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

[ed breya]

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