive pigments and fillers), thick wall Acrylic or Polycarbonate tubing, thick enough to have a dielectric breakdown voltage at least a factor of five greater than the max working voltage, with 3D printing only used for end fittings and grips.N.B. Don't treat the low voltage end of the probe as safe - bad things happen if the ground clip comes off or if the HV resistor flashes over e.g. due to surface contamination or humidity.

author: MarkKruetzmann
author_rank: Contributor
timestamp: October 28, 2019, 11:47:12 pm
content: Next best thing would be an acrylic tube with at least 2mm wall strength, but then I would still have the problem with surface degradation if that thing gets dirty. A printed handpiece wouldn't help - assuming worst case again with too hot printed PLA.Also the use of 20kV or 50kV cabling to the meter would help, with all connections buried deeply in the acrylic. This would make it necessary to put the smaller resistor outside of the probe or using two HV cables.I could alternatively place a hook at the probe tip and just move it around when the DUT is powered off. That would slow down measurements drastically though.But I think what I really should do is not trust cobbled-together high voltage probes and either rely on safety-tested commercial options or just omit measurements above 1kV altogether. Hence this post.Thank you for your input though. I still think that a DIYed probe is "relatively" safe, but as the kids say: "you only live once" and I intend to do that as long as possible.I didn't think of carbonised plastic, but might go the acrylic tube road and make sure it is clean.

author: tautech
author_rank: Super Contributor
timestamp: October 29, 2019, 12:04:35 am
content: It is very questionable to ever need to measure a scopes PDA voltage and when so high it presents several risks.Far better IMHO to just measure the EHT that the PDA is derived from as it's always less than 3 kV.Far simpler to make a resistive divider and measure it with a DMM.

author: joeqsmith
author_rank: Super Contributor
timestamp: October 29, 2019, 12:19:40 am
content: I don't normally work with enough energy to be lethal at real high voltages so I don't think about human safety too much. I'm more concerned with taking out my DSO's front end. If you use the search you will find a fair bit of info on various home made HV probes I have put together.This is the one I use most often. I'm using ceramics for the insulator and copper foil to form the capacitor to achieve some decent bandwidth from it. Buying a good Tektronix probe may have been cheaper but not as much fun.

author: sourcecharge
author_rank: Regular Contributor
timestamp: October 29, 2019, 05:13:40 am
content: Quote from: MarkKruetzmann on October 28, 2019, 08:53:38 pmHigh voltage Oscope probes are expensive.A more indepth understanding can be found hereLhttp://emcesd.com/1ghzprob.htmI have mad a DIY HV probe, but I don't remember exactly what resistors and capacitor I used.I had to delete my discription of the one I made because I just checked the last resistance and it was 4 Mohms, not 100k.So I can't tell you exactly how I made it.

author: MosherIV
author_rank: Super Contributor
timestamp: October 29, 2019, 08:01:07 am
content: HiAs others have pointed out, the material for the outer handle is critical, hence the high price for commercial units.Additionally, use multiple resistors in the divider chain, it will give better immunity to HV break down, see the instructables link.https://www.instructables.com/id/DIY-HIGH-VOLTAGE-MULTIMETER-PROBE/

author: sourcecharge
author_rank: Regular Contributor
timestamp: October 29, 2019, 12:03:51 pm
content: Ok, so I double checked my DIY 100kv probe and found it to be about 1050:1 after putting 100V through it, and measuring the output.I also used the ucurrent to double check it, and found about 4.3 Gig ohms as R1, while R2 was 4 Meg ohms.The 4.3 Gigs were multiple resistors in series, and I'm guessing it was 99, 40 Meg ohm resistors, but I not sure because it was sealed but I remember it was a very long line of them and I made sure each resistor could handle the power that would be across each of them.I used a 2 inch diameter, 3 foot PVC pipe, but I used super corona dope on the resistor chain first.The whole thing was taped up with electrical tape and the pipe caps were sealed with superglue.The resistor chain was also centered in the tube by using styrofoam cutouts, so the only point at which it touches the PVC is at the tip where I have a bolt as the probe tip.I remember using measuring 40kv AC at 10khz, but only DC at about 80kv. I don't think I trusted it too much as I believe I never touch it while the circuit was hot.Take it for what it's worth, but it's possible to do measurements with a DIY HV probe.

author: MarkKruetzmann
author_rank: Contributor
timestamp: October 29, 2019, 12:59:40 pm
content: @tautech:Thanks for the advice. the ~2.5kV or so would have been the first HV I'd have tested anyway. This thread is not about the scope repair though, I am just curious for the techology and details of HV probes. It's all for the fun in the end.@joeqsmith:Now that's an impressive build! I guess I have to watch your whole series on that puppy. Especially the frequency response is remarkable! I hope theory of operation and compensation techniques are described in the other five videos. Hats off to your accomplishment! It sure was a lot of more fun than to just buy a Tektronix probe. You did very good work here. I might try to copy it to some extent.@MosherIV:The guy in the instructables link does claim 15kV, but he totally relies on heatshrink tubing and a used garden hose for insulation. I would not put my life at stake for that. As you pointed out, material is critical, and that just does not look safe.@sourcecharge:The 1Ghz probe you linked doesn't exactly help... I am quite sure that in bigger probes compensation gets increasingly difficult. Physics is the same of course, but undesirable effects will show up way quicker with physically bigger probes and many orders of magnitude greater input impedance.Your description of a HV probe sounds quite promising though, in that even a "simple" resistive divider chain without frequency compensation procedures produce an output usable to some kHz at least. Not fiddling around in a hot circuit is very probably a very good idea in HV measurements anyway.

author: AlienRelics
author_rank: Unknown
timestamp: October 30, 2019, 03:27:22 am
content: I don't think I'd use PVC. PVC absorbs moisture. I recall long ago seeing in something like 73, CQ, or some other ham radio magazine that coil forms made from PVC tended to arc across the surface and form carbonized tracks.

