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Nano current inrush switch/low voltage cutout

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Howardlong:

--- Quote from: David Hess on March 18, 2019, 05:00:44 pm ---
--- Quote from: Howardlong on March 18, 2019, 11:01:08 am ---I used 1N4148 which aren't quite as good as 1N4007 in terms of reverse leakage current. It wouldn't work at 10nA but would at 20nA, although it would take about 150 hours to charge up the reservoir cap sufficiently to trigger!
--- End quote ---

Gold doped diodes like the switching 1N4148 and fast recovery rectifiers have much higher leakage currents.  If you want a low leakage diode, then use a 2N3904 base-collector or base-emitter junction, or something like a BAS116 or BAS416.

--- End quote ---

Thanks for the tip. I did measure the reverse bias current over an indicative range of voltages for both the 1N4148 and 1N4007 as mentioned earlier. It was just a bit easier to breadboard the 1N4148. I could use resistors, but the biggest I have in stock are 100Mohm. Practically speaking, reverse biased pn junctions are going to be cheaper than resistors in this range anyway.

Howardlong:
With a 2N3904, I measure 35pA at 3v BC and 48pA BE, so yes, not bad!

SiliconWizard:
What lab instrument are you using to measure currents in the tens of pA range?

David Hess:

--- Quote from: Howardlong on March 18, 2019, 11:57:07 pm ---With a 2N3904, I measure 35pA at 3v BC and 48pA BE, so yes, not bad!
--- End quote ---

That must be a bad one.  I usually get more like 2pA.   :)

2N3904s from some manufacturers are better than others.

If you want an inexpensive low leakage diode with guaranteed specifications, then the 2N4117 JFET is the way to go.


--- Quote from: SiliconWizard on March 19, 2019, 12:32:45 am ---What lab instrument are you using to measure currents in the tens of pA range?
--- End quote ---

There are a couple of easy ways to make measurements in this range.  A standard voltmeter with 100 microvolt resolution and a 10 megohm input resistance has a resolution of 10 picoamps when used in voltage mode to make current measurements.

If I need something better, then I configure an LMC6081 as an integrator, have it measure its own input bias current which will typically be about 2 femtoamps, and then use it to measure whatever I am interested in.

Howardlong:

--- Quote from: SiliconWizard on March 19, 2019, 12:32:45 am ---What lab instrument are you using to measure currents in the tens of pA range?

--- End quote ---

Keithley 237 SMU with triaxial probing.

I do a bit of nano amp ultra low power microcontroller stuff, I used to make do with DMMs and uCurrent, but an SMU is really the way to go. Recently I bought a Current Ranger, which is OK but it loses accuracy quite badly the closer you get toward the bottom of the uA and nA ranges.

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