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DIY High voltage probe - safety concerns

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MarkKruetzmann:
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_1G


This 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 is really bad 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.

Ian.M:
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.

MarkKruetzmann:
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.

tautech:
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.

joeqsmith:
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.

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