author: floobydust
author_rank: Super Contributor
timestamp: October 30, 2019, 04:30:18 am
content: I'm usingOhmite Slim-MoxSM108 (20kV) HV resistors, they are low cost and small. I also use the cheap chinese red HV resistors, like 5GIG 10W 100mm, but their coating is paint thin. No comp cap for DC work.

author: sourcecharge
author_rank: Regular Contributor
timestamp: October 30, 2019, 04:40:47 am
content: Quote from: AlienRelics on October 30, 2019, 03:27:22 amhttps://omnexus.specialchem.com/polymer-properties/properties/water-absorption-24-hours#PM-PPWater Absorption 24 hoursAbsorption Properties of PolymerWater Absorption Values of Several PlasticsPolymer Name-----------------------------------------------------------Min Value (°C) Max Value (°C)PVC (Polyvinyl Chloride), 20% Glass Fiber-reinforced-----------------------0.01 0.20PVC, Plasticized----------------------------------------------------------------0.20 1.00PVC Rigid-----------------------------------------------------------------------0.04 0.40and as a comparison:PTFE - Polytetrafluoroethylene------------------------------------------------0.005 0.010PI - Polyimide------------------------------------------------------------------1.34 1.43PETG - Polyethylene Terephthalate Glycol------------------------------------0.10 0.10ABS - Acrylonitrile Butadiene Styrene----------------------------------------0.05 1.80ABS Flame Retardant----------------------------------------------------------0.10 0.80ABS High Heat-----------------------------------------------------------------0.10 0.80ABS High Impact-------------------------------------------------------------- 0.10 0.80(No PLA listed)Well the plasticized PVC "Water Absorption Values" seems to be much higher than most plastics, but the rigid PVC look pretty good.I also used super corona dope on the resistor chain, and electrical tape all around the outside of the PVC pipe.Unfortunately, it's been so long since I used this at high voltage, I would verify it was working correctly at high voltages before touching it with a hot circuit.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 30, 2019, 07:01:33 am
content: Putting resistors in series is the way to go. I would use ones with axial leads, so much easier to deal with than the ones with the leads coming out sideways.Use physically large enough, brand name resistors so that you can trust their ratings are real. Derate, add your own margin - then put in series to build up the full rating.PVC heat shrink tubing is your friend. Ones with hot melt glue exist, sealing the connections. Use several layers of tubing to get the physical thickness. Then you can add a 3D printed case for decorative purposes, but I wouldn't trust it as a reliable insulator.The idea of putting resistors in series is you will create a physically long chain. If you divide your 16kV to, say, the length of 32cm, you'll only have 500V/cm, which is much easier to deal with than, say, several kV per cm you get with optimistically rated single HV resistors. High voltage gradients require much more understanding of insulation materials: avoiding trapped air to avoid corona discharge carbonizing the materials slowly, avoiding materials that absorb moisture, and so on.Example:HVA12FA50M0 is 0.37€ at Digikey; 20pcs is 7.40€https://www.seielect.com/catalog/sei-hva.pdf20 in series is 1 GOhm, total 160kV(!) rated working voltage.Body length is 11mm, voltage per resistor is 16kV/20 = 800V, voltage gradient is 727 V/cm. Creepage distance is actually a bit longer.Allowing 7mm for soldering the leads, total solution is 360mm in length.

author: Gyro
author_rank: Super Contributor
timestamp: October 30, 2019, 11:28:17 am
content: Quote from: Siwastaja on October 30, 2019, 07:01:33 amNot necessarily. In my Picoammeter experiments I've found heatshrink to be surprisingly leaky, even at low voltages...https://www.eevblog.com/forum/projects/picoammeter-design/msg790280/#msg790280At the very least, I would say that not all heatshrinks are created equal and that a sample of the one you intend to use should be specifically tested for resistivity.

author: tooki
author_rank: Super Contributor
timestamp: October 31, 2019, 07:54:22 pm
content: Quote from: sourcecharge on October 30, 2019, 04:40:47 amWell, is that surprising, given that we use PVC for water pipes? It wouldn't really work if it absorbed tons of water, I would think.

author: sourcecharge
author_rank: Regular Contributor
timestamp: October 31, 2019, 08:53:24 pm
content: Quote from: tooki on October 31, 2019, 07:54:22 pmNot in the least, but I'm pretty sure when I was making this thing, someone in another forum I was posting at may have commented on that very same issue, and I double checked it then.In any case, the out wall is like 2 to 3 mm thick, and it's not being used as a coil, but as a barrier from the HV on the chain of resistors.Because the resistor chain was first dipped in super corona dope, (4100V/mil), centered with styrofoam cutouts in the tube, and then the tube was sealed and coated with multiple layers of electrical tape, I was pretty sure of myself then about the probe's ability to withstand the 100kv.Not sure if I would trust it now without testing it first though.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 15, 2025, 05:13:14 am
content: HiI know this thread is from ancient times but it is still relevant to me.I have a 25kV commercial 1000:1 voltage probe that is designed for a 10M voltmeter.I also have a 5kV 100:1 scope probe designed for a 1M scope input.The 25kV probe is great because the resistance (~10Gohm) is high enough not to noticeably load down HV supplies on CRTs but it isn't designed for use with a scope.The 5kV probe works as advertised, but it definitely loads down HV supplies which is not so good. This is not a quality issue, it is defined by physics.So I have been looking at making a DIY HV probe. This thread is probably the best info I have found but I think some key info and features are missing from here.The photo shows my commercial 25kV probe. A key safety feature is the metal ring, connected to earth. If there is an arc-over from the tip, along the probe, the ring will short the voltage to earth before it gets to the user. This only works if the probe is earthed.The next safety feature is the insulated handle. If the ring is not earthed, it will still present a barrier to an arc-over. Not as good as an earthed ring, but it might just be good enough to capture an arc before it reaches a hand.As others have pointed out, material selection for a DIY (any) probe is critical. For DIY probes I think virgin PTFE is probably the best choice. PTFE can have a rating of up to 250kV / mm which is quite astounding.It is easy to machine and easy to get.What ever material is used, it should not contain any additives, fillers, colouring etc. They will only reduce the break down voltage.No way would I use heat shrink or 3D printing anywhere near HV.With HV scope probes, the probe should include a copper shield along the length of the probe to reject interference. In addition there is a requirement for capacitance along the length of the probe for frequency compensation. So it should be possible to set the diameter of the copper tube (made from rolled sheet) to get the right capacitance for compensation. That would eliminate the need for HV caps along the probe which would reduce the required diameter and cost. It should also be possible to calculate the required dimensions to achieve the desired results.This method would require a PTFE tube to contain the HV resistors, the copper shielding tube, then an insulating exterior PTFE tube.I think I would use a split copper tube (rolled sheet) to eliminate the risk of inductive interference. That would actually make a rolled sheet tube easier to make.Just my thoughts.

