I was driving home over Bernal Heights, full of pretty much legal amounts of Vietnamese food and beer, when this popped into my head for no reason:
formatting link
It's feasible down to the hundreds of fA maybe. Any fA measurement will be slow. The idea here to integrate the current into a good cap, and use the DUT itself as the sampling switch. And use available stuff.
--
John Larkin Highland Technology, Inc
lunatic fringe electronics
That's more or less how my Footprints sensors worked, except that the cap was a PVDF pyroelectric pixel and the diode was a LED, with dim external light to provide a few picoamps of DC bias.
I think my measurement limit is sheer patience, and the nonlinear capacitance of the diode coupling a spike into the scope, which distorts the real signal.
I already built a fA leakage tester, and have an old Keithley electrometer, so this is just a random idea.
If the leakage:temperature relationship holds up, one could test diodes hot and improve measurement speed and resolution.
Something might be done with a good integrating cap and a reed switch/magnet to sample the cap voltage. But that means building stuff.
Someone here pointed out an opamp with absurdly low bias current. That could snoop the slope of a cap charged by diode leakage. With calibrations, that might get down close to 1 fA resolution.
--
John Larkin Highland Technology, Inc
lunatic fringe electronics
In my experience, simple ceramic NP0 capacitors have leakage well below a fA. I used a 10pF NP0 ceramic 1206 as the feedback cap in an ionization chamber pre-amp with an LPC661 op-amp. It ramped more or less linearly with 150aA into the cap.
Any air that has moisture in it, has a few parts--per-billion ionized particles. Air is free, and is a pollutant. Light also should be excluded, if you care about small leakage currents. A planet like Earth, in vacuum, would be a fine low-leakage capacitor, if there weren't that darned solar thing spewing trash nearby...
Metals like Cu, Ag, Ni, Al are generally not going to have enough unstable isotopes to worry about, but there's some concern about any carbon 14 in the PTFE.
However, I'm not so sure it applies to humidity in air, since the water particles are separated from each other. For example, in sea air with a
or 9e13 ohms per meter. How much of this is due to the salt in the air is not known.
formatting link
9e13 ohms/meter for humid air is much greater than 18e6 ohms/cm for pure water.
OTOH, everyone experiences worse static sparks in winter, when the humidity is quite low. However, this may be due to increased triboelectric effect when walking across a dry carpet.
Another example is lightning in clouds. A cloud has high humidity, yet is capable of building up very high voltages which lead to lightning discharges.
Scrap the last part. Any humidity in thunderclouds is probably frozen due to the low temperatures. The ionized molecules cannot move, so the conductivity is very low. This leads to the triboelectic effects which build up to high voltages.
That's liquid water. Water vapour doesn't self-ionise.
m-1,
What's going to make air conductive is ions created by cosmic rays or local radioactivity (as in trace of potassioun-40), and what affects the conduct ivity is whatever quenches those ions - which is to say provides a route fo r the freed electroncs to get back to the positive ions from which the elec trons had knocked out.
ty
The bit of the thundercloud that charges seems to be cold, and it seems to be ice crystals that carry the charges - large crystals seem to mostly get negatively charged and sink while small crystals get positively charged and get blown upwards.
It's not just liquid and vapor phases, it's also films on surfaces (and mos t metals attract water molecules, which are polar). Because the capacitor has surf aces, water may cling... probably to the electrodes but even PTFE can have active sites.
Charge leakage in humid air is well known, and (like lots of suface effects ) not easily understood. Some quantitative info here:
I don't think the triboelectric effect changes appreciably, but the surface conductivity of objects greatly increases with humidity. That's why you c an shove the end of the anti-static foot strap into your shoe and not down your sock. No matter what the humidity in the air, it is very humid in you r shoe.
Isn't that a counter example? I think the static building process simply o verwhelms the conductivity of the air... at least until the voltage gets hi gh enough, then the air becomes *very* conductive.
Condensed water would be frozen, but water vapor is still a gas at all temperatures. As it gets colder the vapor pressure drops, but it is still a gas.
I have a vague recollection of solid sulfur being used as a low-leakage insulator. This may just be because PTFE (for example) had not yet been invented!
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.