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
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