author: MariuszD
author_rank: Regular Contributor
timestamp: July 15, 2025, 09:08:36 am
content: QuoteThe shield does not facilitate compensation but complicates it. Large HV resistor must be analysed as distributed parameter circuit. Instead of a single capacitor compensation, a multi-stage RC circuit is needed. Look at the attached P6013A probe schematic.My approach to a high impedance DIY high-voltage probe is different.Separate probe for AC and separate for DC. The AC probe is exclusively a capacitive divider, the DC probe is exclusively resistive divider. Sometimes DC level is known or not important.In my AC probe, the capacitor had only 0.1pF; I suspect that no commercial HV probe has such a high impedance. Of course, the capacitance of the probe with a 0.1pF capacitor is greater than 0.1pF but still less than 1pF. I can't compete with Tek trying to build a better copy of the P6013A, but I can achieve a higher impedance in a probe that only transmits AC (or DC).Tek's HV probe has an unusually designed compensation capacitor, this is variable capacitor.https://www.eevblog.com/forum/testgear/teardown-of-tektronix-p6013-high-voltage-oscilloscope-probe/

author: dazz1
author_rank: Frequent Contributor
timestamp: July 15, 2025, 10:45:21 am
content: HiI didn't say the shield made compensation simple. On the contrary, it would require some specific R&D to get it to work. The capacitance between the resistor and the shield would depend on the current density in the resistor. Skin effect will concentrate the current to the outside diameter of the resistor, so that could be used as a starting assumption. The resistors are obviously not perfect conductors so the skin effect won't be perfect.So I would start with a thick film (not composite ceramic or wire resistor) and assume the the current is on the surface. Then calculate the capacitance to the tube. The capacitance will depend on the diameter of the shield.It should be possible to mathematically model a shielded resistor as a high loss coax. Without the shield, it would be an antenna, and performance would vary with what ever was in the near field. Another advantage with PTFE is that it is a high quality dielectric.Ohmite products appear to be reasonably priced, rated up to 50kV.I don't understand your assertion that a multi-stage RC circuit is needed. Discrete components introduce a bunch of parasitics that are not present in a continuous distributed network. Think coax.I do understand the benefits of an exclusive AC capacitive divider. I already have the DC probe so a capacitive AC probe would compliment that but a single AC/DC probe would be more versatile, compromises accepted.

author: MariuszD
author_rank: Regular Contributor
timestamp: July 15, 2025, 11:31:52 am
content: QuoteSkin depth will not be constant across the entire bandwidth, there will be changes in resistance that need to be compensated for.Quote from: dazz1 on July 15, 2025, 10:45:21 amRead from page 108https://w140.com/tekwiki/images/6/62/062-1146-00.pdf

author: tggzzz
author_rank: Super Contributor
timestamp: July 15, 2025, 11:32:10 am
content: EDIT: first check the scope is working sufficiently well that thereoughtto be a visible trace. That means all the PSU voltages and the CRT deflection voltages. Use a standard 10Mohm voltmeter and a slow (or XY) timebase. The differential voltages on the CRT vertical and horizontal plates should vary from -V through 0V when the spot is in the center, to +V. RTFM to find the actual values of V, but +=40V seems reasonable.Consider finding out what will be visible ifonlythe 15kV supply is faulty. Fix everything else before worrying about the 15kV, or replace the 15kV source "just in case".If the -2.4kV EHT is present and correct, then there may be a problem with the grid-cathode voltage Vgk. Realise that the AC voltage change in Vgkis generated at "low voltage", i.e. up to a couple of hundred volts, and then translated/shifted up to -2.4kV. So, use a simple 10Mohm voltmeter to debug the "low voltage" version first; no need to use a scope. Then see if Vgkis the same and changes in the same way. If there is a difference, suspect the HV diodes and/or capacitors. HV diodes cannot be tested with simple component testers.Given that there are only a couple of testpoints you are going to want to test at EHT, consider not making a general purpose test lead/probe with case, but instead soldering or clipping flying (solid core hookup wireliterallyin mid air!) leads to the test points. Attach/remove them only when the instrument is turned off at the mains, and any capacitors have had time to discharge. Stand back before turning the mains on!

