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Learning from North Star HV probes...

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Patrick

Wed Dec 22 2010, 05:37AM

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

ok, here is a pic of the wanted device,

In the above pic, the aluminum turned handle houses the low voltage R and C, and compensation components.
I suspect there is nothing in the blue/clear nylon tube but the HV resistors and at the very tip of the HV resistor a single round plate which acts as acapacitor between itself and the grounded handle. I dont think threre is a shield in the clear/blue tube. But the aluminum handle would act as a static, RFI, EMI shield for the LV section.
Also, the clear/blue part is filled with Shell Diala oil.

Here, above we see a simplified schematic both electrical and mechanical are indicated.

]vmonad.pdf[/file]
]vmonman.pdf[/file]
]pvm5-6.pdf[/file] <- dimentioned mech drawing.

I am trying to figure out how they put it all together, and if we on the forum can make our own to replace the aging P6015A/B tek ones. I already have the HV resistors and will make an attempt soon, please and comments if you have anything usefull to add.

Pinky's Brain

Wed Dec 22 2010, 11:32AM

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

Seems okay, the LV arm capacitance seems on the order of 10 pf ... making the HV arm capacitance 10 fF which with plate capacitor calculations gets near the dimensions of the probe within an order of magnitude or so.

With 20 cm separation it's also a Faraday cage up to 1.5 GHz, so that should be enough.

I guess I grossly overestimated the amount of capacitance necessary in very high voltage dividers in your thread in the project forums, as well as shielding.

Steve Conner

Wed Dec 22 2010, 11:46AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I presume you guys have read and re-read Sam Goldwasser's "Basics of High Voltage Probe Design"?

wrote ...
(From Winfield Hill (**link**).)

A significant part of the design effort (and cost) deals with, the problem of how to go smoothly from a resistive divider at low frequencies, to a capacitive divider at high frequencies, while keeping a constant attenuation value at mid-frequencies. This isn't easy. Consider for example, that an overall shield is clearly needed and must properly prevent the high-Z end of the probe from simply acting as an antenna (as some HV probes do! i.e. ground the tip of the probe and *still* see large signals at the output). This shield acts as a capacitance to ground for the HV resistor, routing some of the high-frequency current which is supposed to go to the output, to ground. Hence at some middle frequency there's a dip! This is solved in various ways - with shields connected to the probe tip (but inside the ground), capacitors bypassing the resistor, special resistor construction, etc. Most solutions can just as easily cause a region with a response hump, as well as a dip, or even both. BTW, these problems are much harder if one seeks to make a probe with very low capacitive loading and high frequency response. The Tek P6015A probe is 3pF, and you'll also note it has a veritable raft of response adjustments on the scope-input end.

Looking at the Northstar, if there were no shield inside the plastic HV tube, I think it would have a lot of "antenna" response and utterly fail the grounded tip test. (The faraday cage assertion is incorrect, it doesn't take account of fringing fields- "evanescent waves" for you quantum weenies)

Yes, that is one of the guys who wrote The Art Of Electronics.

Antonio

Wed Dec 22 2010, 11:50AM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

The capacitance ch is not due to the tip only, but a distributed effect along all the high-voltage resistors. The tip shape was dimensioned so it does not produce corona when properly connected to a HV conductor within the voltage range of the probe. See the probes for higher voltages in the manual cover to see how to avoid corona.

Pinky's Brain

Wed Dec 22 2010, 11:55AM

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

You can't have both a near full length shield on the order of 10 cm, a radius on the order of a couple of cm AND a total capacitance of a couple pF.

The shield alone would be pF range ... and the LV arm capacitance has to be 1000x as much (and determines the capacitance of the probe).

If it's not an effective RF shield, so be it ... there is no alternative.

Steve Conner

Wed Dec 22 2010, 12:01PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well, if you look in the manual, you'll see that the input capacitance is 10-15pF. That's huge by HV probe standards, it suggests that inside the tube is simply a series string of resistors with a capacitor across each, swamping the electrostatic capacitances by brute force.

I also notice that the probes have a 4 meter lead made of regular 50 ohm coax, which also shows that they didn't really care about achieving low input capacitance. (You need more input capacitance to drive the cable capacitance, which adds to the low arm of the capacitive divider, and is of the order of 100-200pF per meter.)

All in all this is probably a realistic design for a hobbyist to copy. I remember Dan McCauley/EasternVoltageResearch made something like it.

Pinky's Brain

Wed Dec 22 2010, 12:18PM

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

Ugh, you're right ... I thought the capacitance was from the point of view of the scope, but it's from the load of course ... the LV arm will have nF range capacitance. So back to needing distributed capacitance ... the problem with distributed capacitance with discrete components is the amount of inductance in the leads ... that's going to sing if you don't damp it.

Steve Conner

Wed Dec 22 2010, 12:30PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes. Regular scope probes have the same problem, the probe's input capacitance and the inductance of the body and ground clip form a resonant circuit. When scoping a fast square wave, the pulse shape you see depends on the placement of the ground clip.

With one of these HV probes connected to a fast HV pulse source, the problem is exactly the same in nature, only worse in amount. There is mention of a series damping resistor of a few hundred ohms, built into the probe tip, in the instructions. (and a warning to remove it for high frequency measurements, in case it burns out)

There is also the ground loop problem of RF current flowing through the probe capacitance and along the probe lead's screen. That screws up your results worse as the division ratio of the probe gets higher, not to mention that it can actually be dangerous. Northstar caution that the bottom of the probe needs to be really well grounded to the HV system, and things might get damaged or destroyed if it's not. I believe this is a standard problem for any HV lab. We use big ribbons of sheet copper on the floor in ours, to join all the grounds of a HV test rig together.

Pinky's Brain

Wed Dec 22 2010, 01:02PM

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

Come to think of it, I don't think there are any resistors in that transparent bit ... just a conductor. I think it's just an air gap from the tip to the grounded base. The actual divider is probably inside the base (which could then act as a Faraday cage). 40 kV isn't that much, plenty of room for insulation there.

Patrick

Wed Dec 22 2010, 06:10PM

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

WOW! ive never had so many responses in such a short time.

But as to Pinky's straight wire no resistor in the tube comment, im worried when i place a shield that close to a resistor i get 14pF ...at least. i think seperation from the ground and shield is how they get such low input C. so i think the resistors are in that plastic blue or clear part.

Also i belive there is a circle of metal connected to the HV tip, which equalizes the field distribution, between that internal circle and the curved lip of that ground cup. So internally it looks like their bigger 100+kV ones, but since its under oil and plastic, we dont get to see it.

Steve McConner wrote ...

I presume you guys have read and re-read Sam Goldwasser's "Basics of High Voltage Probe Design"?

yes, i read this often, it checks my sanity.

This is the kind of stuff that keeps me up at night.

EDIT: e-mail "College student, need probe... " Reply from NorthStar to me:

Dear Patrick,

The PVM-3 is $2400 with a typical 5 - 6 week lead time. We have a
resistor shrouded by a coaxial capacitor and the combination of the
high side of the capacitor and the grounded "Bell" on the bottom
shield stray fields. The capacitance between the resistor and
capacitor is compensated by an RC network. We also offer the PVM-1
with 2000:1 attenuation, at $1850.

As from my contact at NorthStar.

A Drawing based on an interpretation from the above...

if you have a differing idea, please draw it, and upload it.

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