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High voltage probe for O-scope, 1 of 2, ( DEVELOPMENT ).

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Patrick

Tue May 10 2011, 06:16PM

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

Carl Pugh wrote ...

Could you use a capacitive voltage divider with no resistors?
There are applications where a small sphere is put in a tank with high voltage. The sphere is calibrated at low voltage and then used to measure high voltage.

I dont know, i tried this but it seemed difficult to calibrate, and i was getting alot of bogus field division external to the device. And they dont work below 400Hz to well.

Carl Pugh wrote ...

BSEE from Heald College.
Definitely not a communist from Berkeley.

I was being facetious.

Patrick

Thu May 12 2011, 12:33AM

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

[Gen 5, Type 3, Mod 60, Rev I, Ver 3.0, Attempt 11] for HV O-scope Probes. (Most Current)

My planned circuit. [oops! C4 is pico should be nano]

(C1 should be nano) Heres the compensation ive been planning and simulating....(from EVR)
At 54Mhz im getting about -44 degrees on the bode plotter.

Magnitude roll off. (This looks real good)

Phase angle. (This is ok)

EDIT: Im hoping someone else could simulate this circuit here:
, to verify some of the math related to the roll off.

ALL problems *SEEM* to be solved, maybe the end of this project is in sight? Now that ive said this, ill probably discover some other problem.

Pinky's Brain

Thu May 12 2011, 01:52PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Does anyone have a rule of thumb calculation for capacity to environment for a vertical piece of wire at a given distance above a ground plane?

teravolt

Thu May 12 2011, 02:03PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

I like it Patrick and it is agood starting point, C4 should also be variable I think. I was looking into surface mount components witch may have a flatter frequency responce vs descreate components what do you think. just keep the attenuation ratio the same in each branch and reduce inductance any way you can.

Patrick

Thu May 12 2011, 02:16PM

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

I put in an email to Eastern Voltage Research in regards to them re-posting that orginal web page which is incomplete on the Wayback Machine. So hopefully they will put up pics and docs related to how that compensation circuit works. But I have a way better understanding now then I did 2 days ago on this matter, so it does in simulation look like its working.

EDIT: Wow! I have been seeing the "views" number tick up quite a bit on this thread, so there must be some interest in this device.

Pinky's Brain

Sat May 14 2011, 03:17PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Patrick wrote ...

Found It !!!

Dude ... that's what I linked before!!! :)

wrote ...
It is interesting that 82pf x 2 = 164pf / 7 = 23.4pf total input capacity. thats way more than mine. Soo maybe i could increase it quite abit.

I think I said that before, basically the distributed capacitance has to drown out the capacitance of divider to ground.

Found some formulas for capacitance to ground :

1 pF capacitance to ground in between each of the stages of the high voltage arm seems a decent if slightly pessimistic approximation.

Also found a divider which shows nicely why it's better to create the distributed capacitance by structural methods with the shielding than with individual capacitors.

The reason they use so many capacitors in a circular string at each stage is to minimize wire inductance. If you build it structurally like for instance with the dual layer shielding Teravolt proposed you automatically get low inductance (a wide diameter hollow conductor has lower inductance than a narrow wire) at much lower cost. Although it will be hard to control insulation thickness to get equal capacitance per stage, flaring out the shield into two discs with an insulator in between will make this easier.

Patrick

Sat May 14 2011, 04:55PM

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

Pinky's Brain wrote ...

Patrick wrote ...

Found It !!!

Dude ... that's what I linked before!!! :)

I didnt relise what the significance of your link was, until i simulated the compensation circuit alone.

Patrick

Sun May 15 2011, 04:14PM

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

I have just been informed that Daniel McCauley, of Eastern voltage Research that the pics in question are archived and not immeadiatley available. hopefully he can find them when he has some time.

Patrick

Wed Jul 27 2011, 12:14PM

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

New work completed so far...

Insulator is acrylic should be good for 50kV+

Patrick

Sun Sept 04 2011, 02:06AM

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

Ok new construction and assembly is about to begin, so we will see if my probes work or not shortly.

Based on my own research and experiments, Ross Engineerng, Northstar, Tektronics, Caddock and Marco Denicolai's work ive come up with this as the basic building block:

The resistor's V drop is linearized with the cap plates, and grading rings.

Marco Denicolai's explanation of his 20 liter water HV probe (600kV).
[Note what i have highlighted in green.]
His PDF is here:
]hvprobemd.pdf[/file]
Marco's tesla and thesis site:

1315101988 2431 FT107837 Fileld Analyisis1

My Hv resistors.

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