author: tautech
author_rank: Super Contributor
timestamp: July 15, 2025, 10:03:25 pm
content: Quote from: dazz1 on July 15, 2025, 05:13:14 amAs usual overcomplicating things.....as per your Sig.CRT scope manuals specify the voltmeter requirements which in all cases will load the EHT some however the spec for any voltmeter specified can be found online to match your own against.Some later CRO's specified more modern DVM's with lower loadings however an old Kaise MC unit I have with a 30k Ohms/V and 3kV range has been sufficient for EHT measurements.The AVO 8 Mk5 I have only offers 20k Ohms/V (50uA FSD) and a 3kV range yet it has been sufficient for EHT measurements also.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 16, 2025, 09:40:09 am
content: Quote from: MariuszD on July 15, 2025, 11:31:52 amOn thick film resistors, skin effect should be reasonably close to the outer diameter through out the bandwidth.The thing that will have the most effect is the snakes game path of the thick film trace. Definitely not straight lines. Maybe there is an alternative.QuoteYes I have seen that manual before. The flourocarbon 114 has about the same dielectric strength as bulk virgin ptfe. About 40kV/mm.So mechanically replicating something like the Tektronics probe with PTFE would be relatively straight forward, if you have a workshop like mine.Achieving a flat broad band frequency response, not so easy. I'd rather keep the HV end simple and make the LV matching network really complicated.Most electronics is making something from parts made in a factory and sold in a catalogue. The thing I like about RF is that I can make something in my workshop that isn't in a catalogue, and get it to work.

author: MariuszD
author_rank: Regular Contributor
timestamp: July 16, 2025, 10:04:12 am
content: Quote from: dazz1 on July 16, 2025, 09:40:09 amQuote from this documentQuote

author: dazz1
author_rank: Frequent Contributor
timestamp: July 17, 2025, 02:28:29 am
content: Quote from: MariuszD on July 16, 2025, 10:04:12 amYes I read that too. I interpreted this as making the input RC a complex impedance, which needs to be compensated out. The manual is ambiguous.The lowest reactive impedance HV resistors sold by Ohmite are ceramic composite but the HV limit and max resistance are too low for what I am thinking of. Multiple resistors could be chained but that introduces parasitics that negate the benefits of a solid resistor.Ohmite sell film resistors. The Maxi-MOX series have the following characteristics:Series Watts kV Length $USD DigikeyMOX3N 4W 30kV 80mm $33MOX4N 5W 40kV 105mm $57MOX5N 6W 50kV 130mm $70With these being available at reasonable prices, I would not chain lower spec resistors. That would just make the probe build more complex and add points of failure.These are thick film resistors so the convoluted signal path is likely to limit the frequency range more than anything else.My 25kV probe has a 10Gohm input resistor matched to a 10Mohm volt meter (1000:1). A scope has a 1Mohm input, so a 10G resistor would be 10,000:1. Looking at (say) a 50VAC ripple on top of (say) a 10kVDC would only show 50mVAC on 1VDC on the scope. Practically invisible on a DC view and OK an an AC view. Doable, but not that great.Old CRT scopes are cheap enough to buy one and mod the input to 10M dedicated for the probe.The other option would be to include an active pre-amp between the scope and the probe. That would allow range switching.Then all of this comes back to the question of would I use such a probe, and the answer is probably only once, for a fault I am trying to fix now. Building one would give me bragging rights, and probably not much more.I want one but I don' need one or is it I want one therefore I need one.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 18, 2025, 09:56:51 am
content: HiOne of the problems building a HV probe is testing it.Aliexpress to the rescue. Search for a "1000Kv Diy Kit High Voltage Generator 400Kv Dc High Voltage Generator Booster Board Inverter High Voltage Diy Kit Module"An old car ignition coil could probably do something similar if you don't want 1,000,000 volts.Don't ask me if I believe the kit actually produces 1MV but who would know.

author: tautech
author_rank: Super Contributor
timestamp: July 18, 2025, 10:05:25 am
content: Quote from: dazz1 on July 18, 2025, 09:56:51 amWhat do you want your headstone to read ?

author: Gyro
author_rank: Super Contributor
timestamp: July 18, 2025, 07:57:35 pm
content: Yes, that output voltage spec is pure Chinesium, divide the output voltage by 10 - 30. Those things break down without an external spark gap.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 19, 2025, 09:07:07 am
content: HiThere is another feature of these types of HV probe that would limit frequency bandwidth.That is the loop formed by the probe and the earthing lead. My DC probe has a generous loop so the earth lead is safely away from the HV bits and pieces.If it was a constant earth loop area, it could be compensated out to a certain degree, but it isn't.So I think a DIY 25kV HV AC/DC single sided passive probe is likely to be physically limited to around 25MHz to 50MHz or so with reasonably priced off-the-shelf parts.The intermediate step between a fully passive and a fully active differential probe is to insert and active compensating amplifier between the probe and the scope. The more I think about this, the more I think it would be a good thing.If the amplifier output was matched to 50ohm, it would remove the cable between the probe and the scope from the compensation network. With a 50ohm terminator at the scope/meter input, you could use any 50ohm cable at any length.

author: tautech
author_rank: Super Contributor
timestamp: July 19, 2025, 11:32:00 am
content: Quote from: dazz1 on July 19, 2025, 09:07:07 am34kV 50 MHz 1000:1 scope probes are available.https://www.pintek.com.tw/customer/pintek/upload/HVP-39-28-18-10-SPE.pdf

