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

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Proud Mary

Thu Jul 01 2010, 08:14AM

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

Patrick, I was going to suggest you used vacuum capacitors in the input, but didn't do so for two reasons:

1) A 100kV series string would be very expensive, even using Russian ones.

2) Unless you keep the voltage across each one down to a very low level, you will have field emission, and your capacitors will discover their true vocation as cold cathode X-ray tubes.

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

Decent test equipment often has a 'set zero' button or similar control to take account of drift and the real world conditions of the day.

Patrick

Thu Jul 01 2010, 08:22AM

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

yeah, i thought there might be a catch, x-rays are only fun to play with when they dont cause death.
ok, back to olive oil.

well i knew of vacuum caps, but thought them to be low voltage type, i didnt realize they went upto 20-30 kv, and proud mary as you said i wondered if they might spontaneously and suddenly electron avalanche, with no dieletric to impede the little buggers.

Wolfram

Thu Jul 01 2010, 09:59AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

I think the vacuum capacitor idea needs a bit more consideration.

Variable vacuum caps are expensive, but the fixed value ones are really cheap. A quick ebay search found these

, 5 lots of 2 would cost 84 dollars including shipping. With a rated voltage of 10kV per cap, 10 in series would do for 100kV. At only 10kV per cap, any x-rays will be stopped by the glass. The capacitance would be 2.5pf, a bit higher than your planned, is this a problem?

Anders M.

IntraWinding

Thu Jul 01 2010, 11:14AM

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

I don't know much about the X-Ray hazard from vacuum capacitors.

Assuming metal shielding isn't an option for this project, is the only solution to keep the voltage across any vacuum gap below 10KV?

Are electrode shape, material and spacing not also factors?

I suppose making your own custom vacuum capacitor is out of the question?

EDIT:
Here's a 32KV 6pF capacitor on eBay. Will it be built for low X-Ray emission, or intended to be heavily shielded in use?

Fabio

Thu Jul 01 2010, 11:14AM

Registered Member #122 Joined: Fri Feb 10 2006, 12:55PM
Location: Milano Italy
Posts: 148

Hi All!

mmmm.... really interesting and nice theread, i builded a small X100 scope probe many years ago using commercial capacitors and resistors and now i have also a surplus X1000 unit but i ever wanted something bigger.......

...... what about sulphur hexafluoride???
I have 3 cylinders filled of this gas and this capacitive system seems really cheap and easy to build!

Ciao!
Fabio.

Patrick

Thu Jul 01 2010, 12:52PM

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

yeah Fabio i would like to complete this device and thoruoghly explain it too all. many others could then build the same, less, or better for their own needs.

Proud Mary, as i will post the math later, the result of thermal expansion amounts to no significant change of dimension, plastic, metal, and ceramic. like +0.008 to -0.002 inches at the gap, and the gap will be 1.000 inches, so the capacitence which varies as 1/x, for x>0 of the distance will see very little capacitence change from even very much rod movement, and 0.008 inches is not very much. therefore the capacitence change by mechanical means will not be a significant factor though it will be identified and noted.

Thu Jul 01 2010, 01:25PM

Registered Member #2372 Joined:
Location:
Posts: 62

IntraWinding wrote ...

dugg wrote ...

Sorry, I missed why you cant just use a high voltage probe.

Normal high voltage probes have a very low frequency response.

I usually use voltage probes from this company

they can do 8 MHz at 550 kV, I dont know how high your frequency is though.

Patrick

Thu Jul 01 2010, 01:32PM

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

8 (or even ju$t one) of the VD floor standing ones would be nice, but im not made of gold.
dugg if you have used them before how much do yours co$t?

100kvac @ 100khz would be nice but illl be greatful if i get even close to that. if i could measure 550kv at 8Mhz i could ware a pink polka dot dress, and still be happy.

IntraWinding

Thu Jul 01 2010, 02:02PM

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

dugg wrote ...

IntraWinding wrote ...

dugg wrote ...

Sorry, I missed why you cant just use a high voltage probe.

Normal high voltage probes have a very low frequency response.

I usually use voltage probes from this company

they can do 8 MHz at 550 kV, I dont know how high your frequency is though.

Wow! Impressive, and presumably expensive kit!
Here's a permanent copy of the pdf for future reference
]north_star_high_voltage_ac_probes.pdf[/file]

Here's all they say about the internals at the web site:
Low temperature and voltage coefficient resistors and capacitors are used throughout the probe to provide maximum measurement accuracy. The probe has a rugged nylon housing, and it is filled with dielectric oil for maximum high voltage stability.

EDIT: But I just found this Probe Manual there which should make a worthwhile read:
]north_star_high_voltage_ac_probe_- _manual__data.pdf[/file]

Thu Jul 01 2010, 02:29PM

Registered Member #2372 Joined:
Location:
Posts: 62

I usually use the PVM-5 series ones and those are about 4grand. If you are at a university you can just buy one on your grant, assuming it has any money on it (mine never seems to).

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