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

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

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

1 2 3 4 next last

Move Thread

LAN_403

Patrick

Wed Jun 30 2010, 04:29AM

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

Intent of this post:
To seek input/advice as first generation rev 2 construction nears completion. I really hope this crazy idea works, I need these waveforms for deriving phase lag/lead and trigonometric funtions related to HV wave timing. I hope to have effective operation at upto 100Kv. If this becomes a succesful endevour, I will move all this work to the "Projects Forum", for others to build if you choose. Comments wanted.

Purpose and background:
My high voltage divider will be used to support my DeSeversky Ionocraft Ph.D thesis/project.
My device is based on Sam Goldwassers original document here

(Jim Lux is also an important source.)
My basic schematic is most close to the last one:

Specifically, mine is based on the 3 op-amp, instrument amplifier of this type:

(pic credit: allaboutcircuits.com)

1277617526 2431 FT1630 Cap Plates 4hv

Above: 1pF @1 inch gap (under yellow olive oil), HV capacitor plates: long=HV plate, short=low Volt plate.
Made from 6 and 1.5 inch 1/4-20 thread bolts and two 0.5 inch ballbearings.

Above: Wire frame model

Above: Close-up of the terminal and ceramic cup.

Above: A collection of 8 Voltage dividers, each connected to a different HV phase, for the ionocraft, and each of the 8 devices will always be connected to its phase, but will have its BNC connector swapped on-too/off-of a 2 or 4 channel o-scope as needed.

Ballbearings, bolts, HMR brass and stuff

How they're made, notice the right most bearing has a flat spot 0.25" dia. and the shafts are 0.25", then they get case soldered together. notice the top two are dark, cause I overheated them.

Note: Once the Ball bearings reach 940 degrees F, and they start to roll off the table, dont try to catch them in your hand. You wont be happy.

Hangin with Mr. Verne.

EDIT: my major concern right now is the fact that the olive oil dielectric is directly and perhaps non-linearly responsible for the capacitence of the HV cap. The LV cap will be a mylar foil type. thus any change in dielectric properties will show up as a bogus voltage measurement.

GhostNull

Wed Jun 30 2010, 07:19AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

good work so far, hope it works!

Patrick

Wed Jun 30 2010, 07:42AM

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

Yeah, thank you GhostNull, if this volt divider idea doesnt work, I dont know what else will.
I worry about the gain factor and slew rate.

well lets see, the math should be: SR <Greater than, equal to> 2 times PI times f times Vpk
and since i measure in V/uS ... then SR from above should be divided by 10^6.

Where:
SR = Slew Rate, in Volts per second.
2 = a constant
PI = a constant, Mmmm, Pie.
f = Frequency in hertz.
Vpk = Waveform Peak Volts.
10^6 = one million
V/uS = Volts per micro-second.

SR= ( 2 x PI x 100,000 Hz x 10 Vpk ) = 6,300,000 V/s divided by 10^6 which =6.3V/uS !
(these input numbers are way worse then expected, and the output is 6.3V/uS)

Checking some datasheets like the TL082 reveal a SR minimum of 8V/uS and 16V/uS typical,
since 8V/uS is equal to or greater than 6.3V/uS, Im good! (As far as slew rate not bucthering the waveform.)

I may use the low noise, high impedance JFET, TL082 op-amp or a type my professor gave me from back at Shasta College, I cant remember that part number at the moment.

Proud Mary

Wed Jun 30 2010, 11:04AM

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

Hi Patrick,

your capacitive divider is going to be temperature sensitive whatever sort of capacitors you use. With time and trouble, you might be able to compensate for the decline of dielectric constant of the olive oil with increasing temperature by having the rods move closer together as they expand.

CHARACTERISTICS OF VEGETABLE OILS OF SOME SLOVENE
MANUFACTURERSâ€
D. Rudan-Tasic and C. Klofutar
Acta Chim. Slov. 1999, 46(4), pp. 511-521

from the above citation:

Refined olive oil - temperature versus dielectric constant

25.15 C - 3.098
30.15 C - 3.067
35.15 C - 3.036
40.15 C - 3.007

As there is no universal standard 'olive oil', I wouldn't take these figures as if handed down from above on tablets of stone, but as a more general statement regarding the rate of the oil's reduction of dielectric constant with increasing temperature, from which you could develop a calibration curve.