author: MariuszD
author_rank: Regular Contributor
timestamp: July 19, 2025, 02:08:03 pm
content: Quote from: dazz1 on July 17, 2025, 02:28:29 amIt is not ambiguous, look at the schematic. They draw multiple parasitic capacitors around the input resistor.Complex RC time constant doesn't mean impedance is a single complex number. Time constant is a different thing than impedance.The RC time constant is never written as a complex number.I suspect that if you give up the wide band, the compensating capacitor can be a single one.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 20, 2025, 09:53:44 am
content: Quote from: MariuszD on July 19, 2025, 02:08:03 pmThe Tektronix manual is ambiguous because there are different interpretations of what it means.The Tektronix network tunes different parts of the square wave in the time domain. I think the Tektronix network tunes out signal reflected from the mismatch with the scope input. One cap won't achieve that.My HV scope probe is 1000:1 with 100Mohm input resistance. The HV circuit I am fault finding has resistors up to 68Mohms, so a 100Mohm probe just drags down the voltage I am trying to measure. My 1,000:1DC probe has 10Gohm input resistance but there is 100x difference in gain going from a multi-meter to a scope.I notice on the spec sheet you linked, the input capacitance rises with input volt rating. This is to be expected as the increasing volt rating requires a physically larger resistor, and that will have more capacitance.I have also noticed that AC HV scope probes tend to be short and fat. I suspect the length is dictated by the length of the input resistor. I suspect the diameter is relatively large to accommodate a shielding tube and minimise the input capacitance.I have also noticed that no modern probes I have seen includes a bare metal earthing ring between the probe and the handle. If the ring is connected to the earth, then any flash-over from the probe end will be grounded and will not reach the user. If the earth lead is not connected, then the ring will be at the HV input voltage via the input resistor. 25kV / 10Gohm is 2.5uA. Probably enough to give someone a zap (like HV static electricity), but not enough to end a life.In contrast, an arc-over along the probe surface, through a plasma gas arc will have close to zero resistance and the peak currents could reach spectacular values. I am careful to make sure my HV scope probe (rated to only 5kV, no earthing ring) is very clean before each use. I clean it with IPA at least an hour before use.One thing hasn't been discussed is the noise contribution of the input resistor. a 10Gohm resistor at BW 50MHz will contribute 91uV = -20dBV = -60dBkV. Reduced S/N ratio is the penalty for increasing the input resistance.

author: MariuszD
author_rank: Regular Contributor
timestamp: July 22, 2025, 05:41:39 am
content: Quote from: dazz1 on July 20, 2025, 09:53:44 amThey use a resistive wire in the coaxial cable to dampen reflections. Like in all other probes. There is nothing ambiguous, but I see that you really need your interpretation to fit even though many arguments are against it.It all comes down to the amount of work, you assume that you will make a much simpler probe with a comparable results to the Tek's probe by using a worse resistor. If your assumption is not right, you will simply have to create a more complicated probe.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 24, 2025, 10:39:43 am
content: HiI have asked a local PTFE supplier for relevant electrical specs. Silence.I asked Ohmite for frequency related specs on specific resistors. SilenceI am not going to use PTFE unless I know the insulation properties published by the manufacturer.Similarly I am not going to purchase multiple expensive resistors to find the one that works.To build a very high resistance 10Gohm passive probe with a decent frequency response would be difficult.If I was going forward on this, I would look at a hybrid active, single ended probe using a trans-resistance opamp as the grounded sensor.By isolating the probe from the cable and scope input, it would be a lot easier to increase the frequency response. The performance of the probe would not be determined by the coax.The accuracy of the probe would not be dependent on the input impedance of the scope or volt meter.

author: RJSV
author_rank: Super Contributor
timestamp: July 26, 2025, 05:01:16 am
content: In school I was amazed and struck, by the amounts of 3-D graphical relations, in the construction of HIGH VOLTAGE components, and support structures. Especially with optics, but the question here is about safe HV probe construction.One very visible example is those repeating 'egg crate' style surfaces, that are meant to expand the volts per meter, in relation to surface runs (of current leakage). Since these things aren't always intuitive I'd have to ask:"Is your particular design both experimental, AND a casual hobbyist amateur design ?"I know, I can be an A-hole, with rule enforcement, but...That HV Probe, built at home MIGHT be quite safe, but that's not the point. For example; Do you always have a companion nearby, while experimenting ?What comments, did your lawyer issue ? (Snarky, but I needed to go there.)Home owner's insurance ? O.k. that was out of line, there, but just sayin.Aside from the criticism, though, the topic is an interesting one. Especially the weird, 1950's space travel looking hardware, for high voltage isolation, of some hand-held gizmo.There are, besides measurement, applications for a 'Shorting Wand' type of tool, for shutting down HV supplies, motors, etc.

author: dazz1
author_rank: Frequent Contributor
timestamp: July 29, 2025, 09:10:28 am
content: Quote from: RJSV on July 26, 2025, 05:01:16 amI am OK making an HV probe for myself, but I wouldn't make one for someone else.I always have someone else in the house when I am working with anything above 48V.I am somewhat familiar with HV equipment. I have a short length of 200kV (that's not a typo) power cable in my garage from a past job.I am not prepared to use materials (including PTFE) not specifically rated for HV use. If you study semi-conductor materials, tiny amounts of impurities turn an insulator into something that doesn't insulate.In electronics, usually HV supplies are low current. Easy to drag down with a 100Mohm probe.Designing a AC probe with 10Gohm input is a significant challenge.The fault that initiated my investigation looking at HV Probe design is rectified. I am not looking at this problem any more.

author: HighVoltage
author_rank: Super Contributor
timestamp: July 29, 2025, 10:09:38 am
content: Quote from: MariuszD on July 15, 2025, 09:08:36 amVery good, this is a perfect approach !I have been doing this for many years with great success.If I compare my signals with the Tek 6015A, they are perfectly matching on the scope screen and all values can be set to match the measurements.

