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High voltage probe and high voltage resistors

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joeqsmith:
Shown is a 20kV differential probe in apple jelly jars.   Wonder what ever happened to this channel. 

TimFox:
The first high voltage divider I built (in grad school) was carefully constructed in an oil-filled glass housing with the HV connections on top (not hermetically sealed, with a substantial air gap above the transformer-oil level), for 100 kV.
That became a good demonstration of dielectric forces:  the oil flowed upwards to leave the region of non-uniform field and leaked out the top.

xzswq21:

--- Quote from: TimFox on August 01, 2021, 07:47:36 pm ---Choosing resistors with a specification (even “typ”) on VCR is a good idea.  Note that the spec is “ppm/V”, so the ppm (10-6) must be multiplied by the voltage across the resistor to get the fractional deviation (this can be derived from a quadratic function of current vs. voltage, rather than linear).

--- End quote ---

What do you think about this capacitor? But the price is 40$ and the diameter is 21mm

https://www.mouser.com/ProductDetail/Vishay-Cera-Mite/715C40KTT10?qs=TiOZkKH1s2TbBg3om3yqqQ%3D%3D

https://www.mouser.com/datasheet/2/427/715c_kt-1762164.pdf



TimFox:
For obvious reasons, that’s called a “doorknob” capacitor.
The typical curve of capacitance vs. applied voltage (% of rated max) is shown below the table.
Does this meet your requirements?
N4700 is probably the “worst” of the Class-1 dielectrics.  The dielectric materials get “worse” as their dielectric constant increases, but high voltage capacitors need a thick dielectric for insulation, based on their dielectric strength.

HVStuff.com has some interesting high-voltage polystyrene capacitors.  They cater to the Tesla-coil enthusiasts.  http://hvstuff.com/high-voltage-capacitors/film/polystyrene  They also have HV ceramic capacitors, but don't specify the dielectric  (some have Q about 250, which is probably N4700 again).  Their prices are reasonable.  The polystyrene capacitors should be good, but their values are a bit high.

MaxGain Systems have some interesting HV NP0 ceramic capacitors:  https://mgs4u.com/product/npo-rated-doorknob-capacitors-5-pf-5-kv/  , used, at quite good prices.

xzswq21:
Consider I have a 1000:1 voltage divider and the input voltage is 20kV, if I Want to use ten 100MegOHM in series, the differential voltage across the first resistor would be only 2kV but the absolute voltages across the resistor would be 20kV and 18kV! I think I should use a resistor with the voltage rating of 20kV. What do you think? I think there is the same philosophy for the capacitors. Thanks

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