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

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Patrick

Tue Jul 20 2010, 01:00AM

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

this kind of resistor would be used for final calibration.

]high_voltage_dividers_300.pdf[/file]

Proud Mary

Tue Jul 20 2010, 08:23AM

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

These application notes from Ross Engineering may be worth a look, Patrick:

For best AC accuracy HV clearances should be a radius equal to twice the height of the unshielded divider....If in a confined area with proximity clearances marginal, the AC divider should be calibrated in place or in a simulated area. DC accuracy is not affected unless corona is present.

Patrick

Tue Jul 20 2010, 10:21PM

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

@Proudmary: i have been reading the ross engineering stuff, but mine is shielded , and the 8 dividers will be in close proximity, so i need to think about clearances.

@intrawinding: i now see your point in keeping R6 at 10M, while changing the ratio to 6000, at 150 KV this equals the absolute max differential voltage at input (30v), however then i need the op-amp to have a gain of less than one, the only way to do this is to use a divider at one of the inputs, then set gain for graeter than one. only this way can a 5000:1 or 6000:1 ratio work, also, allowing a more than doubling of the HV cap (maybe i can get it to 3pF)

here was the real start of my wanting a CVD, after i read this PDF.
]cvd400.pdf[/file]

IntraWinding

Wed Jul 21 2010, 10:43PM

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

Patrick wrote ...

@intrawinding: i now see your point in keeping R6 at 10M, while changing the ratio to 6000, at 150 KV this equals the absolute max differential voltage at input (30v), however then i need the op-amp to have a gain of less than one, the only way to do this is to use a divider at one of the inputs, then set gain for graeter than one. only this way can a 5000:1 or 6000:1 ratio work, also, allowing a more than doubling of the HV cap (maybe i can get it to 3pF)

I got my figures for the division factor wrong: I kept thinking your network was designed to divide by 1000 instead of the correct figure of 10,000. When I suggested changing the capacitors to give a division by 1500 and then bring this back to 1000 with gain from the op-amp, I should have said, change the capacitors to give a division of 15,000 and use the op amp to bring this back to 10,000.

Bandwidth: I'd be tempted to try the capacitative divider connected directly to 1M Ohm input of your oscilloscope thus eliminating any performance bottleneck the op-amp might introduce. I think the main reason for using the op-amp in Sam Goldwassers circuit was to isolate the scope from potentially damaging high voltages if you forget to earth the lower end of C2. Another method is just to make sure you never forget to earth C2!

I'd start with your original 0.7pF, or there abouts. The smaller this capacitance the lower the load the probe makes on the signal being measured. If such a low value proves problematical, then increase it as required as a last resort, but I think the shielding should help a lot if not completely solve that.

Patrick

Wed Jul 21 2010, 11:57PM

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

IntraWinding wrote ...

Bandwidth: I'd be tempted to try the capacitative divider connected directly to 1M Ohm input of your oscilloscope thus eliminating any performance bottleneck the op-amp might introduce. I think the main reason for using the op-amp in Sam Goldwassers circuit was to isolate the scope from potentially damaging high voltages if you forget to earth the lower end of C2. Another method is just to make sure you never forget to earth C2!

yes i was tempted and may skip the op-amp, but remember my calibration is done through the opamp, otherwise i will have a +-10% accruacy, becuase of the 2 sig figs from my BK precision 875, the divison ratio might be as low as 9,500 or as high as 10,500.

IntraWinding

Thu Jul 22 2010, 03:59AM

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

Calibrate by using a slightly undersized C2 and adding small capacitors in parallel.

Patrick

Thu Jul 22 2010, 10:14PM

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

im not sure about the math on that, the sig figs of my calcs seem to be the most deciding factor.
ill look at my meter and the math again to see if that will work.

NOTE: i have 4 sig figs for my 200pF range, and 125-175pF or so is the most accurate region on the 200pF range. maybe getting rid of the opamp is a good idea.

so, 1.500nF x 7 caps = 10.50nF

i am also trying to figure out absolute accuracy on a measurement device , does any one know what this means?
"Accuracy is Â± ( 1% of readingï¼‹number of digits )."

Proud Mary

Fri Jul 23 2010, 12:30AM

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

Patrick wrote ...

im not sure about the math on that, the sig figs of my calcs seem to be the most deciding factor.
ill look at my meter and the math again to see if that will work.

NOTE: i have 4 sig figs for my 200pF range, and 125-175pF or so is the most accurate region on the 200pF range. maybe getting rid of the opamp is a good idea.

so, 1.500nF x 7 caps = 10.50nF

i am also trying to figure out absolute accuracy on a measurement device , does any one know what this means?
"Accuracy is Â± ( 1% of readingï¼‹number of digits )."

ISO GUM: Guide to the expression of Uncertainty in Measurement

Patrick

Fri Jul 23 2010, 07:12AM

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

Proud Mary wrote ...

ISO GUM: Guide to the expression of Uncertainty in Measurement

thats a long pdf ill read it this weekend.

also, great knowledge gained from this prototype, i did a full copper shield capacitence test, under corn oil
the capacitence went to 2.6pF right away! after the gapwas set too 0.1inch the cap was at 3.8pF
ok heres the table:

0.1" @ 3.8 pF
0.4" @ 2.5 pF
0.8" @ 2.2 pF
1.1" @ 1.2 pF

i will get olive oil tomarrow. and try again.

i can also verify resistence to stray fields with the existing copper sheild diementions. (the ball is 0.500" dia, and at least two diameters(1.0") below the copper upper edge (while the gap is at 1.1"). so the shield cofiguration works well.

Proud Mary

Fri Jul 23 2010, 08:53AM

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

For the sake of interest, I read the chapter on High Voltage Probes in

Zucconi, B. Probes New York 1955, p. 99 et seq

Zucconi gives an illustrated practical account of constructing a 100:1 capacitive divider using the inter-electrode capacitance of an unheated 1X2-A EHT diode rectifier as the sampling capacitor - an improvised vacuum capacitor of 1pF according to the General Electric 1963 datasheet.

The anode top cap of the valve is used as the actual probe tip, and the measuring capacitor consists of a trimcap in parallel with the capacitance of the coax leading from the probe to the oscilloscope.

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