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

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Mattski

Fri Jul 02 2010, 09:13PM

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

Klugesmith wrote ...

I think not, because the shield is not floating-- it's grounded. So the second red cap will have almost no voltage across it, and will not significantly perturb the attenuation ratio.

You're right, I overlooked that fact. So the first red cap will mainly serve to decrease the input impedance of the probe by provided an additional path for reactive current to ground.

Patrick

Fri Jul 02 2010, 09:20PM

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

wow! 3 pages!

ive never had a thread go past 1 page, this one is past 3 now!

Patrick

Sat Jul 03 2010, 12:48AM

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

Klugesmith wrote ...

Regarding the question about how to calibrate the gain of Patrick's HV probe, which has no response at DC.
The same question applies to fancy probes such as Dugg pointed out -- how does the designer or user adjust and verify that the HF attenuation is the same as the DC attenuation?

We can independently measure high voltage with an electrostatic voltmeter like:

(which looks big enough to handle many tens of kV if filled with oil).

The force on capacitor plates is proportional to square of electric field strength.
So the instrument responds equally to DC of either polarity, or to the true RMS value of an AC waveform.
A homebrew instrument could be calibrated by measuring the change of capacitance as the plates move.
Or by applying high voltage DC, independently measured with a trusted HV DC probe.

i fail to understand, please eleaborate.

klugesmith

Sat Jul 03 2010, 04:41PM

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

Patrick wrote ...

Klugesmith wrote ...

We can independently measure high voltage with an electrostatic voltmeter.
... A homebrew instrument could be calibrated by measuring the change of capacitance as the plates move. Or by applying high voltage DC, independently measured with a trusted HV DC probe.

i fail to understand, please elaborate.

Suppose your capacitive divider probe & buffer are predicted to have flat frequency response down to 1 kHz, but the attenuation ratio is known only within +-15% because of geometric & material uncertainties.
You could calibrate the probe with an accurately known AC source of suitably high frequency.
Could be less V and F than your end application, if you trust the linearity & flatness of your probe.
For example, 5 kV at 5 kHz, obtained by under-driving your fancy transformer.

How do you know the value of your AC calibration voltage, if the only trusted instrument on hand is a HV DC voltmeter?
a) get an electrostatic voltmeter like

b) make one & calibrate it with an accurately measured HV DC source.

I'm talking about voltmeters that register the mechanical attraction between capacitor plates.
The two specimens I pointed to are configured like old fashioned rotary variable capacitors.
When the cap is charged, there is a torque tending to turn the shaft in the direction of increasing capacitance (like a magnetic reluctance motor). Torque constant can be figured from slope of capacitance vs. shaft angle.
Instantaneous torque is proportional to square of instantaneous voltage. So average torque is proportional to square of RMS voltage, for any frequency until second-order effects interfere.

Patrick

Sat Jul 03 2010, 08:27PM

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

so maybe i should use a gram mass scale? since electric force increases as a square while the distance increases linear. thus the acccuracy increases rapidly towards 100kv, but diminishes towards 10kv. also gram force might be more easy to measure.

Patrick

Mon Jul 12 2010, 06:45AM

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

semi-final Schematic is below:

This is no longer the most current schematic.

also, the 8.5nF cap will now be polyethylene, not mylar, on account of dissipation factor.

IntraWinding

Mon Jul 12 2010, 11:13AM

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

Looks good. Presumably the op amp feedback will be adjustable to calibrate the meter in use, right?

Is the connection to scope input C just for testing purposes?

Take a look at the meter in this post. It has a pair of vacuum capacitors as inputs and reads to 100KV.
There are no indications that the designers though X-Rays would be a hazard, which seems surprising. I'd like to know more about when and when not to worry about X-Rays from vacuum capacitors.

Patrick

Mon Jul 12 2010, 07:00PM

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

Yes IntraWinding there will be a calibration pot, and yeah tha o-scope channel C is for testing/simulation purposes only. It will be removed on the final prototype.

Yes I have seen the vacuum volt meter and am intrigued, I have questions as how not to end up Curied' by X-rays, if you see my meaning.

the ceramic insulator drying out, to be fired and glazed this week.

IntraWinding

Tue Jul 13 2010, 10:12AM

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

I used to do a bit of pottery - your insulator is impressive! Can you say more about its construction?

Patrick

Tue Jul 13 2010, 09:18PM

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

yes intrawinding, i love to make my own ceramic, i dropped it off 30 mins ago to be fired. i dont have to depend on what i can scavenge, i can make unique sizes and shapes, and these are ceramic and porcelian. they both have very high volumetric resistivity, though that decreases with increasing temp. its very nice to be able to make what ever shap and size i need, i can choose colors too 65 colors though i try to use the common wire colors like the colors on the old rainbow ribbon cable.

first the pic you see in the above post, is ceramic, it is made by layering tempates 0.375 inches thick, slip and pressed together.

above are the cardboard templates.

my standard thicknesses are 0.250, 0.375, 0.500 while wet, they shrink from the widest part (not the thin-ness, or narrowest parts) between 3% and 22% which must be calculated, and experimented with. a small circle 1 inch dia, will shrink 22% of is diameter while a 5 inch diameter circle will shrink 7% so its counter intuitive. (your clay my vary)

the insulator above was paid for today to be fired for 3$ (2 cents per cubic inch)

the first firing vitrefies the clay for 3$
the second firing will be for colored glaze for 4.50$
the third firing will be a glossy clear glaze for 4.50$
plus 3.00$ for color glaze
plus 0.75$ for clay
thus the total is about 16$ for the insulator seen in the above post.

as soon as i get my lite membership, my costs will be about a third of the above.( so about 4.80$ for each insulator )

so this is the cheapest, and best materal for HV work, i used to make acrylic and polycarbonate insulators for much more money and limited shapes, and i had to CNC them too. the ceramic and porcelain i like much better.

also, i am always interested in pics of insulator shapes to copy, and the math that describes size/shape for the field being held off.

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