author: sourcecharge
author_rank: Regular Contributor
timestamp: September 11, 2025, 06:48:03 pm
content: So, I have a need for my high voltage, high impedance probe again.I was mainly trying to use it for up to 1.5kv but I need the high impedance for the measurement.I decided that I wanted the output of the probe to be fed into a non-inverting opamp as a high impedance buffer so that the output can be used with any meter, and oscope.After doing this, I ran into mains line noise trying to measure AC and DC measurements, and I figured out that this was not only noise that I was seeing from the mains but also the signal noise.Meaning the 10khz that I thought I was measuring was actually only noise that bled through the same way as the mains line noise.I messed around with noise canceling circuits, but I found that the best way to get rid of it was to shield the probe (steel sheet metal tube), the wire (BNC), and the buffer circuits (steel sheet metal box).I found that the probe was only usable for DC and low frequency (5hz) AC measurements as the compensation was completely off.Although it will allow me to measure a ripple voltage, I still would like a HV high impedance AC probe.I think this is the best approach for DIY:Quote from: MariuszD on July 15, 2025, 09:08:36 amComponent cost at digikey right now for 100x 1600VDC PP (0.0006 DF) 100pf caps are about 20 to 30 bucks.I'm wondering, what type of caps (and how many) did you use for your 0.1pF probe?Did you shield that probe and how did it react with or without the shielding?Is this the reason why the capacitance was higher than 0.1pF?Did you have to compensate C2 after the C1 was completed?Did you wind up using a small variable cap in series or parallel with C2?What was it's final cutoff frequency and rated voltage limit?I'm interested in knowing how well this turned out because it really does seem like the best option.

author: MariuszD
author_rank: Regular Contributor
timestamp: September 12, 2025, 12:07:55 pm
content: I needed this probe to build a device, but the probe wasn't a priority, which is why the final version hasn't been completed to this day.QuoteDIY capacitorThe input capacitor was a PTFE tube with a metal rod inside and a copper ring on the outside.QuoteMy device was giving 5kVp and the PTFE tube was 1mm thick, so I had a margin of safety. On the 10ns rise time waveform, I didn't see any significant discrepancies compared to the P6013 (25MHz).QuoteUnder favorable conditions, measurements without a shield are possible, but it's better to use one.QuoteCapacitance to shieldQuoteThe division coefficient needs to be trimmed.One of the first versions of this probe used a capacitor with alumina ceramic plate, and I observed corona discharges on the plate, this is unacceptable. Later, I filled the empty spaces with silicone grease.

author: sourcecharge
author_rank: Regular Contributor
timestamp: September 12, 2025, 06:59:59 pm
content: The following electrical characteristics of PTFE come from a datasheet from professional plastics, here in the US.Unfilled PTFE has basically the best DF of most known materials (<0.0002), and also it's dielectric constant is only at 2.1, both at 1 Mhz.Unfilled PTFE has a breakdown voltage of 285 V/mil, which puts your probe at about 11kv.Sourcing unfilled (white, not clear) PTFE is as easy as 1.75mm 3d printer filament tubing if it is truely unfilled ptfe, as there are glass filled and carbon filled ptfe which I'm guessing is either clear or black.Your design is pretty good (if not one of the best), and I like the use of PTFE as it really is the best material for this.Just a couple of questions though.Was the shielding grounded, or was it C2?If not, how could you differentiate between them if C2 is grounded in the C1:C2 dividing network.Did you use a seperate grounded shield, with a virtual ground on C2 in your one of you latest versions?I'm assuming that you used the output to an active adaptor because the impedance of C2 has to be so great that any meter would ruin it.I'm also assuming that you are using BNC cables for probe outputs.That would mean if the probe was shielded with a ground tube, the output of C1, and C2 would both require a BNC output terminals with the grounded shielding not connected to C2 which is on a virtual ground.At these low of capacitances, the BNC cables are now part of the dividing network too, are they not?IDK.Alot of the problems faced with a multi series capacitor DIY probe is solved with this design.I think the virtual ground on C2 would work with a grounded shielding.Putting an active buffer between the HV and the oscope is benificial too.

author: intabits
author_rank: Frequent Contributor
timestamp: September 14, 2025, 04:47:32 am
content: Quote from: MarkKruetzmann on October 28, 2019, 08:53:38 pmYes, do include a worst case bottom divider resistor, selected to give the correct division ratio when in parallel with the 10M DMM input.If it's in a GOhm input impedance range, you'll just get the wrong reading, but no damage.See the values shown in the Fluke 80k-40 manual:https://assets.fluke.com/manuals/80k40_6_iseng1000.pdf

author: MariuszD
author_rank: Regular Contributor
timestamp: September 16, 2025, 05:34:07 am
content: Quote from: sourcecharge on September 12, 2025, 06:59:59 pmIt was done very simply. The shield and capacitor C2 were connected to a common ground. The capacitance between the C1 second plate and the and shield, and the load capacitance required a reduction in C2.Quote from: sourcecharge on September 12, 2025, 06:59:59 pmUltimately, it's better to use a high-impedance buffer. I was only interested in high frequencies, so I connected my probe to the input of the factory made 1:10 probe. C2, at nearly 100pF(with 1:10 probe capacitance), loaded with a 10MOhm resistance, limits the bandwidth from below to 160Hz.Quote from: sourcecharge on September 12, 2025, 06:59:59 pmI was experimenting with buffers for the active probe. Fast operational amplifier, noninverting, with 50 Ohm cable matchingQuote from: sourcecharge on September 12, 2025, 06:59:59 pmI don't see a reason to use virtual ground, but buffer is beneficial

