Electronics > Projects, Designs, and Technical Stuff
Open HV Probe 40kV
ArthurDent:
I was only using that video to show how the Fluke probe was constructed and not anything else, although his using pill bottles for insulators is a nice touch!
When the metal piece the coax is crimped into is pushed into the front part of the probe, the end of the resistors look like they are almost an inch inside and the two metal tabs on either side of that piece makes good contact with the two metal threaded halves and pretty much enclose any live parts inside the tube. All metal shell parts and the coax shield inside the handle are at ground potential. At 1000:1 the maximum voltage at that end should be less than 40 volts so it should be fairly safe as long as it is connected to a meter. I assume the second (ground) lead going to the resistor assembly is to a safety resistor across the output to limit the maximum voltage if there is no meter connected.
I don't know how far the shield goes inside the front part of the probe but the drawing of the Tektronix probe shows that that one goes about 3/4 of the distance to the tip. The coax connects to the end of that assembly with a BNC connector and that probe is assembly is sealed. To achieve the full voltage rating with the Tektronix p6015 (non "A" version) you had to open the sealed assembly and pour freon in it. The "A" version used more environmentally friendly silicone oil. I had used one of the old versions and they worked well but were a pain to maintain.
beanflying:
Had a look at the space within the current tip and roughed in the planned Resistors as per the cutaway below and have shown the maximum voltage and distance from the hand. There would be room to allow a thin copper tube perhaps covering up to 1/2 way into the 90MOhm resistor. Inside that tube would still be the multilayer wrapped PLA tube already made. Another option would be some tinned copper sleeve which I already have from some RFI shielding, it could easily be expanded over the last section of inner tube then soldered onto the common Earth/0V wires.
I most likely won't see the Resistors this side of the coming weekend so plenty of time to tweak and remake a section or two.
** I am getting 90, 400 and 500 MOhm resistors so it will be 100:1 not 1000:1
PartialDischarge:
--- Quote from: ArthurDent on April 23, 2019, 04:02:17 am ---I don't know how far the shield goes inside the front part of the probe but the drawing of the Tektronix probe shows that that one goes about 3/4 of the distance to the tip.
--- End quote ---
The workings of the p6015 series are different, it is a compensated oscilloscope probe and the shield is not there for safety but to 1) isolate the internal high impedance resistor from external fields, and 2) create a internal cylindrical volume where fields are controlled and HF compensation can take place.
I do not think the fluke dmm probe has the resistor covered but theres an easy way to know. Connect the probe to an oscilloscope using a banana-bnc adapter with vert scale set to 10mv/div. Now grab the probe top part with your hand and see if you can see a 50Hz signal in it. Given the thin diameter of this probe I do not think it has this part shielded.
It is mostly forgotten that any high valued resistor is a 50Hz nice pickup.
vk6zgo:
--- Quote from: beanflying on April 22, 2019, 12:13:50 am ---
--- Quote from: mnementh on April 21, 2019, 11:17:23 pm ---Please understand... my primary concern here is the "open" part of this project. As soon as you put that label on something, the fuckwits start to come out of the woodworks. Building it for yourself is one thing; sharing the blueprints online is something I'd consider to be roughly akin to all the imbeciles sharing plans for 3D-printable firearms, all of which are far more likely to kill the user than anyone more deserving such fate.
I've committed numerous Darwin-award-worthy acts of stupidity with HV transformers; my grand-dad started me early helping him wind a 3KV transformer for a 160m transmitter when I was 10. :scared:
There's a reason I don't document the details of those adventures publicly; and as anyone in the TEA will tell you, it isn't embarrassment because I have not so much as a single shred of it left in my being. ;)
mnem
*tzzzt*
--- End quote ---
There is a counter argument to that non discussion of potentially hazardous projects given the mass idiocy on youtube with a simple search on High Voltage. Discussion here in the open is educational, imformative and generally carried out by people with a greater level of understanding than the potential Darwin Award Winners.
The maths in this one is generally ok but the practical is horrifying as are the comments below it in general :scared:
https://youtu.be/RAOw0mHQRvk
--- End quote ---
I also noted he determined the series resistor value on a low voltage range.
I seem to remember that many DMMs have lower internal resistance on such ranges, with the high range resistance around 10/11M .
As far as I know, my old Fluke 77 has the high value on all ranges, but ! may be wrong.
bd139:
Fluke 7x and 8x meters have variable impedance on each range. It's "around 10-11 meg" and varies from meter to meter and range to range. This is one reason I originally bought a Keysight U1241C as it has constant across all ranges. Shame it otherwise sucked.
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