3.1 @ 20 C is the figure commonly given in tables.

IntraWinding

Wed Jun 30 2010, 11:16AM

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

This would be really useful!

I guess you need an insulating oil which is inert for longevity, and with a low temperature coefficient of dielectric constant for accuracy despite environmental temperature variation. Perhaps Olive Oil would work well, I don't know, but I've always wondered why those Tektronix high voltage probe designers went to the trouble of using freon gas as insulation. Perhaps it was to meet these criteria?

In the new version of their high voltage probe they've replaced
"The CFC 114 dielectric often used in high-voltage
applications ... with a silicone compound."
I've attached the datasheet for permanent reference.

1000x, 75MHz, 20KVac (40KVdc), 3pF, 100Mohms, US$1,570
]tektronix_p6015a_high-voltage_probe_.pdf[/file]

Silicone oil is certainly inert, but I have no idea about its temperature coefficient of dielectric constant . The other particularly inert type of insulating oils are perfluorocarbons but they are pricey unless you happen to get lucky.

Patrick

Wed Jun 30 2010, 06:28PM

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

well dam, thats what i was worried about, as per Proud Mary's comment, i will compensate using a thermistor-driven op-amp then. As i dislike moving parts.

Mattski

Wed Jun 30 2010, 09:43PM

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

Another option would be to have the high oil held at a constant temperature. You can run a heater with a thermistor and PID controller (you can hack one together with a PIC) to keep it at a constant temp which is a few degrees above your expected maximum room temperature. That's how oven controlled crystal oscillators work. The disadvantage is the fact that you now need to turn your probes on and let them warm up.

So the way this works is that ball bearing gap is the high voltage capacitor, and you have a large external mylar foil cap?

Regarding nonlinearity, is there any way you could do this with air gaps or would the size end up being ridiculous to avoid breakdown/corona?

According to this palm oil does have a nonlinear dielectric constant at high electric fields, starting at about 1.25MV/m:

Proud Mary

Wed Jun 30 2010, 10:24PM

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

Patrick, perhaps you should first calculate the capacitance variation caused by the change in dielectric constant to see what effect this will have on the divider ration across the range of ambient temperatures likely to be encountered in actual service.

At the same time you should also consider the linear temperature expansion coefficients of the both the metal rods and the housing of your oil dielectric capacitor to see whether the gap between the spheres becomes larger, smaller, or stays the same across the range of ambient temperatures used in your dielectric constant versus capacitance calculation.

Patrick

Wed Jun 30 2010, 11:32PM

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

Yes Mattski, I thought of the oven idea, and you raise a good point about field intensity even though I cant read your cited article.

Proud Mary, yes i was going to make those calculations last night but i was too tired.

so here we go, the math is as follows:

C = (epsilon1) times (epsilon0) times ( A / d )

where:
C = Farads
epsilon1 = dielectric (permittivity) constant
epsilon0 = Electric field constant (Faraday)
A = area (meters)
d = distance (meters)

then we set C = 1pf, epsilon1= 3.098, epsilon2= Farady Cons., (A/d) = 0.036457 (fixed as if a constant)

thus:
@ 25.15 degrees C and 3.098, C=1.0000 pF (Assumed standard, 1.00pF, 100%) measured with BK precision 875B
@ 30.15 degrees C and 3.067, C=0.9899 pF (0.99 %)
@ 35.15 degrees C and 3.036, C=0.9800 pF (0.98 %)
@ 40.15 degrees C and 3.007, C=0.9707 pF (0.97 %)
four sig figs everbody!

so for every 5 degrees C rise (above 25 C), 1% of capacitence is lost.
and for every 5 degrees below 25 C 1% is gained.

however the difference between 3.098, to 3.007 in 5 C increments is slightly non linear but the approximation still has validity. I will refine this later. ProudMary saved my ass with that PDF.

well this doesnt seem overwhelming or impossibly difficult now. maybe just a trim pot, marked in 5 degree c increments ?

IntraWinding

Thu Jul 01 2010, 12:02AM

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

The large capacitor will have a temperature coefficient dependant on its dielectric, and this might be used to advantage to correct the effect of the olive oil temperature coefficient.

Perhaps it's worth building the probe and testing it to see what the overall effect of all the competing effects are before attempting to fix anything!

1 2 3 4 next last

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob