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

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Patrick

Sat Apr 16 2011, 05:14PM

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

Pinky's Brain wrote ...

He was at Eastern Voltage Research I see ... they had a HV divider on the old website resources, but that is no longer there. I don't remember exactly what was on the page and the wayback machine isn't cooperating. They do have schematics for a high voltage probe. Of course a physical description is more important than the schematic with just electronic components, but it's something :

Ok, in that case I have seen his work related to this device. Maybe Teravolt will look at that compensation section.

OOooo Teravolt got in a ninja post!

I have seen all of the above links many times before teravolt except for the 88 page PDF one.

teravolt

Mon Apr 18 2011, 04:33AM

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

well you obviously have much more experience with building hv probes so I shall be the student until you have not read what I can find on the net. these probes are not easy to design or they would not sell for 1000 plus. regretfully I have not built one of these probes but it would be nice to.

Patrick

Mon Apr 18 2011, 07:30AM

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

teravolt wrote ...

regretfully I have not built one of these probes but it would be nice to.

Then we shall do so.

As one experiment is worth a thousand expert opinions. We will see if I know as much as I hope I do.

Patrick

Tue Apr 19 2011, 04:24AM

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

ok here is my most current attempt.

I was trying to PM Pinky's Brain, but get the "User not found" message?

This is an example of a standard probe modded to take BNC's on both ends, as per the blue boxes in the schematic pic below.

Basic physical layout, though now there would be three park RC's not the two seen above.

Pvc pipe, 2.4" OD, schedule 40, 0.160" wall thickness, 10.3 inches tall. this probe would be more similar to the VD NorthStars (not hand held like the Tek's or PVM's) as per Pinky's opinion, with 3mm per kv seperation from the terminal to nearest ground.

Pinky's Brain wrote ...

To me it seems that a handheld probe is significantly harder to design than a standalone probe connected through a low induction copper strap. With a stand alone probe you can keep the electric field across the HV resistor uniform (like your capacitive hoops in the other thread) which simplifies modelling it's operation.

The moment you put a grounded metal shield around it which goes close to the tip the behaviour of the system becomes much harder to predict.

Notice that in the Teks (any of the p6013,14,15) they all share an internal capacitor leaf, plus the full length cylinder grounded shield. They also conduct the main current from the tip all the way through the cable to the comp box circuit.
The NorthStars, mine and the patent
all have the main current sent to a nearby ground, with the small current signal then going to the far off O-Scope.

Schematic. Hopefully this will be flat from DC-5Mhz, even though the probe is rated past 150Mhz.

I hope the use of the standard modded probe will solve the coupling problem ive been having. I have been able in real circuits, and simulated circuits to get the initial division done, but then trying to get the signal to the O-scope has always involved other comp circuits which caused bazarre behavior, like frequency peaks, drops, tail offs on square waves, droops and so on.

teravolt

Tue Apr 19 2011, 07:47PM

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

I like the one on the bottom it is simple and streight forward. in my nija poast one of those papers say that the capacitance has to have the same division ratio as the resistors wich makes sense. The P6013 are a bitch to calibrate and the frequency specrum on those are suto flat and probibly need special equipment. the copper peaces that you have added to your resistors are exalent electrostac shields but offer no parallel capatance to carry the AC component that we are trying to mesure. if you added a ring of capacitors around each oppen ereas betwean the electrostatic shields, say 15kv 30-100 pf disk or chip, and then used the example in the last schematic you might have a starting point you could put the bottom part of the divider at the base of your pvc tube. on the side the cabe that goes beween the p3016 and its compinsation box is a special foam coax and the center is like 3-5 mill to I asume to reduce the capance in the connction between them

Patrick

Tue Apr 19 2011, 08:32PM

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

Teravolt, read this patent with regards to the capacitience of each seriesed stage.

30-100pF is to much to get the 5Mhz, 70nS I need, capacitence is the enemy,I need only enough to equalize the division of the e-field.

then look at these:

The dinosaur P6013, P6014, P6015 probe series.

Northstar, Ross engineering, the patent, and my type of probe.

teravolt

Wed Apr 20 2011, 05:01AM

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

I talked to one of our engineers today and the variable cap is for undershoot and overshoot adjustment. apparently you don't need much capacitance to get the effect you want. For an example he showed me a weird probe that has some cadock resistors surrounded by two large bowls and he said that by adusting the size of the bottom peace he can adjust the over or undershoot. The p6013 above uses the same princable. If you had a peace of metal tubing around the outside of your pvc tubing connected to ground. that may work to create the capacitance you need. or half and half where the top half of your probe connects to the top peace of tubing and the bottom half connected to ground. The northstar probe looks like the capacitors are physical capacitors in parallel with the divider resitors. If they are not then some how the capacitanceis created physicaly like you are trying to do and they are in parallel with the resistors.
The probe in the first page has resistors in series with each capacitor, this is for dampining ringing, probibly because the lenght of the probe probibly has a inductive component that becomes notiable at higher freq combined with the capacitor you are trying to create. He said the best way to lern about this stuff is by doing. I hope my ramblings make some sense so I the student will sit now.

teravolt

Wed Apr 20 2011, 05:27AM

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

maby something like this

1303277240 195 FT107837 Hv Divider Concept

Patrick

Tue May 10 2011, 06:06AM

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

Found It !!!

Eastern Voltage Research 50MHz, 60kV probe.
]60kvdivider_fullschematic.pdf[/file]

I must admit it does look like it has modeled after this patent....

The only problem is -- I dont have any idea how the math works for the compensation section, or how to modify it for my purposes. Totally guessing and simulating at the moment.

EDIT: Im getting a -3dB roll off at 53MHz and -5 degrees at 5MHz in simulation, is this just a phase shift type circuit? Whatever it is, it works well and has only one adjustment, instead of 6 trimmers.

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.

Carl Pugh

Tue May 10 2011, 03:01PM

Registered Member #1064 Joined: Tue Oct 16 2007, 05:04PM
Location:
Posts: 42

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.

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

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