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 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.