diode leakage tester

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:

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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.

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John Larkin         Highland Technology, Inc 

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John Larkin
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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.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

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.

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John Larkin         Highland Technology, Inc 

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John Larkin

Huh, OK. You need to load enough charge through the diode I guess.

What a good cap? George H.

Reply to
George Herold

Where can I buy a capacitor with 10x less leakage than a fA?

Reply to
John S

I suggested this be usable to hundreds of fA. I don't think the cap is the limit.

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John Larkin         Highland Technology, Inc 

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John Larkin

That involves a lot of waiting.

Polystyrenes are good, if you can still get them. It's not hard (but takes a while) to check the cap leakage.

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John Larkin         Highland Technology, Inc 

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John Larkin

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.

It took three days to run into the rails.

Jeroen Belleman

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Jeroen Belleman

900 electrons per second.
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John Larkin         Highland Technology, Inc 

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John Larkin

Teflon is sold in rolls as sheet goods. So, you can build your own!

Extra points for enclosing it and pumping out the air...

Reply to
whit3rd

Air is free!

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John Larkin         Highland Technology, Inc 

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John Larkin

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.

Reply to
whit3rd

That applies to water, which self-ionizes:

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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.

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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.

Reply to
Steve Wilson

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.

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Steve Wilson

One fA is 6250 electrons per second. That's actually a lot.

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John Larkin         Highland Technology, Inc 

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John Larkin

hms/cm.

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.

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Bill Sloman, Sydney
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bill.sloman

gohms/cm.

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:

Reply to
whit3rd

hms/cm.

m-1,

ty

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.

Rick C.

Reply to
gnuarm.deletethisbit

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.

Rick C.

Reply to
gnuarm.deletethisbit

(and most metals

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!

Mike.

Reply to
Mike Coon

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