author: sourcecharge
author_rank: Regular Contributor
timestamp: September 16, 2025, 09:39:28 pm
content: The reason why I ask about a virtual ground is because of the active adaptor's input offset and the possiblity of limiting the common mode capacitance thats grounded, due to the BNC cable on the non inverting input.The high impedance non inverting buffer (HINIB) of the active adaptor also have parameters of input bias current (Ib), input offset current (Ios), and input offset voltage (Vos).3 seperate parameters, all contributing to the total offset.I'm not perfect with the calculation, but B2spice is saying that if the Ib and the Ios are 25pA each, then the total current that is calculated for the offset is 37.5pA.Let's say for instance that impedance of C2 is 1Mohms, even if its much higher.That's 1Meg ohm x 37.5pA which equals 37.5uV offset extra from the Vos, so that precision 1uV Vos opamp for example, now has 38.5uV offset on the output.The impedance of C2 is much higher though, so the offset is going to be much higher too.In fact, the impedance of C2 is so high that the HINIB of the active adaptor requires a resistor from the noninverting input to ground in order to not fry, which is now our R2.R2 will have to be used in the compensation equation: R1xC1=R2xC2, but it can be anything you want, although I'm not sure what it should be in relation to C2's DF or if it matters.So this offset on the input of the HINIB is there if you ground C2 with R2 grounded as well.The reason why I say to use a virtual ground is to null the total offset.In fact, if you are using a quad package or same opamp that came from the same lot#, it's possible to null the total offset using three opamps to null the first.It works like this:A HINIB as the first of three extra opamps, has it's output to C2 and R2 as the virtual ground.Instead of rail splitting and using a pot to manually adjust the offset on the bias of the HINIB, which can vary due to rail voltages and temperature, two other opamps are included.The first of the two opamps, starts off with as a HINIB with a resistor from the non inverting input to ground that is 2x the impedance of C2 and R2.An extra pot can be included to this resistance, for nulling out the final Vos.This creates the uV (or mV) offset needed to feed into an inverting buffer with a gain of 1, and biased to ground.The output of the inverting buffer is fed into the virtual ground buffer's non inverting input, and nulls the total offset of the 1st opamp that is affected by C2's, and R2's high impedance due to the input bias current (Ib), input offset current (Ios), and input offset voltage (Vos).I'm not sure if using a virtual ground though will limit the C(cm).That's why I asked you about it.Here's the schmatic that shows the difference with and without the virtual ground using the TP-17 (It's a cheap, mid ranged precision single opamp):Of course this can be solved by using a jfet or cmos opamp as the first HINIB that has a typical 0.5 to 1pA Ib or Ios with low Vos, but those that have the bw and 36V Vcc max are more expensive.Beware though, while doing a search on digikey (or where ever you shop), some opamps may say 5pA Ib, Ios, typical, but they can swing wildly to a max of +/-500pA.Edit:Just wanted to add that if the Ib, Ios, or Vos is opposite polarity, the resistor could be only 1x the C2, R2 impedance.The key point is that if all 4 op amps have about the same offset, you can use their own offset to null the total offset.

author: MariuszD
author_rank: Regular Contributor
timestamp: September 18, 2025, 10:32:08 am
content: Now I understand that you want to compensate for the offset resulting from the polarization currents. Resistor methods were used for bipolar amplifiers where the current Ib was the base current, making it large (nA) and repeatable, while the current Ios was small and resulted from the asymmetry of the input stage. In FET amplifiers, the Ib current doesn't flow into the gate; instead, it's the difference in current from the ESD protection diodes. Therefore, it can vary significantly for both inputs, even flowing in opposite directions. Ios>Ib is not surprising. Diode leakage currents increase exponentially with temperature, so they can be many nanoamps at 125°C. This is why the maximum from the datasheet can be so high. However, the probe will not be used above room temperature.Your compensation method should be tested in practice. A simulation won't tell you if it works.I needed a fast probe, so I was only considering amplifiers >50MHz. These amplifiers have an offset >200uV. Above 70MHz, it's difficult to find an offset smaller than 2mV. That's why I wasn't expecting microvolt accuracy. It is known that a capacitive probe does not transmit DC, so if anything appears at the output, we can consider it to be an amplifier offset or a leakage current from the input capacitor. We can even block the DC component at the buffer output.QuoteWithout R2, the circuit will saturate and won't work.Due to the polarization current, a resistor R2 is necessary, the disadvantage of which is that it limits the bandwidth from below. That's why I'm considering bootstrapping to increase the effective value of this resistor.QuoteThe capacitive divider will not be compensated (R1 non existent), but that doesn't matter because the bandwidth limitation will be <200Hz. In theory, a capacitive divider has no upper bandwidth limit, but it is known that the stray inductance of real capacitors will prevent "microwave" operationDC can be transferred using a separate low frequency resistive probe if needed. DF of PTFE should be low.QuoteI didn't plan for the cable to be before the buffer because if the cable is after the buffer, it can be matched to 50R.I don't know what the parameters of the capacitive divider directly connected to the cable will be. The RG-58 50R cable has about 100pF/m, but I'm worried about reflections. I'd have to try. Maybe it's better to cannibalize the cheap probe and get a lossy cable from it.I came up with the idea that I could suppress reflections with a resistor inserted between the capacitive divider and the cable, an example from the simulation is shown below. The total capacitance of C2 is 200pF, of which 100pF is the cable. It even works somehow, but I don't expect this simulation to accurately represent reality. Beside, a 30ns ringing decay time is not a good result, my primitive probe without an amplifier and cable was better.I don't see a capacitive divider or cable in your diagram, so the details are unclear to me.QuoteI don't quite understand. I was only considering single-ended probe. I don't see anything that justifies introducing a common-mode/differential-mode distinction. Do you mean the amplifier's internal capacitance?QuoteAmplifiers in a single package do not have identical offsets. And there's no reason they should. Beside, we don't know if the observed offset is a real asymmetry in the input transistors or the remaining inaccuracy after the manufacturer performed offset trimming.

