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

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Patrick

Thu Jul 01 2010, 01:02AM

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

IntraWinding, well the above math indicates a simple slight calibration curve will cover the unwanted change region, thus the same trim pot for initial calibration, could also be marked with a temperature label when high accuracy is needed.

And yes I bet the freon/silicone is used so as to be resistant or preditictable for this type of problem.

Mattski, i wish i could read that article. 1.25Mv/m = 1.25kV/mm, I am at 100kv/inch = 3.9kV/mm, yikes!

It has been so useful to consult with my fellow high voltage aces.
Im glad I made this thread, please keep the replies comming.

klugesmith

Thu Jul 01 2010, 04:11AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Patrick wrote ...
Mattski, i wish i could read that article. 1.25Mv/m = 1.25kV/mm, I am at 100kv/inch = 3.9kV/mm, yikes!

It has been so useful to consult with my fellow high voltage aces.
Im glad I made this thread, please keep the replies coming.

And please keep posting your reports of real hands-on stuff!

Your kV/mm values sound pretty conservative, if they are for breakdown field strength. Dry air is 3 kV/mm!
The attached brochure for Biotemp "biodegradable dielectric insulating fluid" claims 32 kV at 0.04 inch gap per ASTM D1816 test method.
]biotemp-abb.pdf[/file]
Lots to find by googling for "dielectric oil testing", for example

[edit] on closer inspection, looks like you're just worried about voltage sensitivity of the dielectric constant. Ever occur to you that big ol' HV pulse discharge caps, and HV resistors, have specified voltage coefficients that can amount to significant percentages at rated voltage?
Can you finish your thesis work using a home-made instrument that has 10% absolute accuracy and 5% linearity?

Patrick

Thu Jul 01 2010, 04:41AM

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

Klugesmith wrote ...

[edit] on closer inspection, looks like you're just worried about voltage sensitivity of the dielectric constant. Ever occur to you that big ol' HV pulse discharge caps, and HV resistors, have specified voltage coefficients that can amount to significant percentages at rated voltage?
Can you finish your thesis work using a home-made instrument that has 10% absolute accuracy and 5% linearity?

yeah i may be able to get by with +10%,-0% absolute, and +-5% relative, but i would feel better with +-1%.
(100kvAC! @ +- 1% the dream, and rantings of a mad man i know)

Thu Jul 01 2010, 05:41AM

Registered Member #2372 Joined:
Location:
Posts: 62

Sorry, I missed why you cant just use a high voltage probe.

Patrick

Thu Jul 01 2010, 06:05AM

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

in the bottom most pic, i cant figure out why there is clipping on the pink trace?

These are no longer the planned schematics

IntraWinding

Thu Jul 01 2010, 06:10AM

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

dugg wrote ...

Sorry, I missed why you cant just use a high voltage probe.

Normal high voltage probes have a very low frequency response.

IntraWinding

Thu Jul 01 2010, 06:11AM

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

How about making the input capacitor a vacuum capacitor?

Patrick

Thu Jul 01 2010, 06:48AM

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

intrawinding, will the electrons not liberate themselves from the metal, then fly across the vacuum? al la a vacuum tube?

EDIT: wholly crap! your right vacum tube 20kv ones would be better i must be able to make one for 100kv!
i just wiki'ed it! i still worry about "libbing" some electrons though.

below: the clipping still occurs, even with one op-amp. why? i need sleep!

this schematic is no longer useful
asyou can see the blue sine wave is input, and fine, but the amp output is clipped.

Mattski

Thu Jul 01 2010, 07:29AM

Registered Member #1792 Joined: Fri Oct 31 2008, 08:12PM
Location: University of California
Posts: 527

Patrick wrote ...

Mattski, i wish i could read that article. 1.25Mv/m = 1.25kV/mm, I am at 100kv/inch = 3.9kV/mm, yikes!

The focus is actually on nonlinear surface tension effects of the palm oil under high field, and they specifically avoid discussing the dielectric constant except to show the value and mention that it is changing under high field.

You could try testing it, maybe running some impedance tests by biasing the oil cap with a DC voltage in a setup something like this:

Gnd--(SigGen)--(Current Sense Res.)--(DC blocking cap, large, HV)--+-(Oil capacitor)----Gnd
                                                 Gnd--DC voltage---|

Vacuum would probably work well. There is always a small leakage current no matter if you have vacuum, oil, mica, or whatever. If you don't heat up the capacitor (think cathode in a vacuum tube) and don't run the voltage too high I don't see why it would cause a problem in a vacuum cap.

Side question: what circuit simulator are you using? One of my friends was asking me for a good free/open source one.

Edit: In your simulation try turning the scope to DC, make sure there isn't a DC bias on your output which is making it clip to the limit of the negative power supply.

Patrick

Thu Jul 01 2010, 07:58AM

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

Mattski, i use multisim 10 an illegal cracked torrent.
also changing the o-scope and checking for bias didnt help.

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