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Capacitive Voltage divider, for O-scopes (10,000:1)

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IntraWinding

Sun Jul 18 2010, 11:40PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

How fast do you need this to go?

Antonio

Mon Jul 19 2010, 12:10AM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

It will be interesting to see how your ceramic insulator behaves. My experience with ceramic insulators shows that they only work well when the air is very dry. Otherwise they are as hygroscopic as glass and inferior to plastics. I have a Van de Graaf generator that uses a big ceramic insulator as support for the terminal. It works well, but the air must be reasonably dry.

Patrick

Mon Jul 19 2010, 03:57AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

@antonio : thats interesting, i need to research that then, i use a dense-medium clay ceramic, or a high alumina content porcelain (i like porcelain better, its twice the cost.)

either ceramic or porcelain are dried,
then fired to vitrify,
then triple-layer color glazed and fired for second time,
then two layers of thick clear flux glazed and fired for the third time.
[all fires will be cone 5 or 6.]

the glaze is important antonio, otherwise the base vitrfied porcelian or ceramic are porous to air or fluid.
also, i belive that the glaze-atomosphere surface interface (Humidity or dust) is where you have detected current leakage. Not through the bulk of the insulator material itself. ProudMary is the master of the PDF's perhaps we could freeload off him on this topic. i wonder Antonio, if your insulator has micro-cracked, is dirty with dust or coated with low resistence chemistry (bad oil vapors, sometimes), can you post on these factors regaurding your insulator?

@intrawinding: if i could find a 100kHz opamp with 15v/uS slew rate and +-15V inputs, that would meet my needs, but i may have to settle for less.

i got the container made, with the oil section, next up will be the copper flashing.
will post pic in a few hours.... need sleep....ZZzZZZzzzzZzzz

Proud Mary

Mon Jul 19 2010, 10:12AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Glass electrolysis can and does occur in thermionic valves*, especially those operating at high temperature and high voltage, so might represent a risk in glazed ceramic insulators.

I can imagine a scenario in which a surface track occurs due to moisture or contamination, and heats up due to I squared R losses, until electrolysis begins, and ions in the glaze begin to get their own ideas about their place in the world.

Fabio's point about hygroscopic ceramics would have been of less concern in the Thermionic Age, when equipment bins and cases often worked at high temperatures.

I use ceramic stand-offs and tag strips a lot, for aesthetic reasons, because they're easy to solder around without leaving unsightly melted burns all over them, like the edge of a bench where smokers have left their cigarettes to smoulder away.

* Vacuum Tube Design, RCA, 1940

IntraWinding

Mon Jul 19 2010, 12:00PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

Patrick wrote ...

@intrawinding: if a could find 100kHz opamp with 15v/uS slew rate and +-15V inputs, that would meet my needs, but i may have to settle for less.

100KHz? Did you mean 100MHz?

Antonio

Mon Jul 19 2010, 05:13PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

My VDG with a long ceramic insulator.

If kept clean, in a room with conditioned air, it works very well.

Patrick

Mon Jul 19 2010, 08:47PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

@Antonio, you biuld good stuff, but i think your ceramic insulator is fine... maybe the humidity bleeds charge of your belts. there long and fully exposed to air on 4 sides. also the PVC at the center maybe storing charge through long term molecular charge polarization, which PVC is notorius for.

@intrawinding: i need a good 100kHz wave form, the slew rate cuases a square wave to be more trapezoidal on the oscope.in my case a 100kHz 50Duty cycle square wave cuases a 980nS rise lag, and another 980nS for the fall, which amounts to almost 2uS distortion for a 5uS+,5uS- square wave.(10uS total=100kHz and TLO82CP)

@proudmary: why is the problem you describe not a problem for 500kV triple phase ceramic insulators i see here in california all the time? also, what is a "high temperature"? 200 degrees F or 1000 degrees F ? i know that ceramic a glass can become ionically conductive at 600-1000 degrees F, (like Solid oxide fuel cells). my conditions will be room temp tomaybe 150 F.

Proud Mary

Mon Jul 19 2010, 09:57PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Patrick wrote ...

@proudmary: why is the problem you describe not a problem for 500kV triple phase ceramic insulators i see here in california all the time? also, what is a "high temperature"? 200 degrees F or 1000 degrees F ? i know that ceramic a glass can become ionically conductive at 600-1000 degrees F, (like Solid oxide fuel cells). my conditions will be room temp tomaybe 150 F.

I think I may have over-egged the pudding there,

but constructive imagination is always required to anticipate and model possible failure scenarios.

Needless to say, pollution flashover is a common failure mode in 500kV power line insulators, as is the formation of sub-millimetric breakdown channels,
which often begin around non-conformities such as inclusions, undetected cavities, and locked-in thermal stress in the insulator material.

Patrick

Mon Jul 19 2010, 11:00PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Proud Mary wrote ...

but constructive imagination is always required to anticipate and model possible failure scenarios.

yes proudmary, agreed. i need to find out volumetric resistivity of ceramic, glass, and porcelain.

also pics, work so far.
the dimensions are : 6.5 inches tall, and 4.5 inches outside diameter. 0.25 inches thick.
hopefully the insulator has been fired, and if so i will get it tomarrow.

above is the ABS container, top and oil filled section.

above: bottom section, for electronics and coax.

above: figuring out the copper foil.

above: temporary fit of copper and cold ball.

above: copper roll, 8 inches by 20 feet and 0.0042 inches thick(3oz.), its paper backed but thats easy to peel off.

Proud Mary

Mon Jul 19 2010, 11:53PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

This paper clarifies the limitations of Paschen's law when thinking about surface discharges on an insulator, which we could talk about as a breakdown at the interface of dissimilar dielectrics.

Feser K., Influence of the source impedance on the breakdown behaviour of air spark-gaps
By Kurt Feserâ€™)

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