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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
[Gen 4, Type 3, Mod 40, Rev H, Ver 2.0, Attempt 9] for HV O-scope Probes.
Ive been reading this... ]us5107201hvprobe.pdf[/file]
14awg wire copper. 75M ohm, 20kV, 3.4W resistor with the main resistive material cut.
I presume the curve will decrease field intensity better then the angled one.
2.0pF in air, 4.0pF in olive oil. I had to destroy this resistor to make the capacity measurement, I ground through the Ruthenium-oxide resistive element. before with a good resistor my C meter said 0.8pF - 0.0pF (@ 2Vrms into 75M ohms!) after cutting it, it read a stable 4.0-4.2 pF in oil.
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837
If you make a divider as tall and for as much voltage as the North Star VD probes you will be left with << 1 pF ... whereas North Star has ~20 pF, and that capacitance isn't there for naught.
This seems only enough for a handheld probe with one or 2 resistors.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Pinky's Brain wrote ...
If you make a divider as tall and for as much voltage as the North Star VD probes you will be left with << 1 pF ... whereas North Star has ~20 pF, and that capacitance isn't there for naught.
This seems only enough for a handheld probe with one or 2 resistors.
Two in series was my target. I was warned that too much capacity would limit bandwidth or rise time.
Proud Mary wrote ...
It suggests a skeletonic version of the individual resistor shielding arrangements in the Park Resistor standard.
I technically wasnt shielding the resistor with the cap, have'nt got that far yet.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Pinky's Brain wrote ...
To me it seems that a handheld probe is significantly harder to design than a standalone probe connected through a low induction copper strap. With a stand alone probe you can keep the electric field across the HV resistor uniform (like your capacitive hoops in the other thread) which simplifies modelling it's operation.
The moment you put a grounded metal shield around it which goes close to the tip the behaviour of the system becomes much harder to predict.
The NorthStar's do have a large topand bottom ring, to equalize the field and as you said I guess thats where some capacity comes from too.
I believe the Tek P6013, P6014, P6015 all have that metal shield so they can treat the whole path as a lossy-transmission line?
Im aiming for these: 300M for 24kV, 2 Probes 600M for 45kV, 3 Probes (0ne of these will be a +-1% calibration standard) 800M for 60kV, 2 Probes 1.40G for 100kV, 1 probe
What about this...
Yellow are copper "Park-like" Caps, Red are resistors. The bottom most section are where compensation and LV componets live. ( Approx. 14" tall )
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Updates and Pics.
Ive had real problems with the caps here, please comment. ive had series capacitence be larger than the parallel cap ? what, how is that possible?! And, ive had each cap very from 1.1 pF to 3.3pF, now i think there stable at 2.3 to 2.8 pF each. I think there both actually close to 2.5 pF, and the dimensions are withins thousandths in any dimension, so i dont see how it can vary unless its other objects on the table that bias them. Also, the tubes have the same half circle radius as the pipe wall thickness (0.040")
Please comment and advise me if im totally wrong, or if you have a better set up for measuring pF's in the single digit range.
Basic Components, tubes for caps, and two 200Mohm resistors.
resistors were ground through till open circuit, so as to not effect the Picofarad measurements.
Assembly and scale.
Olive oil, 2.4 pF, and cap check at 0.570" gap. (copper spool support)
Left cap at 2.8 pF, 0.570" gap. (copper spool support)
Right cap at 2.5 pF, 0.570" gap. (kool-aid support)
Series cap is 1.1pF. (kool-aid support)
Left cap at 2.5 pF, 0.570" gap. (kool-aid support)
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Have you measured the capacitance between the two meter leads on their own - terminated in the oil bath with nothing attached to their ends? You have to account for, or null out, stray capacitance somehow.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Proud Mary wrote ...
Have you measured the capacitance between the two meter leads on their own - terminated in the oil bath with nothing attached to their ends? You have to account for, or null out, stray capacitance somehow.
yes, nulling was the first thing I did, multiple times throughout the day/experiment. And got multiple consistent results, so I belive the meter. (i even was careful with how close and the shape of those two wires)
I think pF measurements are just prone to alot of crap?
Also, proud mary this is my interpretation of the "park" concept.
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