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" <[email protected]>
Date: 3/22/18 8:58 PM (GMT-06:00) To: kc9ieq <[email protected]>, Discussion of precise voltage
measurement <[email protected]> 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 <[email protected]> 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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