[volt-nuts] How can I make a 2000 V DC meter with an input resistance of at least 100 T ohms?

[ed breya]

I'm guessing the application relates back to your leaf electrometer 
project discussed earlier - trying to assess how the bias charge on the 
capacitor holds up from leakage and use of the instrument. If this is 
the case, then it's for a one-time use for design of the item, so 
shouldn't be too fancy or expensive. I believe the original goal was to 
have the cap get charged up and then not need any electric support for 
the leaf electrometer, appearing totally passive, for some amount of 
operating time. If built-in monitoring of the cap voltage is now 
desired, that's a different story.

If the measurement is just for design, to roughly see the cap 
charge-holding time situation, then I'd recommend using methods that 
Chris described, comparing to a variable HV supply at various times and 
settings - all manual iterations, but doable. You can always say, 
recharge the cap, then guess what the voltage may be after so much time, 
then set the test supply and compare - over and over and over.

If continuous, long-term, fairly accurate monitoring is desired, then 
you'd have to go with some sort of non-contact electrostatic voltmeter 
or such, as others have mentioned.

Relating back to recent discussions, it's pretty clear that you're not 
going to find an actual specified resistor in the hundred T-ohm region. 
You can certainly make your own from T-ohms to infinite, but you won't 
be able to know the "exact" value. The commercial instruments that have 
say "200 T-ohms" input R don't actually have that resistor value inside 
- it's an "effective" or "equivalent" derived value that depends on a 
real resistance of maybe E11-E12, multiplied by system gain.

Some electrometers like the old Keithleys have a voltage mode where the 
high-Z input amplifier is bootstrapped up as a voltage follower, but 
have less range than you want. It's conceivable that you could build the 
same thing, but with a HV amplifier follower that can reach the desired 
level. This would not be trivial.

Again, if the purpose is just to measure the droop in bias voltage of 
the charged cap over certain time intervals, there may be another 
option. Since this is a dv/dt rather than DC measurement, you could 
possibly set up an electrometer to view the change of the bias voltage 
via current through another capacitor, and conceivably even rig it up to 
directly measure the total change in cap voltage over a given time.

Let's say the charge storage cap is 1 uF, and you put a much smaller, 
less leaky, test cap plus some protective series R from the HV node to 
the input of the electrometer, and also clamp the input with a low 
leakage diode circuit. The test cap could be say 100 or 1000 times 
smaller than the main cap, so its effect will be small. This could be in 
the 10 nF or less range, where it should be fairly easy to find 3 kV or 
so rated metalized film plastic capacitors with suitably low leakage. 
Any constant DC leakage from the cap could be zeroed out or accounted 
for, at least for short-term measurements.

The electrometer could then read the test cap current directly 
proportional to dv/dt, or integrate it back up to delta V in the charge 
mode. There are limits to the reasonable measuring ranges, of course. 
For example, 1 nF would provide 1 nA at 1V/sec - a fairly easy 
measurement. But 1V/1000 seconds could be tricky - only 1 pA to work with.

Ed


On 3/22/2018 7:12 PM, kc9ieq via volt-nuts wrote:
> I guess I don't see what the issue is.  No, impedance is not infinate when not nulled, but this is why V supply #2 Is adjustable by whatever convenient means.  Rough adjust, connect, adjust for null, measure.  Rinse and repeat.  If it were my project, I'd just run up an HV transformer on a variac, with a rectifier, cap, and probably some series R thrown at it to limit current through the meter.  Curious to know what the application is, if this will not work.
> Good luck with whatever solution you choose.
> Regards, Chris
>
>
> Sent from my SMRTphone
> -------- Original message --------From: "Dr. David Kirkby" <drkirkby at kirkbymicrowave.co.uk> Date: 3/22/18  8:58 PM  (GMT-06:00) To: kc9ieq <kc9ieq at yahoo.com>, Discussion of precise voltage measurement <volt-nuts at febo.com> Subject: Re: [volt-nuts] How can I make a 2000 V DC meter with an input resistance of at least 100 T ohms?
> On 23 March 2018 at 01:49, kc9ieq via volt-nuts <volt-nuts at febo.com> wrote:
> How about using (or building) an additional 2kV power supply and a sensitive meter movement like a differential voltmeter, adjusting for/measuring the null?  Impedance at null will be theoretically infinate, current will be theoretically zero, and you can measure/monitor the voltage of your second supply directly with the probe/meter of your choice.
>
> Regards,Chris
>
> No, that will not work for me, as while the impedance at null is infinite, it is not when not nulled, and that will mess up the measurements.
>
> Absolute accuracy is not important. +/- 10% or even 20% would be okay. I want to measure a couple of voltages and compare them. As long as the meter reads the same with identical input voltages, that is fine.
>
> Dave
>
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