author: MariuszD
author_rank: Regular Contributor
timestamp: September 18, 2025, 11:19:02 am
content: My working prototype was a capacitive divider 1000:1 connected to a commercial 10:1 probe (probe input impedance of 15pF||10MOhm) which allowed me to avoid the problem of reflections in the cable and loading the divider with a capacitance greater than C2. The improved version with a buffer can't be worse.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: September 18, 2025, 01:58:27 pm
content: One of these should be used, regardless of which probe is chosen:https://www.amazon.com/Enespro-Universal-One-Piece-Electrical-AGHS-U6/dp/B0D1SJ47V8/ref=sr_1_2_sspa?adgrpid=1241349616540143&dib=eyJ2IjoiMSJ9.4D9XaCNcEItaxhgQpPZqZbfeNRMy0hARrFyg4kJQEXmAGlM-TwsStJctHU4hxV-X2dr2YoIxjn1PMBimrcXdufrUlBrZEdmaXQ3k489iUwRPfagXOjFlMqgXr5NxxjW5oDs1Vi--5SgZjc0Uuh6sY-DJj-USSpuUSqAGOA-_XBAitvpDAm-jfhcKGqgXanY_bXGBZjsj5YS7PjPEX0hrXke1HIsKmKQULr6jQDVLSFwF-cZcCt2eMzwR1QKd_uch-Ay__k1iJVxX26Zos_UoKz-IBd9T3F1TbsU4auhPyy4.ONRJ504l8IXA4Y0LXqU5jfW1S2MdL9JowRLIDEE92no&dib_tag=se&hvadid=77584480973536&hvbmt=bp&hvdev=c&hvlocphy=105222&hvnetw=s&hvqmt=e&hvtargid=kwd-77584596179876%3Aloc-190&hydadcr=20635_13426491&keywords=lineman+hot+stick&mcid=e59332a8ba3b3414941008e56fd76e88&msclkid=02a5290da45e1ecf91a041c1a98d3e57&qid=1758203536&sr=8-2-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1

author: oz2cpu
author_rank: Frequent Contributor
timestamp: September 18, 2025, 02:57:50 pm
content: here is my own DIY high voltage and fast 100:1 scope probehttps://webx.dk/oz2cpu/hi-volt-probe/hi-volt-probe.htmfree gerber files, free 3D print files, free schematics it is all there

author: dazz1
author_rank: Frequent Contributor
timestamp: September 22, 2025, 09:27:57 am
content: Quote from: oz2cpu on September 18, 2025, 02:57:50 pmI would suggest adding a flange to the 3D print to prevent your hand sliding down to the pointy end where all the voltage is located.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 24, 2025, 12:44:28 am
content: Quote from: MariuszD on September 12, 2025, 12:07:55 pmI like this design but I think it needs to be partnered with a DC probe with matching performance.I am trying to diagnose an HV fault at present where the circuit is producing a kV spike where should only be DC. The capacitor probe would detect that. In addition, the voltage is flickering at about 100Hz to 200Hz. Probably too low for the capacitor probe to detect.My existing commercial DC probe does not show the spike or the flicker. Its frequency response (unmeasured) is too low. So I am thinking that this capacitor probe needs to be partnered with a DC to (say) 1MHz probe. The frequency response is likely to be dictated by the available HV high value resistors, like those made by Ohmic, plus the performance of the electronics signal processing.

author: dazz1
author_rank: Frequent Contributor
timestamp: September 24, 2025, 01:08:17 am
content: HiI have read on this thread how some have used, or proposed, a range of materials to make HV probes. People making these choices do so at their own risk, and that is fine. I am conscious that people may follow without fully understanding the risks. Basically it comes down to materials properties.The properties of insulators are greatly affected by impurities included at manufacture. Silicon semiconductors work because of impurities added to pure silicon. The manufacture of plastics is a chemical process and all chemical processes include impurities. Virgin PTFE is a great insulator, but if it is made in an anonymous factory where lowest cost is the priority, it is unlikely to be suitable for HV applications. I wouldn't bet my life on it. It would be advisable to buy PTFE from a reputable supplier who can tell you who manufactured the product. You can then trace the origins of the product see what properties/quality the product has. If they don't quote HV performance, at least get food grade ptfe where there should be less impurities than ungraded product.None of the DIY probes I have seen attempt to test the limits of their probes. It wouldn't be too difficult to set up a test rig, based on a car ignition coil, to test a design. Of course any probe tested to arc-over should probably be thrown away. Arc-over is likely to cause damage, even if it is not visible. Testing would provide data on safety margins.So I am saying that materials selection and origin is really important. Testing to failure is important to measure the safety margins.Keep it safe.

author: joeqsmith
author_rank: Super Contributor
timestamp: September 24, 2025, 03:54:12 am
content: Quote from: dazz1 on September 24, 2025, 01:08:17 amhttps://www.eevblog.com/forum/projects/diy-high-voltage-probe-safety-concerns/msg2759134/#msg2759134As I mentioned in the post, I have little concern for safety and you are correct, that is on me. As far as your not seeing any DIY probes being tested to failure, that's on you. I have certainly posted about it and provided videos of the test jig.This was the last video I have made showing the internals of my high voltage probe after I repaired it. It's not been apart since. There is a whole series though for anyone interested.

author: RJSV
author_rank: Super Contributor
timestamp: September 24, 2025, 05:47:49 am
content: Actual GLUE, the noun, or did you mean 'attach' like glue.Because, I'm relieved that you've purchased 'high voltage' resistant glue, that won't be a current flow path. (Sarcasm).Do you lock up that exp probe, at night, or can the neighbor's kid wander in, and try it out, unsupervised.Probably never happen, like safely leaving a loaded gun on the bench,...only safer. (More sarcasm).But, at least my text following has the good, responsible safe stuf

author: dazz1
author_rank: Frequent Contributor
timestamp: September 25, 2025, 04:04:58 am
content: Quote from: joeqsmith on September 24, 2025, 03:54:12 amI have watched quite a few of you videos and I like them. I haven't seen the one you linked before. There are probably others out there showing probes being tested, but I haven't found them.I just think it is important that those who are concerned for safety, and might not have your knowledge or experience, approach this type of project with some caution. There is some great info on this thread, but there is also stuff I would never do, like 3D printed probes or use cheap anonymous PTFE, but that's me.