Re: [volt-nuts] How can I make a 2000 V DC meter with an input resistance of at least 100 T ohms?
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@kirkbymicrowave.co.uk Date: 3/22/18 8:58 PM (GMT-06:00) To: kc9ieq kc9ieq@yahoo.com, Discussion of precise voltage measurement volt-nuts@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@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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On 23 March 2018 at 18:49, ed breya eb@telight.com wrote:
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.
Yes it is.
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.
Built in monitor is not required, but if I could design something that has
a performance to allow that, I would be interested to see exactly when the
voltage drops (or rises).
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.
Part of my reason was to know if its possible to connect two electrolytic
caps in series to increase the working voltage, without any parallel
bleeder resistance. In one test, I tried charging a 600 V cap up to 1000 V,
using the power supply in my 4339B high resistance meter, which is limited
to 1 mA. The voltage would not rise above about 700 V, suggesting to me
that perhaps the leakage might increase as the voltage rises, so maybe
bleeder resistors are not required, apart for safety reasons. Safety could
be addressed other ways.
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.
So how does one make ones own resistor? I was thinking of perhaps nails in
wood, where the moisture content would control the resistance. I suspect
that idea would fail because DC would polarise the water molecules. But it
did cross my mind as a possible way.
The highest value commercial resistor I have found at a sensible price is
10 T ohms for £41 from Mouser, but that is on a 2 month lead time.
I do have the Agilent 4339B high resistance meter, so can measure high
value resistors. The basic uncertainty of that meter is 0.6%. Measuring 10
T ohm, I calculate the uncertainty would be 4.5%, so more than adequate as
a starting point. Later a DVM could calibrate a setup.
For the case of a 47 uF cap charged up, if I used a commercial 10 T ohm
resistor, then the time constant is 15 years. So a 10 T ohm input R would
be fine. For a 2.2 nF cap, which is one of which I have a 15 kV model, 10 T
ohms would give a time constant of 6 hours, which would mean the load is
not be negligible.
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.
Most/all the Keithleys do 200 V, which is outside the range of most
semiconductors directly.
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.
My main issue was to measure the voltage across two series connected
capacitors, to find out how equally it split.
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
Regarding making your own extreme high-value resistors - any object that
has insulators and leads but with nothing connected inside will have
some high R that can be perhaps be measured, but won't be stable against
environment effects on the outer surfaces. There's not much point to
carbonizing things for home-made ones, except for curiosity.
You can, however, use existing things that are fairly stable internally,
have hermetic seals, and can be treated externally to reduce environment
issues. I mentioned that reed relay capsule that I used as an unknown,
but very high, yet not infinite R. Burned out light bulbs, vacuum tubes
(especially something like a 5642 HV rectifier - fairly small, lots of
glass), and xenon flashtubes are other examples of common hermetic
glass/metal parts that can be used. But, the R is what it is, and can't
readily be adjusted, only measured and maybe used in circuits that can
accommodate the value. Also, along with the R, there will be some C that
depends on the structure of whatever is used. The C can be good or bad,
depending on the application.
At extreme values, the surface characteristics will dominate, so the
glass envelope would have to be silicone treated. Then the measured R of
the device will be almost all intrinsic. So, you can measure it, but you
won't know how stable it may be with temperature and voltage and time,
for example, so don't expect much precision.
Regarding over-voltaging electrolytic caps - you can reform caps to
somewhat higher voltage, given enough time. They are formed
electrolyitically to begin with, so the dielectric layer thickness is
right for the rated voltage. If you gradually up the voltage, the
thickness will increase and the C will go down over time. It's best to
just use them only up to the design rating though, or the leakage will
become unpredictable.
A good way to do voltage splitting/protecting on medium-high voltage
series connected electrolytic caps with low leakage, is with an
appropriate high voltage "Zener" (actually an avalanche device, not
truly Zener) across each one. The Zeners will prevent over-voltage of
the caps in the normal direction, and reverse protection in the diode's
forward region. Look for transient voltage suppressors (TVS or TVSS)
devices to get into the hundreds of volts region, and of course they can
be stacked for more. Unipolar ones will provide intrinsic reverse
protection for the cap, while bipolar ones will not. They are usually
specified fairly loosely in terms of leakage current, but it should be
possible to find ones in the low nA region at applied V reasonably below
the knee, at room temperature. That sounds like a lot in a High-Z
context, but it's almost certainly much less than the leakage of a
typical electrolytic cap.
Ed