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A 75kV 1000:1 Voltage Divider With 3G Input Impedance

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Proud Mary
Tue Sept 07 2010, 12:38PM
Proud Mary Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Patrick wrote ...

hey Proud Mary... i got out my old 2 giga ohm, 1000:1 resistive voltage divider and connected it up too a 270vDC (+- 1.2 %)source then used a miliVolt meter to mearsure the low side but it wasnt stable it should have read 27mV but instead it would go down to 9mv and up to 41mv very slowly, no loose leads seemed to be the cause,i have no idea if its become and antenna or not? i need help. any ideas?

With only 135 nA bottlenecking through your 2G @ 270V, you'll get erratic performance alright, Paddy.

Firstly, the the input offset current of the DVM input device will become significant in the nA regime.
Then the input of your meter should be considered as Rin parallelled by Cshunt, a combination which has a time constant, as Cshunt takes time to charge.

With only 135nA to be split between Rmeasure, Rin, the offset current and the charging of Cshunt, no wonder it takes time!

If you measure charging time t, then you'll be able to back calculate Cshunt, and see whether the answer looks plausible or not.

Leakage currents and input offset current will be signicant when contrasted with your miserable 135nA, and perhaps induced exogenous noise currents too, I would think, as you suggest, so you're most likely looking at a multifactorial error.

My advice: put an electrometer transconductance amplifier ahead of your DVM, but failing that, put some real voltage across it! cheesey


Added Much Later:
Pity I didn't think of all this when designing my own voltage divider! cry
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Patrick
Tue Sept 07 2010, 02:23PM
Patrick Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
ok ill turn the dial up then, i forgot about burden current, and the input draw of the meter.

and ill be waiting to see your finished device, i would like to have another instrument and person to duplicate test result to see how predictable our stuff is, though the HV source would play a roll in this.
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radhoo
Fri Dec 10 2010, 06:09PM
radhoo Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
any news on this?
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Patrick
Fri Dec 10 2010, 07:38PM
Patrick Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
i didnt mean then or now to hijack Proud Marys thread, but my resistor string failed in this attempt.
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Arcstarter
Fri Dec 10 2010, 09:10PM
Arcstarter Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
I love you Stella cheesey.

I am going to use bigger EHT supplies so i can pull plenty of current for testing tongue. I hope yours works well with such high dropper resistance!
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Patrick
Thu Dec 16 2010, 09:11AM
Patrick Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
I am approaching a build soon, I would like to know if you have any new info for this thread, Proud Mary?
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Proud Mary
Thu Dec 16 2010, 09:37AM
Proud Mary Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Patrick wrote ...

I am approaching a build soon, I would like to know if you have any new info for this thread, Proud Mary?

I'm just checking to make sure it works across the entire range 10kV - 75kV with just 2GΩ rather than the 3GΩ originally planned, which let so little juice through on the low end that I couldn't measure it accurately, as we've discussed.

It's a perverse world where the measuring instrument draws more current than the application, but there it is!
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Patrick
Thu Dec 16 2010, 09:49AM
Patrick Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
yes, I thought you would reduce the impedance. And yes it is quite a punch to the brain, to make that realisation, I was so ticked off with myself.
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